A REFERENCE FOR THE REST OF US!
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** About the Author
At the time of this writing, the author's body temperature was 98.6 degrees Fahrenheit, and he was breathing normally. But beyond the physical characteristics, the author of this book has also has written over a dozen computer books including MORE Visual Basic For Windows For Dummies, Microsoft Office 97 For Windows For Dummies, and MORE Microsoft Office 97 For Windows For Dummies. When he's not writing computer books, he does what other computer authors usually do and writes articles for computer magazines instead. He writes a monthly column for Boardwatch magazine and has written articles for The Net magazine as well.
When not writing computer books, he often refers to himself in the third person singular form, such as in this current sentence. While such self-referential sentences tend to look authoritative and profound when appearing in print within a book's "About the Author" page, they tend to make people stare in a most peculiar way when spoken out loud in public. Not many people introduce themselves at parties as "Hi, his name is Wallace," and this author, of course, is no exception.
In his spare time, Wallace can often be seen watching the demolition crews knock down the buildings in San Diego that used to house his former employer. Wallace's only regret is that the demolition crews didn't start much earlier when his former boss and coworkers would still have been inside.
** Dedication
This book is dedicated to all the wonderful people I've met during my mad pursuit in the world of stand-up comedy:
Budd Friedman, for giving me my first national TV appearance on his show A&E's Evening at the Improv.
Mark Kuker, for offering delightfully strange, one-nighter gigs in Ruidoso and Deming (home of the annual duck races), New Mexico. Mark and his wife Amanda are two of the best people a comedian could ever hope to work for.
Steve Schirripa and Don Learned, for giving me my first Las Vegas gig at the Riviera Comedy Club, located in the Riviera Hotel & Casino. The next time you're in Las Vegas, drop by, see a show, and dump some money in the Riviera Casino slot machines. Who knows? Maybe you'll win enough money to buy yourself a new computer.
Patrick DeGuire and Dat Phan who helped me put together Top Bananas, our company devoted to booking the three of us in corporate comedy events around San Diego. Thanks also goes to Fred Burns, Leo (the man, the myth, the legend) Fontaine, Ron Clark, Dante, Frank Manzano, Chris (the Zooman) Clobber, Bob Zany, Tony Vicich, and George Hirschmann.
Final thanks go to Gene Perret, Linda Perret (and the rest of the whole happy Perret clan), Liz Sage, and everyone else who has helped me navigate my way through the minefield of inflated egos and hair-trigger temperamental personalities known as show business.
** Author's Acknowledgments
Nobody writes and publishes a book without the help of other people, and this book is no exception. It goes without saying (although I'm going to say it anyway) that two of the most important people responsible for this book are Matt Wagner and Bill Gladstone of Waterside Productions. Thanks guys. I'd give you more than your usual 15 percent cut but if I did, then I wouldn't have anything left over to pay for my groceries.
Two other people who deserve thanks include Brian Kramer of IDG Books Worldwide, Inc., and Allen Wyatt of Discovery Computing, Inc.
Next, I have to acknowledge Cassandra (my wife) along with Bo, Scraps, Tasha, and Nuit (my cats) for their support during the long hours I've spent glued to my computer instead of doing anything else around the house.
A final acknowledgment goes to the friendly people at Complete Design & Remodeling including Wes McKusick and Tom DeLisle who did all the remodeling of my new house while I just wrote them checks.
** Publisher's Acknowledgments
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The publisher would like to give special thanks to Patrick J. McGovern, without whom this book would not have been possible.
++ CONTENTS AT A GLANCE
Reader's Note:
INTRODUCTION
PART I: CREATING A VISUAL BASIC 6 PROGRAM
Chapter 1: How Visual Basic Works
Chapter 2: Using the Visual Basic User Interface
Chapter 3: Designing Your First User Interface
Chapter 4: Writing BASIC Code
PART II: CREATING USER INTERFACES
Chapter 5: User Interface Design 101
Chapter 6: Forms and Buttons
Chapter 7: Boxes and Buttons for Making Choices
Chapter 8: Text Boxes for Typing and Showing Words
Chapter 9: Scroll Bars and Labels
Chapter 10: Pretty Pictures and Objects from Geometry
PART III: MAKING MENUS
Chapter 11: Creating and Editing Pull-Down Menus
Chapter 12: Submenus, Growing Menus, and Pop-Up Menus
Chapter 13: Dialog Boxes
PART IV: THE BASICS OF WRITING CODE
Chapter 14: Event Procedures
Chapter 15: Using Variables
Chapter 16: Responding to the User
Chapter 17: Math 101: Arithmetic, Logical, and Comparison Operators
Chapter 18: Strings and Things
Chapter 19: Defining Constants and Using Comments
Chapter 20: Killing Bugs
PART V: MAKING DECISIONS (SOMETHING YOU STOP DOING WHEN YOU GET MARRIED)
Chapter 21: The If-Then and If-Then-Else Statements
Chapter 22: The Select Case Statement
PART VI: GETTING LOOPY
Chapter 23: The Do While and Do-Loop While Loops
Chapter 24: The Do Until and Do-Loop Until Loops
Chapter 25: For Next Loops That Can Count
Chapter 26: Nested Loops and Quick Exits
PART VII: WRITING SUBPROGRAMS (SO YOU DON'T GO CRAZY ALL AT ONCE)
Chapter 27: General Procedures (Subprograms That Everyone Can Share)
Chapter 28: Passing Arguments
Chapter 29: Functions, a Unique Type of Subprogram
Chapter 30: Getting Some Class with Object-Oriented Programming
Chapter 31: Managing Files
PART VIII: DATABASE FILES AND PRINTING
Chapter 32: Creating Database Files
Chapter 33: Using Files from Database Programs You'd Rather Not Use
Chapter 34: Making Your Program Print Stuff
PART IX: THE PART OF TENS
Chapter 35: The Ten Visual Basic Topics That Didn't Fit Anywhere Else
APPENDIX A: ABOUT THE CD
++ Reader's Note:
This book has been scanned and edited by Jane Etherington. She has marked the beginning of each chapter with a double plus [++], The beginning of each section in the chapters with a double star [**] and each sub-heading with a single star [*]. These markings are to assist the reader in finding their way around the book more easily and rapidly
The reader may come across such mistakes as: The letter "I" for a number 1 or the letter "L" or any combination of these. Other mistakes may also have been missed. These have been corrected when detected, but inevitably, some may have been missed. It is our hope that these minor errors do not detract from the overall enjoyment that this book gives?
++ PART I - CREATING A VISUAL BASIC 6 PROGRAM
In this part ......
Writing your own program isn't hard. If you've always been curious about computer programming but wer intimidated by the hard-to-read books, less-than-useful softward, or obtuse and convoluted "explanations" from "experts," then this book is for you.
Rather than impress you with matematical proofs and theoretical background about computer programming, this book (with the help of Visual Basic) lets you jump right in and start finding out about programming on your own computer all by yourself.
So grab some snack foods, a few carbonated beverages, a comfortable seat, and get ready to program your computer and make it finally to what you want it to do.......
++ CHAPTER 1
HOW VISUAL BASIC WORKS
In This Chapter:
Looking at the Visual Basic development cycle
Creating a Visual Basic user interface
Understanding what BASIC code does
The whole purpose of writing a program is to make your computer do something useful. People often spend thousands of dollars for one of these machines, so it's important that the computer does something more than consume electricity and take up space on a desk.
Before you delve into the world of programming, keep two thoughts in mind. First, anyone can write a program. Programming is just a skill, much like swimming, sailing, or shoplifting. If you've ever taught yourself a new hobby or skill, you can teach yourself how to write a program without an extensive mathematics background or a fancy college degree that puts you in debt for the next ten years.
Second, the key to programming is defining exactly what you want your program to do. Defining what you want your program to do is half the battle (a battle that governments and large corporations routinely lose all the time). The other half of the battle involves taking the time to write your program and making sure it works correctly. (This part of the battle is another place where governments and large corporations fall flat on their faces on a fairly regular basis).
You can write a program to make your computer do anything you want, short of launching nuclear missiles at your next-door neighbor. (Of course, if you write a program for the Air Force's computers, you may even be able to do that).
** Writing a Visual Basic Program
There is no single correct way to write a program. Theoretically, you can use a million different ways to write a program correctly, just as you can travel from New York to Los Angeles a million different ways. Some people may fly, others may take a train or drive; the more adventurous may walk, hitchhike, or hijack a vehicle. Similarly, you can write the same program a million different ways. However, no matter how you write the program, the result can always be the same.
As a programmer, your job is to write a program that works correctly and is easy to use. If your program doesn't work, nobody can use it (although you often can sell a few thousand copies to unsuspecting individuals first). If your program isn't easy to use, nobody will want to use it, even if it works perfectly.
Testing whether your program works is usually simple enough. If your program is supposed to print mailing labels but erases the computer's hard disk instead, then your program obviously doesn't work correctly.
However, determining whether your program is easy to use is a bit more difficult. What you may consider easy to use may be almost impossible for someone else to understand.
To create programs that everyone can understand how to use, Visual Basic helps you to easily produce windows, pull-down menus, dialog boxes, and command buttons. These features are the same ones found in Windows 95/98/NT programs. Visual Basic helps you write programs that look and act like other programs on the market.
Making your program look and act like an existing program can help others learn your program faster. For example, most people can drive a Toyota or a Ford without any problems; the steering wheel and brakes always look and work the same way, even if the windshield wipers and horn may not. The same goes for programs. Pull-down menus contain a program's numerous commands, and you can always use the mouse to highlight and choose commands or objects. So while each program may work differently, they all look and work in similar ways.
* The Visual Basic Development Cycle
Before writing a Visual Basic program (or any program for that matter), get away from your computer and plan your program using an old-fashioned paper and pencil. After you know what you want your program to accomplish and how you want it to look, then you can start writing your program. Skipping this crucial first step is like building a house without blueprints. You can do it, but it will probably take you longer.
Writing a Visual Basic program requires nine steps - three steps fewer than those required to overcome and addictive habit. The first eight steps are what programmers call the development cycle. The ninth step is what programmers call job security.
1. Decide what you want the computer to do.
2. Decide how your program will look on the screen. (The appearance of your program is its user interface).
3. Draw your user interface using common parts such as windows, menus, and command buttons. (The parts of a user interface are objects or controls).
4. Define the name, color, size, and appearance of each user interface object. (An object's characteristics are its properties).
5. Write instructions in BASIC to make each part of your program do something. (BASIC instructions are commands).
6. Run your program to see if it works.
7. Cry when your program doesn't work perfectly. (Required).
8. Fix any errors (or bugs) in your program.
9. Repeat steps 6 through 8 over and over again until you get tired of searching for more bugs.
Although you don't have to memorize these nine steps, you do have to follow them. Shortcuts aren't an option. Trying to skip from Step 1 to Step 4 is like trying to start a car by using the gas pedal but forgetting to turn the ignition key. You can try it, but you're not goint to get anywhere.
Believe it or not, Step 1 is actually the hardest and most important step of all. After you know exactly what you want your program to do, it's just a matter of finding ways to do it. Persistence and creativity are helpful, as are lots of caffeine-laden beverages and plenty of sleepless nights in front of the computer screen.
* Why Programs Don't Work (Part I)
Writing a program that works 100 percent correctly all the time is mathematically impossible. First of all, if you write a program that works 100 percent correctly today, there's no way you can guarantee that it will work 100 percent correctly on future computer brands, models, processors, and accessories. As a result, you can never guarantee that your program will work correctly on all types of computers unless you exhaustively test every possible computer configuration in the world.
Second, not only do you have to test your program with the latest and greatest products (including the ones invented after you wrote your program), but you also have to consider the virtually infinite number of possibilities that your program must face during everyday use.
For example, your program needs to behae correctly if the user presses any key and then clicks the mouse anywhere on the screen. What if the user clicks the mouse by mistake while tapping a key? What if the user pounds the keyboard in frustration? What if another program happens to interfere with the computer's memory, thus affecting your program? What if..
(well, you get the idea).
Unless a programmer can plan for an infinite number of possible problems and situations that a program may face during its existence, then writing a program that works 100 percent of the time is impossible.
What's scarier is that this scenario holds true for every computer operating system in the world (such as Windows 95/98/NT). Therefore, you'll always be writing programs to run on an operating system that doesn't work 100 percent correctly either. This situation is like building a house on a foundation of quicksand and then wondering why your house keeps failing apart.
Because no one has an infinite amount of time to test an infinite number of possible problems, computer programs always (yes, always) will have bugs that keep them from working 100 percent correctly. That includes every program you write and every program that Microsoft's millionaire programmers may write. That's why when you write a program, set aside plenty of time for testing so you can kill any potentially fatal bugs before you give your program to someone else.
So the next time you're using a program that doesn't work right, now you'll know that ifs not your fault; it's the programmer's fault.
* Making a neat user interface
The user interface is what someone sees when your program is running. Every program has a user interface in one form or another. Some programs have elaborate, colorful windows, while other programs have a sparse appearance - as if the programmer were afraid that screen phosphor may be in short supply one day.
A Visual Basic user interface consists of forms and objects. A form is nothing more than a window that appears on the screen. Most Visual Basic programs have at least one form, although most programs likely will use several forms.
Objects are items that appear on a form, such as a command button, scroll bar, option button, or check box. An object lets the user give commands to your program. If you really wanted, you could create a program with only one form and no objects, but it wouldn't be very useful or interesting.
* Defining properties to make your user interface unique
After you create a form and draw some objects on it, the next step is to define the properties of each form and object. An object's properties determine the object's name, color, size, location, and appearance on the screen.
Different objects have different properties. Each time you draw an object on a form, Visual Basic assigns default property values, which define a generic object that no one can really use. If you want to customize an object, you need to define one or more properties for each object that your program uses. Chapter 3 provides a quick introduction to changing an object's properties.
* Writing BASIC code
When you're happy with the way your program looks, the next step involves writing BASIC commands (also known as code) to make your program actually work. (Don't worry. If you change your mind and want to edit the appearance of your user interface, you can go back and alter it at any time.)
The whole purpose of Visual Basic code is to tell objects on a form what to do when the user does something. For example, if the user clicks on an OK or Cancel command button, nothing happens unless you've written BASIC commands to tell your computer exactly what to do.
Any time a user presses a key, moves the mouse, or clicks the mouse button, it's called an event. Whenever an event occurs, your BASIC commands tell the computer, "Hey stupid, something just happened. Let's do something about it!"
Essentially, writing a Visual Basic program means drawing your user interface and then writing BASIC code to make it work. If you can handle these two steps without losing your mind, you can start writing your very own programs using Visual Basic. Chapter 4 provides a short introduction to writing real-life BASIC code.
** Why programs don't work (Part II)
Most programs are written by professional programmers who may have studied programming for years. Does this mean that every program they write will always work? Of course not. This is the world of computers where nothing works right, remember?
Besides the fact that the skill level among professional programmers can vary widely, professional programmers are often called upon to write programs for tasks that they don't understand themselves. For example, programmers may know nothing about accounting yet be hired to write a program to control a bank's electronic-fund-transfer system. Likewise, programmers with no skill or experience in flying may be hired to write a program to control the landing, takeoff, and flight of a 747 jumbo jet. Programmers with no knowledge of medicine may be required to write a program to control a medical instrument that administers doses of radiation to cancer patients. How can you work in a field where you have no experience and still get paid a lot of money? Easy, you become a programmer.
Hiring a programmer with no knowledge of the task she's trying to solve is like hiring a translator to translate Greek into French without that person's knowing how to read or write in either language. Given this paradox in the programming world, is it any wonder that planes crash, banks lose money, and hotels can't keep our reservations straight?
++ CHAPTER 2
USING THE VISUAL BASIC USER INTERFACE
In This Chapter:
Loading Visual Basic
Getting to know the Visual Basic user interface
Opening, closing, and moving windows around
Quitting Visual Basic
Before diving into the depths of Visual Basic programming, take a deep breath and examine the program's user interface. After all, if you don't know how to use the Visual Basic user interface, you won't be able to write your own Visual Basic programs.
To use Visual Basic, you need to know the following three functions:
How to load Visual Basic from Windows 95/98/NT
How to use Visual Basic to write your own programs
How to exit out of Visual Basic
** Loading Visual Basic
To load Visual Basic, just follow these simple steps:
1. Click on the Start button of the Windows 95/98/NT taskbar. A pop-up menu appears.
2. Click on Programs, click on the Microsoft Visual Basic 6 folder (or the Microsoft Visual Studio folder), and then click on Microsoft Visual Basic 6.
Visual Basic displays a New Project dialog box.
To load Visual Basic quickly, create a desktop shortcut by following these steps: Right-click on the Windows desktop, click on New, click on Shortcut, click on the Browse button, search for the VB6.EXE file, click on the Next> button, type Visual Basic 6, and click on the Finish button.
After the New Project dialog box appears, you have two options. You can
Start writing a brand new program.
Load an existing program so you can modify it.
* Starting a new program
After you have the New Project dialog box displayed, you have several choices as to the type of program you want to create. (The Learning and Standard Editions of Visual Basic do not display all the following types of programs in the New Project dialog box.)
Standard EXE: Creates a stand-alone program that you can copy, give away, or sell to others. Examples of stand-alone programs are Microsoft Word, Lotus 1-2-3, and Netscape Navigator. Stand-alone programs have an EXE file extension.
ActiveX DLL: Creates a file that has a .DLL file extension. ActiveX DLL files are not meant to be used by themselves. Instead, these types of files contain subprograms designed to function as building blocks when creating a stand-alone program.
ActiveX EXE: Creates a file that has an .EXE file extension. Unlike a stand-alone EXE file, an ActiveX EXE file is designed to work as an OLE server, which is nothing more than a program designed to share information with another program.
ActiveX Control: Creates a file that has an .OCX file extension. Unlike an ActiveX DLL or ActiveX EXE file, an ActiveX Control file usually provides both subprograms and a user interface that you can reuse in other programs.
ActiveX Document DLL: Creates a file that has a .DLL file extension. An ActiveX Document DLL file is designed to help you run programs on a Web site.
ActiveX Document EXE: Creates a file that has an .EXE file extension. An ActiveX Document EXE file can display a Visual Basic form within an Internet Web browser.
AddIn: Enables you to create an addin program specially designed to work with the Visual Basic user interface.
VB Application Wizard: Helps you create a skeleton Visual Basic standalone EXE program quickly and easily.
Data Project: Creates a program for devising a database report, which lets you see your database information in a pretty and organized way.
DHTML Application: New to Visual Basic 6, this program creates a DHTML (Dynamic HyperText Markup Language) document suitable for posting on a Web Site.
IIS Application: Creates a file for use with Microsoft Internet Information Server - another new Visual Basic 6 feature.
To choose one of these options from the New Project dialog box, just click on the option you want and click on OK.
If you choose File->New Project (or press Ctrl+N), Visual Basic displays a New Project dialog box that does not contain the Existing or Recent tabs.
Creating ActiveX, DHTML, IIS, and Data Project files is fairly advanced, so don't worry about such files until you figure out how to create a simple (Standard EXE) Visual Basic program first.
* Loading an existing program
Most of the time you are goint to want to load an existing program so you can modify it. To load an existing program, click on the Existing tab. Visual Basic displays folders so you can choose the specific Visual Basic program you want to load.
If you want to load a program that you've loaded in the past, click on the Recent tab. Visual Basic politely displays a list of all the programs you recently loaded. Just click on the Visual Basic program you want to edit and then click on Open.
If you choose File->Open Project (or press Ctrl+O), Visual Basic displays an Open Project dialog box that does not contain the New tab.
If you click on the File menu, Visual Basic kindly displays a list of programs that you last edited. By clicking on one of these program names, you can load the program without going through the Open Project dialog box.
Welcome to the Visual Basic User Interface
After you decide to start a new Visual Basic program or edit an existing one, the Visual Basic user interface appears in its full glory. Of course, before you can draw your program's user interface and write BASIC code, you have to know how to use the user interface of Visual Basic itself. The eight main parts of the Visual Basic user interface appear in Figure 2-2, although all parts don't necessarily have to be visible at any given time.
1. Pull-down menus: Provide access to every available Visual Basic command, although these menus can be confusing and intimidating to use.
2. Toolbar: Displays icons that represent the most commonly used Visual Basic commands - which may still be confusing and intimidating to use.
3. Toolbox: Displays the types of objects (such as a command button or check box) that you can draw on a form. The Toolbox may not list all possible objects you can draw on a form. To get a complete list of objects you can display in your Toolbox, press Ctrl+T.
4. Project explorer: Lists all the files that make up a single Visual Basic program.
5. Properties window: Displays the properties of the currently selected form or object.
6. Form layout window: Enables you to arrange the location where your forms appear on the screen.
7. Form: Provides a window where you can draw objects to design your program's user interface.
8. Immediate window: Enables you to debug your Visual Basic program.
When you're designing your user interface, you use the Toolbox to draw objects on a Form. After you draw your objects, the next step is to customize the appearance of each object by using the Properties window. Finally, after you're happy with the way your program's user interface looks, you can define the location of the interface on the screen by using the Form layout window.
To see more of the Visual Basic user interface, change your screen resolution by following these steps:
1. Click on the Start button on the Windows 95/98/NT taskbar.
2. Choose Settings->Control Panel. A Control Panel window appears.
3. Double-click on the Display icon. A Display Properties window appears.
As an alternative to Steps 1 through 3, just point the mouse cursor anywhere on your Windows 95/98/NT desktop, click the right mouse button, and click on Properties.
4. Click on the Settings tab.
5. Click on the Desktop area horizontal scroll bar and drag the slider toward the right.
The screen resolution (such as 800 x 600 pixels) appears underneath the horizontal scroll bar.
6. Click on OK.
* Closing and opening windows
If the Visual Basic user interface seems cluttered, don't be afraid to modify it. Because most parts of the Visual Basic user interface consist of windows, you may want to close them to give yourself more screen space. Then, when you need to use the interface windows again, just open them back up again.
To close a window in the Visual Basic user interface, click on the Close box of the window you want to close.
To open a window and make it appear in the Visual Basic user interface, choose View and then click on the window you want to open, such as the Code or Properties window.
* Resizing and moving your windows
Rather than close a window and make it disappear completely, you may prefer to resize or rearrange the window instead.
To resize a window, move the cursor over the edge of the window until the cursor turns into a doubIe-headed arrow. Next, hold down the left mouse button, drag the mouse, and release the left mouse button when the window is the shape you want the window to be.
To move a window, move the cursor over the title bar of a window, hold down the left mouse button. Drag the mouse and then release the left mouse button when the window is in the desired location.
* Docking your windows
With so many windows cluttering up the screen, you can see how easily the windows overlap one another and just get in the way. To help solve this problem, Visual Basic offers a docking feature.
Docking simply means that Visual Basic neatly stacks your windows to one side of the screen. To dock (or undock) a window, follow these steps:
1. Move the mouse cursor over the title bar of the window you want to dock (or undock).
2. Double-click the left mouse button. Visual Basic automatically docks (or undocks) your chosen window.
In case you don't want Visual Basic to dock (or undock) your windows at all, you can turn off the docking feature. To turn the docking feature on or off for a particular window, follow these steps:
1. Choose Tools->Options. An Options dialog box appears.
2. Click on the Docking tab.
3. Click on a check box to place a check mark in or remove a check mark from that check box. A check mark means that the docking feature is turned on for that particular window. An empty check mark means that the docking feature is (surprise!) turned off.
4. Click on OK.
** Quitting Visual Basic
No matter how much you may love using Visual Basic, eventually you need to turn off the computer and go to sleep (or at least pass out on the keyboard for an hour or two). To exit Visual Basic, use one of these three methods:
1. Choose File->Exit.
2. Press Alt+Q
3. Click on the Close box of the Visual Basic user interface window.
If you haven't saved the currently displayed Visual Basic program, Visual Basic displays a dialog box, giving you one last chance to save your work before the material is gone for good. Just click on Yes to save your work (or No to lose any changes you have made since the last time you saved the file).
As long as your computer hasn't crashed, Visual Basic smoothly exits and dumps you back to the Windows 95/98/NT desktop.
++ CHAPTER 3
DESIGNING YOUR FIRST USER INTERFACE
In this chapter:
Understanding the common parts of a user interface.
Drawing a user interface.
Changing the properties of your user interface.
As I discuss in Chapter 1, you must go through a number of steps in order to create a Visual Basic program from start to finish. While the earliest of these steps deal with designing and creating the user interface, this chapter focuses on the fundamental steps in writing a Visual Basic program:
Drawing the user interface
Defining the user interface properties
Writing BASIC code
So before you can write your first program in Visual Basic, you need to know how you can create a user interface.
** Common Parts of a User Interface
Despite the different varieties of user interfaces available, most graphical user interfaces share similar features such as displaying text or pictures in a window. A window can fill the entire screen or just part of it. Two or more windows can appear on the screen at the same time, either overlapping like cards or side by side like tiles. For some odd reason, Visual Basic calls a window a form.
When you first create a form, it's entirely blank. To make your form useful, you have to draw objects on the form. An object can be a command button, a text box, a picture, or an option button. The user communicates with your program by clicking, typing, or manipulating the objects displayed on a form.
By themselves, objects do absolutely nothing but look nice. To make them functional, you have to write BASIC code (Which you can read about in Chapter 4).
** A short history of user interfaces
In the old days of computers (back in the '50s), using a computer meant opening up the computer and rearranging some wires. Not only did you have to know how to program a computer to use it, but you also had to know how to connect the computer's wires together without electrocuting yourself in the process.
To use a computer in the '60s, you had to type commands to punch holes in cards and then feed the stack of punch cards into the computer. Because typing commands perfectly on cards wasn't something that most people considered exciting, using a computer was slow, tedious, and boring.
In the 70s, scientists connected a TV set to the computer and called the whole thing a computer terminal. For the first time,you could type a command directly into the computer and the computer could respond right away. This was the first attempt at creating a user interface that people could actually use and understand.
Of course, these first crude user interfaces consisted of nothing more than a blank screen and a blinking dot, called a cursor. To get the computer to do anything, you had to type the proper commands. Unfortunately, if you didn't know the right commands to type, the computer would refuse to work and make you feel stupid. Once again, using a computer became slow, tedious, and boring.
In a desperate attempt to make computers simpler to use, computer programmers soon invented something called a graphical user interface or GUI (pronounced "gooey"). Basically, a GUI displays menus and icons that the user can choose by clicking on commands with the mouse.
Apple Computer created the first commercial GUI when it introduced the Macintosh, but Microsoft quickly created its own GUI (dubbed Microsoft Windows). Unfortunately, GUls can still make a computer slow, tedious, and boring to use, so when this happens, blame the computer industry. This won't solve any problems, but at least you can make yourself feel better emotionally for a minute or two.
** Drawing objects with Visual Basic
To draw objects on a form, you need to use the Toolbox, which normally appears on the left side of the screen (unless you move it somewhere else). The Toolbox contains little drawings that represent the various objects you can draw on a form.
To draw any object on a form, you always have to follow these steps:
1. Click on the object in the Toolbox to tell Visual Basic what you want to draw on a form.
2. Move the mouse pointer onto the form where you want to draw the object.
3. Hold down the left mouse button and drag the mouse to draw your chosen object on the form.
As a fast way to draw an object on a form, just double-click on an Icon in the Toolbox and Visual Basic draws your chosen object in the center of the form automatically.
Designing your user interface is a lot like doodling. Although Part II of this book explains more about using specific objects and how they work, for now just remember two things: All programs need a user interface, and Visual Basic uses forms and objects to help you create a user interface quickly and easily.
* The two ways to create a user interface
When you want to create a user interface with Visual Basic, you can either:
Use the VB Application Wizard to create user interface for you automatically
Create your own user interface from scratch
So which way is better? If you need to create a Windows 95/98/NT program that offers standard File, Edit, Window, and Help pull-down menus, let Visual Basic create your user face for you automatically.
However, if you re just creating a simple program that doesn't need pull-down menus, creating your own user interface all by yourself is going to be easier.
Whichever way you decide to create a user interface, you can always add, delete, or modify your user interface at a later date.
* Drawing your first user interface from scratch
In case you don't feel like going through the following 14 steps, the enclosed CD-ROM has the HELLO1.VBP file stored in the Chapter 3 directory for you to examine and study.
To get acquainted with Visual Basic right away, the following are some steps you can use to create a real-life user interface from scratch:
1. In Windows 95/98/NT, start Microsoft Visual Basic if you haven't done so already. If Visual Basic is running already, then choose File->New Project.
Visual Basic displays a New Project dialog box, asking you what type of program you want to create.
2. Click on the Standard EXE Icon and click on Open.
Visual Basic displays a blank form titled Form 1. (If you want Visual Basic to create a user interface for you automatically, click on the VB Application Wizard icon at this step, but don't click on the icon now. The end of this chapter explains how to use the VB Application Wizard.)
3. Move the mouse cursor over the bottom right corner of the form (directly over the small rectangle, called a handle, that appears In the center of the right edge) so the mouse cursor turns into a left and right pointing arrow. Hold down the left mouse button and drag the mouse to make the form larger.
4. Choose View->Toolbox to make the Toolbox appear on the left side of the screen.
(Skip this step if the Toolbox is visible already.)
5. Click on the Command button Icon In the Visual Basic Toolbox.
6. Move the mouse over the form, then drag the mouse to draw a command button.
7. Click on the Option button icon in the Toolbox and draw an option button. Repeat this process two more times to draw three option buttons.
8. Click on the Image box icon and draw an image box. Repeat this process two more times to draw three image boxes.
9. Click on the Text box icon and draw a text box. Repeat this process two more times to draw three text boxes.
10. Choose File->Save Form 1 or press Ctrl+S. A Save File As dialog box appears, asking what you want to name the file. (You may want to click on the Save In list box to choose a specific folder to save your Visual Basic project and forms.)
11. Type HELLO and select Save. This action saves your form in a file called HELLO.FRM.
12. Choose File->Save Project. A Save Project As dialog box appears, asking what you want to name your project.
13. Type HELLO and click on Save. This action saves your entire Visual Basic project in a file called HELLO.VBP.
14. Choose File->Exit (or press Alt+Q) if you want to exit from Visual Basic.
Congratulations! You've just created a generic Visual Basic user interface. If your user interface looks a little less than impressive, this is because you haven't customized the user interface for your program.
To customize a Visual Basic user interface, you have to define the properties for each object on your forms.
** Defining the Properties of Your User Interface
Drawing your user interface creates the initial appearance of your Visual Basic program. To finish defining your program's user interface, you next have to define the properties for each object on your user interface.
But do you really need to define these properties? Yes and no. Visual Basic automatically sets default property values for all the objects on your user interface. However, these default values make your program look pretty ugly. So if you want to make your user interface more attractive, you must define the properties for your objects.
While each object typically has 10 to 30 different properties that you can change, you don't have to modify every single property. Most of the time, you just need to modify two or three properties of each object.
* What properties do
Before you change the properties of any object, you may want to know what the heck properties do in the first place. Essentially, properties define the characteristics of an object such as its name, size, shape, and color on the screen.
The name of an object is for your convenience only. Visual Basic automatically gives all objects boring names like Text1 or Command3. However, when you need to refer to specific objects, it's much easier if you have descriptive names for each object used by your program.
An object's size, location, and color define its appearance on the screen. The whole purpose of an object's appearance is to make your user interface look pretty.
* Changing property settings
You can change the property settings of an object at two separate times:
During design time
During run time
Design time is when you're drawing your user interface but before you actually run your program.
Most of the time, you want to change an object's property settings at design time. The most important property to change at design time is the name of the object.
Run time is when your program uses BASIC code to change an object's properties while your program is actually running. Of course, before your program can change an object's properties, you have to write BASIC code that tells your program exactly which object's properties you want to change.
Changing a property during run time enables you to create animation or display messages on the screen, such as error messages, program status messages, or warning messages alerting the user that the computer is going to blow up in ten seconds.
Changing property settings at design time:
Several properties are assigned to every object. To change the property of an object, make sure you have a form displayed within Visual Basic and then follow these steps:
1. Click on the object whose properties you want to change.
2. In the Properties window, click on the property that you want to change. If the Properties window is not visible, press F4 to make the window appear.
3. Type or choose a new setting for the property.
Simple, don't you thing? When you need to change multiple properties for one or more objects, this book displays a table similar to the following:
Object Property Setting
Form Name frmHello
Caption Hello, world!
The following steps detail what this table is telling you to do:
1. Click on the Form object.
2. Click on the Name property in the Properties window.
3. Type frmHello to change the value of the Name property.
4. Click on the Caption property in the Properties window.
5. Type Hello, world! To change the value of the Caption property.
Defining the properties of your first user interface:
In case you don't feel like following the next 17 steps, the enclosed CD-ROM has the HELLO2.VBP file stored in the Chapter 3 directory for you to examine and study.
To define the properties of your user interface, follow these steps:
1. In Windows 95/98/NT, start Microsoft Visual Basic.
Visual Basic displays the New Project dialog box. If Visual Basic is already running, choose File->Open Project.
2. Click on the Recent tab.
3. Click on the HELLO file (which is the HELLO.VBP file) and click on Open.
Visual Basic loads your HELLO.FRM form on the screen.
4. Click on Option 1 to highlight the option button, which causes little rectangles to appear around the edges of the option button.
5. Click on the Properties window, click on the Name property and type optGrin. Click on the Caption property and type I'm happy!
6. Click on the Image 1 image box. (This appears near the top of the form.)
7. Click on the Properties window, click on the Name property and type imgGrin.
8. Click on the Properties window and click on the three dots (...) in the Picture property. Visual Basic displays a Load Picture dialog box.
9. Open the Icons folder. (You may have to dig through the Graphics folder in your Visual Basic folder to find the Icons folder.) Next, open the Misc folder within the Icons folder. Visual Basic displays a Load Picture dialog box.
10. Double-click on the FACE03 icon. Visual Basic displays a really happy face in the image box.
11. Click on the Visible property and then click on the downward-pointing arrow and choose False.
12. Click on the Text1 text box.
13. Click on the Border Style property and then click on the downward-pointing arrow and choose 0-None.
14. Click on the Name property and type txtGrin.
15. Double-click on the Text property and then press Backspace to clear the Text property so no text appears.
16. Finish changing the properties for the rest of the objects according to Table 3-1.
17. Choose File->Save Project to save all your changes.
Congratulations! You just defined all the necessary properties for your first user interface.
||
TABLE 3-1
Properties to Change to Finish Designing Your User Interface
Object - Form
Property - Name; Caption
Setting - frmHello; Hello, world!
Object - Option 2
Property - Name; Caption
Setting - optSmile; I'm okay.
Object - Option 3
Propety - Name; Caption
Setting - optFrown; I'm sad
Object - Image 2
Property - Name; Picture; Visible
Setting - imgSmile; FACE02; False
Object - Image 3
Property - Name; Picture; Visible
Setting - imgFrown; FACE01; False
Object - Text2
Property - Name; Border Style; Text
Setting - txtSmile; 0-None; (Empty)
Object - Text3
Property - Name; Border Style; Text
Setting - txtFrown; 0-None; (Empty)
Object - Command1
Property - Name; Caption
Setting - cmdExit; Exit
** Test your newfound knowledge
1. What are the two common parts of almost every user interface?
a. The easy-to-use interface and the 500page manual that explains how easy the interface is to use
b. Menus that nobody can understand and commands that nobody can use
c. Useless icons that don't make any sense and text that doesn't explain anything
d. Forms and objects
2. How can you change property settings of an object?
a. You can't, Property settings have to want to change first.
b. Use constant threat and intimidation.
c. Use the Property window while designing the user interface (known as design time) or write BASIC code to change the properties while the program is running (known as run time).
d. Stick a magnet next to your monitor and watch the images warp and wreck your computer screen.
** Letting Visual Basic Create a User Interface Automatically
If you actually went through the steps to create a user interface from scratch, you can see how tedious and time-consuming this process can be. Because computers are supposed to save you time (so you have more time to play computer games), Visual Basic offers a VB Application Wizard.
The VB Application Wizard creates a skeleton program complete with pulldown menus, toolbars, and dialog boxes. If you want to create a program that requires a user interface similar to Microsoft Word, Excel, or PowerPoint, you can save time by letting the VB Application Wizard make your user interface instead.
To see how the VB Application Wizard works, follow these steps:
1. In Windows 95/98/NT, start Microsoft Visual Basic. (If you already have Visual Basic running, choose File->New Project.)
Visual Basic displays the New Project dialog box.
2. Double-click on the VB Application Wizard icon.
The Introduction window of the Application Wizard appears. This window asks for a profile to use. Profiles are skeleton settings so you can customize the VB Application Wizard. For now, just keep the profile option as (None).
3. Click on Next>. The Application Wizard - Interface Type window appears
4. Click on the Multiple Document Interface (MDI) option button and click on Next>.
The Application Wizard - Menus window appears. (If you want, click on a different option button for Step 4 to see what a Single Document Interface [SDI] and Explorer Style user interface look like.)
5. Click on Next> to accept the default selection.
The Application Wizard - Customize Toolbar window appears. By dragging and dropping icons, you can create your own toolbars.
6. Click on Next> to accept the default selection.
The Application Wizard - Resources window appears. Resource files can help you develop foreign language versions of your program. Rather than force you to retype your program's text (such as manus and dialog box captions) in different languages, a resource file stores the information in a separate file so you can just use a different resource file for each foreign language.
7. Click on Next> to accetp the default selection.
The Application Wizard - Internet Connectivity window appears. This is where you can specify whether you want your program to be able to access the Internet.
8. Click on Next> to accept the default selection. The Application Wizard - Standard Forms window appears.
9. Click on the About Box check box and then click on Next>.
The Application Wizard - Data Access Forms window appears. This is where you can specify what type of database files you want your program to use.
10. Click on Next> to accept the default selection. The Application Wizard - Finished! Window appears.
11. Click on the Finish button. Visual Basic displays an Application Created dialog box.
12. Click on OK.
13. Press F5 to run the skeleton program that the VB Application Wizard created for you. The program appears.
14. Choose Help->About. An About Project1 dialog box appears.
15. Click on OK.
16. Click on any icon on the toolbar or on any of the pull-down menus to see what happens.
17. Choose File->Exit when you're done playing around with this skeleton program.
18. Choose File->New Project. A dialog box appears, asking if you want to save your newly created user interface.
19. Click on No.
As you can see, the VB Application Wizard can quickly create a user interface that includes pull-down menus and a toolbar. However, you still need to customize the user interface and write BASIC code to make the program do anything worthwhile. The VB Application Wizard simply gives you a head start in creating your user interface.
++ CHAPTER 4
WRITING BASIC CODE
In This Chapter:
Discovering the basics about BASIC code
Choosing an object and writing a procedure
Writing BASIC code for your program
To have your computer do anything, you have to give it step-by-step instructions. If you skip a step or give unclear instructions, your computer doesn't know what to do. (Actually, the computer knows what to do - it just doesn't do what you want it to.)
Programmers call a single instruction a command. A typical BASIC command looks like the following:
Taxes = Income * FlatTaxRate
You call a series of commands code. A typical series of commands looks like this:
Income = 90000
FlatTaxRate = .95
Taxes = Income * FlatTaxRate
A collection of code that makes your computer do something useful (such as play a game, calculate your taxes, or display flying toasters on your screen) is called a program.
If you want to speak the language of programmers (even though programmers are notorious for never saying much of anything), you have to know programming etiquette.
You never write a program; you write code. Heaven forbid if you say, "Let me look at your series of commands." Cool programmers are likely to blush at your faux pas. Instead, you ought to say, "Let me look at your code."
** What Is BASIC Code?
To get your computer to do anything, you have to give the machine instructions that it can understand. Because you're using Visual Basic, you have to use the BASIC programming language.
Like all computer languages, BASIC has special commands called reserved keywords. Some examples of reserved keywords are as follows:
Loop Function Sub End
Do Integer Case If
Else Select Then For
BASIC code consists of nothing more than BASIC reserved keywords creatively strung together to form a program. Whenever the computer sees a reserved keyword, the computer automatically thinks, "Oh, this is a special instruction that I already know how to obey."
A program can be as short as a single reserved keyword or as long as several million reserved keywords. Short programs generally don't do anything more interesting than display something such as Hello, world! on the screen. Long programs usually do much more, but these programs are often as confusing to read as an IRS tax form.
Theoretically, you can write one long program consisting of a million or more reserved keywords. However, any programmer attempting to do so is likely to go insane long before completing the task.
* Writing a program one step at a time
To make programming easier, most programmers divide a large program into several smaller ones. After you finish writing each of the smaller programs, you paste the pieces together to make a complete program.
When you divide a large program into several smaller ones, these smaller programs are subprograms. In Visual Basic lingo, subprograms are event procedures (although some programmers may call them subroutines). Visual Basic also has special subprograms called functions, which you can read about in Chapter 29, and general procedures, which you can read about in Chapter 27.
Procedures tell each object on your form how to react to something that the user does. Each object can have zero or more procedures that tell the object how to respond to the user.
One event procedure may tell the computer what to do if the user clicks on an object (such as a command button) with the mouse. Another event procedure may tell the computer what to do if the user presses a certain key while the object is highlighted.
Not every object needs event procedures. The only objects that need event procedures are those that the user can click on or choose in some way, such as command buttons, check boxes, or radio buttons.
* Choosing objects and Events
Before you can write an event procedure for an object, you have to tell Visual Basic:
The name of the object to use
The event you want the object to respond to
Visual Basic gives you two ways to choose an object:
The simplest way is to click on an object (such as a command button) on your form and press F7 (or just double-click on the object in the first place) to switch to the Code window. Visual Basic then kindly displays the most likely procedure that you need for that object.
The second way is almost as easy. Each time you draw an object on a form, Visual Basic stores the name of that object in a list called the Object list. The Object list appears at the top of the Code window. To open this list, press F7. Next, scroll through the Object list until you find the object for which you want to write a procedure.
To choose an event to use, click on the Procedure list, which also appears at the top of the Code window next to the Object list. The Procedure list contains all possible events that you can write a procedure to respond to.
Just scroll through this list until you find the event for which you need to write a procedure. The most common event to use is the Click event.
After you choose an object from the Object list and an event from the Procedure list, Visual Basic displays the first and last lines of the procedure. You're now ready to start writing code for this procedure.
** Writing Visual Basic Event Procedures
Before you can write a Visual Basic event procedure for an object, you have to draw the object on a form first.
Next, you have to change the properties of each object to give them unique names you can remember. If you don't do this, you're stuck with the generic names that Visual Basic provides by default for everything, such as Option1 or Text3.
Do you know what happens if you write an event procedure for a particular object and then later change that object's name? Visual Basic gets confused and thinks that you've created a brand-new object, which means that the renamed object won't have any event procedures attached to it. So if you're going to rename an object, do this renaming before you write any event procedures for that object.
To write a Visual Basic event procedure, click on the object that you want to write an event procedure for and then open the Code window. (To open the Code window, press F7, choose View->Code, or double-click on the object.) After the Code window appears, you can start typing your procedure or code.
But wait! Visual Basic doesn't just display a blank window. Visual Basic automatically types Private Sub, which is followed by the object's name, an underscore, an event (such as Click), and a set of parentheses ( ) that may be empty or that may contain stuff inside of it. (You can find out more about the stuff inside of parentheses in Chapter 28).
If you click on an object named cmdExit and then open the Code window, Visual Basic displays the following:
Private Sub cmdExit_Click( )
End Sub
The first line of this Visual Basic procedure begins with Private Sub, which is short for a subprogram that belongs exclusively to a specific object. In this case, the subprogram belongs to the object named cmdExit.
Next, Visual Basic types your object's name. If you forgot to change the name property of your object, Visual Basic uses a default name such as Text2. Otherwise, Visual Basic displays your object's name (for example, cmdExit).
Following your object's name is an underscore, which separates your object's name from its event. An event is something the user does to communicate with the computer. In this example, the event is Click, which means that the user clicked the mouse on the object named cmdExit.
Next comes an empty set of parentheses. Sometimes, the parentheses contain data that the subprogram uses from the routine that called this subroutine. An empty pair of parentheses says that this subprogram does not need any special data passed to it from another part of your program.
Translating this procedure into English, the first line means, "This is a subprogram for the object named cmdExit, and the subprogram tells the computer what to do if the user clicks on the cmdExit object."
The last line of this Visual Basic event procedure consists of two words: End and Sub. This line tells the computer, "This is the end of all the commands that belong in this subprogram." Rather than type all that, Visual Basic uses the simpler
End Sub
Right now, this Visual Basic procedure does nothing. To make the procedure do something, you have to add commands between the first line and the last line. Before you start adding commands, you need to know what BASIC commands (also known as code) can do.
** Test your newfound knowledge
1. What are reserved keywords?
a. Words you say when you have a reservation in a fancy restaurant where water costs $25 a glass.
b. What shy people want to say.
c. Special instructions that every programming language has.
d. Words that you waritto sayto the face of someone you don't like.
2. How can you write a large program without losing your mind?
a. Divide the program into subprograms, otherwise known in Visual Basic as procedures (such as event or general procedures).
b. Watch others write the program and then steal their homework.
c. You mean I actually have to write a program?
d. If you're thinking of writing a large program, you've probably already lost your mind.
* What can BASIC code do?
BASIC code is generally used to do the following:
Calculate a result
Modify the properties (appearance) of another object
If you want to calculate the number of people who live in wooden cabins, subscribe to Soldier of Fortune, and own cats, Visual Basic can calculate this as long as you provide all the necessary data.
After you calculate a result, you probably want to show the result on the screen. To do so, you have to modify the properties of an object on your user interface. For example, if you want to display a message on the screen, you first need to draw a text box object on a Visual Basic form.
You then have to name this text box with something such as txtMessage. Finally, to display anything in this text box, you have to modify the Text property of the txtMessage text box, such as:
txtMessage.Text = "This is hard to explain."
This command displays the message "This is hard to explain" in the txtMessage text box on the screen.
Of course, Visual Basic code can't change all the properties of an object. You can only change some properties (such as the object's name) during design time by using that object's property window.
* How a Visual Basic event procedure works
In Visual Basic, the instructions in an event procedure run only when a specific event occurs, such as when the user clicks on an object. The same set of instructions can run over and over again each time the user clicks on an object. The only time a Visual Basic program ends is when an object's procedures specifically tell the program to end.
As an example, look at the simplest Visual Basic procedure that is necessary for every program to stop completely. For a simple "Hello, world!" program, this procedure looks like the following:
Private Sub cmdExit_Click( )
Unload Me
End Sub
The code in this procedure contains a Visual Basic reserved keyword called Unload and a variable called Me. The Me variable represents the form, so this procedure tells Visual Basic, "Unload the currently displayed form from memory." Because this program only consists of one form, this command effectively stops the program from running.
This procedure runs only when the user clicks on the cmdExit object. If you look at the cmdExit object on your user interface, you see that the object is a command button labeled Exit.
As an alternative to using the command Unload Me to end aVisual Basic program, you can also use the command End instead, like in the following:
Private Sub cmdExit_Click( )
End
End Sub
So which method should you use? The End command forces your program to stop running immediately, which can be like stopping your car by crashing it into a wall. For a gentler, kinder approach to stopping your program, Microsoft wholeheartedly recommends that you use the Unload Me command instead.
If you run this program, the following is what happens:
1. Visual Basic displays your user interface on the screen, including a command button named cmdExit. This name does not appear on the user interface. Instead, the command button's Caption property appears, which is the word Exit.
2. Clicking on the Exit button causes Visual Basic to ask, "Hey, what's the name of this object on which the user just clicked?"
3. In a huff, Visual Basic quickly notices that the Exit button's name is cmdExit.
4. Then Visual Basic asks, "Are there any instructions here that tell me what to do if the user clicks on the cmdExit object?" Happily, Visual Basic finds the Private Sub cmdExit_Click ( ) procedure.
5. Visual Basic then examines the first instruction of the cmdExit_Click ( ) procedure. In this case, the instruction is Unload
Me, which tells Visual Basic to unload the form. Because this is the only form in the program, this effectively stops the program.
6. Visual Basic stops running the program and removes it from the screen. Naturally, all this happens in the blink of an eye, and your computer looks as though it's responding instantly.
** Writing BASIC Code for your First Visual Basic Program
For those who just like jumping right into the program without typing all the BASIC code, load the HELLO.VBP program from the enclosed CD-ROM in the back of the book.
Because experience is always the best teacher, the following steps show you how to write real-life BASIC code that you can use to impress your friends. You can find out how to write BASIC code for the HELLO.VBP program in Chapter 3.
Don't worry about understanding everything you're typing. The purpose of this exercise is just to show you how simple creating a program in Visual Basic can be.
1. Start Microsoft Visual Basic if you haven't already done so. (Or choose File->Open Project.)
Visual Basic displays a New Project or Open Project dialog box.
2. Click on the Recent tab, click on HELLO, and click on Open.
If the HELLO.FRM form does not appear on the screen, click on the frmHello form in the Project Explorer window and click on the View Object icon. (You can skip this step if you already have the form displayed on the screen from Chapter 3)
3. Click on the optGrin option button displayed in the upper-left corner of the form. (This is the button that says "I'm happy!")
4. To Open the Code window, press F7 or choose View->Code.
5. Type in the Private Sub optGrin_Click( ) procedure so that it looks like the following:
Private Sub optGrin_Click( )
imgFrown.Visible = False
imgSmile.Visible = False
imgGrin.Visible = True
txtSmile.TEXT = ""
txtGrin.TEXT = "I'm going,to Disney World!"
txtFrown.TEXT = ""
End Sub
6. Click on the downward-headed arrow in the Object list box at the top of the Code window and then choose the optSmile object.
Visual Basic displays an empty Private Sub optSmile_Click() procedure.
7. Type in the Private Sub optSmile_Click( )procedure so that it looks like the following:
Private Sub optSmile_Click( )
imgFrown.Visible = False
imgSmile.Visible = True
imgGrin.Visible = False
txtSmile.TEXT = "Hello, world!'
txtGrin.TEXT = ""
txtFrown.TEXT = ""
End Sub
8. Click on the downward-pointing arrow in the Object list box at the top of the Code window and then choose the optFrown object.
Visual Basic displays an empty Private Sub optFrown_Click( ) procedure.
9. Type in the Private Sub optFrown_Click( ) procedure so that It looks like the following:
Private Sub optFrown_Click( )
imgFrown.Visible = True
imgSmile.Visible = False
imgGrin.Visible = False
txtSmile.TEXT = ""
txtGrin.TEXT = ""
txtFrown.TEXT = "Good-bye, cruel world."
End Sub
10. Click on the Object list at the top of the Code window and then choose the cmdExit object.
Visual Basic displays an empty Private Sub cmdExit_Click( ) procedure.
11. Type in the Private Sub cmdExit_Click( )procedure so that it looks like the following:
Private Sub cmdExit_Click( )
Unload Me
End Sub
12. Press F5 to run your program or choose Run->Start.
If you typed everything correctly, Visual Basic displays your user interface on the screen.
13. Click on the option button next to the label "I'm sad."
Visual Basic displays a face on the screen, along with the message Good-bye, cruel world.
14. Click on the option button next to the label "I'm okay."
Visual Basic displays a smiley face on the screen, along with the message Hello, world!.
15. Click on the option button next to the label "I'm happy!"
Visual Basic displays a really happy face on the screen, along with the message - I'm going to Disney World!.
16. Click on the Command button labeled Exit.
Visual Basic quits running your program and returns you to Visual Basic.
You finally completed the Hello, World! Example. Now, you can see how you can use Visual Basic to create a friendly user interface quickly and easily
++ PART II
CREATING USER INTERFACES
In this part ..........
A user interface allows other people to use your program. The clumsier the user interface, the harder your program is to use. So if you make your user interface simple and logical, you can bet that more people will be able to use your program.
This is the fun part of the book. You aren't forced to type any bizarre code, comprehend arcane commands, or memorize ridiculous keystrokes. In this part of the book, you get to doodle on your computer screen while actually finding out how to write your own programs at the same time.
++ CHAPTER 5
USER INTERFACE DESIGN 101
In this chapter .......
Detailing tips for creating a user interface
Drawing objects on the user interface
Moving, deleting, and copying objects
Making objects unavailable
To make oneself understood to the people, one must first speak to their eyes.
Napoleon Bonaparte
First of all, nobody really wants to use your program. Most people would rather play at the beach, watch TV, or make out. However, people do want the results that your program can produce. If they could get these same results by other means with less work, they would. But because they can't, they're willing to use your program.
This means that people really want your program to read their minds and then magically do all their work for them automatically. Because that's not possible, the best you can hope for is to make your program as easy to use as possible. If a completely incompetent moron (your boss) can use your program, then most other people are going to be able to use your program as well.
** Before You Create Your User Interface
Creating a user interface doesn't mean just slapping together some pretty pictures in a colorful window and hoping that the user can figure out how your program works. Your program's user interface must make your program easy to use. To help you create a user interface, here are some points to keep in mind.
* Know your user
Before designing your user interface, ask yourself who is going to use your program. Are your typical users data-entry clerks who understand computers, or managers who understand only paper procedures and are learning to use a computer for the first time?
When you decide who your users are, design your user interface so that it mirrors the way the users already work, regardless of whether the user interface seems totally inefficient or alien to anyone else. Accountants readily accept spreadsheets because the row-and-column format mimics green sheets of ledger paper. Likewise, typists prefer word processors because a word processor mimics a blank sheet of paper.
But imagine if all word processors looked like spreadsheets with rows and columns. Any typist trying to use this kind of word processor would quickly feel lost and confused (although accountants may feel right at home with such a word processor).
The more a programmer understands the user, the more likely the interface is going to be used and accepted. The only person the user interface really has to satisfy is the user.
* Orient the user
Not surprisingly, people get lost wandering through today's supermalls, which contain multiple levels and two different time zones. How do you feel when you have no idea where you are and no idea where you can go from your current position?
This feeling of helplessness is the reason why lost kids cry uncontrollably and confused computer users curse under their breath. (This is also the reason why malls install directories with the big red X that says, "You are here.")
A good user interface must orient people so that they know where they are in your program and how to get out if they want. Some user interfaces display a message at the bottom of the screen, such as "Page 2 of 5." In this case, the user knows exactly how many pages are available for viewing and which page currently appears on the screen.
Your user interface is a map to your program. Make sure that your user interface shows just enough information to orient users but not too much to confuse them.
* Make the choices obvious
In addition to letting users know where they are in a program, a good user interface must also make all choices obvious to the user. If your user interface displays "Page 4 of 25" at the bottom of the screen, how can the user know what to do to see the next or previous page? One solution may be to show forward- and backward-pointing arrows in each bottom corner of the page. Another solution may be Next Page and Previous Page buttons.
As long as your program shows the user which options are available next and which keys to press or where to click the mouse, the user feels a sense of control and confidence when using your program.
* Be forgiving
The key here is useful feedback. If your program takes an arrogant attitude and displays scolding messages like "File MPR.DLL missing" whenever the user presses the wrong key or clicks the mouse in the wrong area, the user may feel intimidated if your program doesn't explain what the error message means and how he or she can avoid the error in the first place.
So be kind. Have your program hide or dim any buttons or menu commands that are unavailable to the user. If the user does press the wrong key or click the mouse in the wrong area, have your program display a window and explain what the user's options are. Users love a program that guides them, which means you can spend a lot less time answering phone calls for technical support.
* Keep it simple
Most programs offer users two or more ways to choose a specific command. You can click on a button, choose a command from menus, or press certain keystroke combinations (Ctrl+F2, for example). Of these three methods, clicking directly on the screen is the easiest procedure to remember and pressing bizarre keystroke combinations is the hardest.
Make sure that commonly used commands can be accessed quickly through a button or a menu. Not all commands must be or need to be accessed through a keystroke combination.
Although keystroke combination commands are faster to use, these commands are harder to learn initially. Make keystroke combinations easy to remember whenever possible. For a Save command, Ctrl+S is easier to remember than something totally abstract like Shift+F12. People can easily remember that S stands for Save, but who has any idea what F12 represents?
** Designing Your Visual Basic User Interface
When you write a Visual Basic program, you first have to design the user interface. Essentially, a Visual Basic user interface consists of objects that you place on the screen and arrange in some semblance of organization so that the screen looks pretty.
The common elements of a Visual Basic user interface consist of the following:
Forms (also known as windows)
Buttons (such as command buttons and radio buttons)
Boxes (such as text boxes and check boxes)
Labels
Pictures (such as icons and graphics)
To design your user interface, follow these steps:
1. Create a form.
2. Choose the object you want to draw from the Toolbox. (See Table 5-1.)
3. Draw the object on the form.
* TABLE 5-1
Tools in the Visual Basic Toolbox
Tool Name - Pointer
What this Tool Does - Selects objects
Tool Name - Picture box
What this Tool Does - Draws a box to display graphics
Tool Name - Label
What this Tool Does - Drawa a box to display text
Tool Name - Text box
What this Tool Does - Draws a box that can display text and let the user type in text
Tool Name - Frame
What this Tool Does - Groups two or more objects together
Tool Name - Command button
What this Tool Does - Draws a command button
Tool Name - Check box
What this Tool Does - Draws a check box
Tool Name - Option (or radio) button
What this Tool Does - Draws a radio button
Tool Name - Combo box
What this Tool Does - Draws a combo box
Tool Name - List box
What this Tool Does - Draws a list box
Tool Name - Horizontal scroll bar
What this Tool Does - Draws a horizontal scroll bar
Tool Name - Vertical scroll bar
What this Tool Does - Draws a vertical scroll bar
Tool Name - Timer
What this Tool Does - Places a timer on a form
Tool Name - Drive list box
What this Tool Does - Draws a drive list box that displays all the disk drives available
Tool Name - Directory list box
What this Tool Does - Draws a directory list box that displays a directory on a particular disk drive
Tool Name - File list box
What this Tool Does - Draws a file list box that displays files in a specific directory
Tool Name - Shape
What this Tool Does - Draws a geometric shape such as a circle or a square
Tool Name - Line
What this Tool Does - Draws a line
Tool Name - Image box
What this Tool Does - Draws a box to display graphics
Tool Name - Data control
What this Tool Does - Draws a control to link a program to a database file
Tool Name - OLE
What this Tool Does - Draws a box to insert an OLE object
* Drawing an object
To draw any object, follow these steps:
1. Click on the icon in the Visual Basic Toolbox that represents the object you wan to draw (command button, picture box, label, and so on).
2. Move the mouse to the place on the form where you want to draw the object.
The cursor turns into a crosshair shape.
3. Click and drag the mouse to where you want to draw your object and then release the mouse button.
If you want to draw an object on a form in a hurry, double-click on the object's Icon in the Toolbox. For example, if you want to draw a command button quickly, just double-click on the command button icon in the Toolbox, and Visual Basic draws the command button for you automatically.
* Using the Properties Window
After you draw an object on a form, the next step is to define the properties for that object. To define an object's properties, you have to use the Properties window. In case the Properties window doesn't appear on your screen, you can make the window appear by using one of the following three commands:
Press F4.
Choose View ->Properties Window.
Click on the Properties Window Icon in the toolbar.
The Properties window can display properties in two ways:
Alphabetical: Lists an object's properties alphabetically from A to Z.
Categorized: Organizes properties into categories, such as properties that affect an object's appearance, behavlor, or position on the screen.
After you open the Properties window, follow these steps to view the properties of any object in your Visual Basic program:
1. Choose one of these methods to open the Project Explorer window.
Press Ctrl+R.
Choose View->Project Explorer.
Click on the Project Explorer icon in the toolbar.
2. Click on a form name in the Project Explorer window that contains the object whose properties you want to examine and then click on the View Object lcon.
3. Click on the downward-pointing arrow of the Object list box in the Properties window and choose the object whose properties you want to examine.
4. Double-click on the property name that you want to change.
If you click on an object displayed on a form, the Properties window automatically displays all the properties for that object.
* Naming objects
Every object you draw has a Name property, which Visual Basic uses to identify the object. (That's the same reason your parents gave you a name - so people don't say, "Hey, you!" all the time to get your attention.) If you click on the Alphabetic tab in the Properties window, the Name property appears as (Name) at the top of the list.
Every Visual Basic object must have a unique name. If you try to give the same name to two different objects, Visual Basic complains and refuses to let you make such a horrid mistake.
When you create an object, Visual Basic automatically gives your object a boring, generic name. For example, the first time you create a command button, Visual Basic names the button Command1. The second time you create a command button, Visual Basic names this new button Command2, and so on.
The name of an object never appears on the screen. Names can be up to 40 characters long, but they cannot contain punctuation marks or spaces. You can name your objects anything you want, but Microsoft recommends that Visual Basic programmers all over the world use Visual Basic three-letter prefixes, as shown in Table 5-2. If everyone in the world uses these prefixes, it makes modifying another person's Visual Basic program easier to read.
To change the name of an object, follow these steps:
1. Click on the object that you want to name.
Handles appear around the object. (To name a form, click anywhere on the form, but do not click on any objects on the form.)
2. Open the Properties window.
To do so, press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.
3. Double-click on (Name) and type a new name.
* TABLE 5-2
SUGGESTED PREFIXES WHEN NAMING OBJECTS
Object - Check box
Suggested Prefix - chk
Example Name - chkYourZipper
Object - Combo box
Suggested Prefix - cbo
Example Name - cboBLT
Object - Command button
Suggested Prefix - cmd
Example Name - cmdOpenSesame
Object - Data
Suggested Prefix - dat
Example Name - datFanOverThere
Object - Directory list box
Suggested Prefix - dir
Example Name - dirTree
Object - Drive list box
Suggested Prefix - drv
Example Name - drbLikeMad
Object - File list box
Suggested Prefix - fil
Example Name - filDocuments
Object - Form
Suggested Prefix - frm
Example Name - frm1040Tax
Object - Frame
Suggested Prefix - fra
Example Name - fraGroupedButtons
Object - Horizontal scroll bar
Suggested Prefix - hsb
Example Name - hsbTemperature
Object - Image
Suggested Prefix - img
Example Name - imgPeace
Object - Label
Suggested Prefix - lbl
Example Name - lblFakeName
Object - Line
Suggested Pefix - lin
Example Name - linBorder
Object - List box
Suggested Prefix - lst
Example Name - lstCandidates
Object - Menu
Suggested Prefix - mnu
Example Name - mnuHamandEggs
Object - Picture box
Suggested Prefix - pic
Example Name - picPrettyPictures
Object - Radio button
Suggested Prefix - opt
Example Name - optStation101
Object - Shape
Suggested Prefix - shp
Example Name - shpUpOrShipOut(circle, square, oval, rectangle, rounded rectangle, and rounded square)
Object - Text box
Suggested Prefix - txt
Example Name - txtReadStuffHere
Object - Vertical scroll bar
Suggested Prefix - vsb
Example Name - vsbMoneyWasted
* Creating captions for objects
In addition to a name, most (but not all) objects also have a caption. An object's caption appears as text on the screen.
By default, an object's caption and name are the same until you change them. So the moment you draw a check box on a form, the check box's caption is something dull like Check1, and the check box's name is also Check1.
The caption for a form appears in the title bar of that form. The caption for an object (such as a command button, label, or text box) appears directly on that object while a caption for a check box or radio button usually appears to the right of the check box or option button.
Captions are meant to help the user figure out how to use your program. A caption can be blank or up to 255 characters long, including spaces, punctuation marks, and four-letter words. The following are valid captions:
Hello
Hello, buddy!
Do I really know what I'm doing?
* To change the caption of an object, follow these steps:
1. Click on the object whose caption you want to change so that black handles appear around the object. (To select a form, click anywhere on the form, but do not click on any objects on the form.)
2. Open the Properties window (by pressing F4).
3. Double-click on the Caption property and type a new caption. Notice that Visual Basic displays your caption on the screen as you type.
* Adding hot keys to a Caption:
Besides looking pretty and displaying information to the user, captions can also be used to create hot keys so that the user can choose an object without having to click on the object with the mouse.
To add a hot key to a caption, you have to put the ampersand character (&) into an object's caption. You may be wondering, "Why the heck do I want to use an ugly symbol like that?" The answer is to give users yet another way to choose an object on your user interface.
For example, to push a command button, users can either:
Click on the command button with the mouse.
Press Tab until the command button appears highlighted and then press the spacebar or Enter (real obvious, huh?).
However, if you use the ampersand in the command button's caption, the user can push that button by pressing Alt plus whatever letter the ampersand is in front of. For example, if a command button has a caption of &Exit, it appears in the command button with the E underlined, as in Exit. To push this button, you can simply press Alt+E. If the command button has a caption of E&xit, however, the caption appears in the command button with the x underlined, as in Exit, and you can use Alt+X to press this command button.
* Changing the font of an object's caption:
When you create a caption, Visual Basic displays your caption on the screen in a plain type style. For you creative types who want to make your captions look spiffier, you can change the font, type style, and size of captions to give them more pizzazz.
Fonts are different ways to display text. Normally, Visual Basic uses the MS Sans Serif font, but you can use any font stored in the memory of your computer. (MS Sans Serif is similar to the Helvetica font, and the Visual Basic MS Serif font is similar to the Times Roman font.)
To change the font of a caption, follow these steps:
1. Click on the object whose caption you want to modify.
2. Open the Properties window (by choosing View->Properties Window).
3. Double-click on the Font property.
Visual Basic displays a dialog box of all the fonts you can use.
4. Click on the font you want and click on OK.
Visual Basic immediately changes the font of the caption.
Fonts give you a chance to be creative, but they can also disorient the user, especially if you choose bizarre fonts that don't look like anything normally found in nature. To avoid confusion, let Visual Basic use the default font of MS Sans Serif, unless you have a really good reason to use a different font.
if you choose a really bizarre font, that font may not exist on other people's computers. When using fonts, try to use the most common ones found on all computers or else your program's captions may look really weird.
* Changing the size of an object's caption:
You can also change the size of your caption by making it smaller or larger to fit inside your object. Depending on the font you choose, Visual Basic gives you a variety of font sizes to choose from.
For example, if you choose the MS Sans Serif font, Visual Basic gives you the following choices of font sizes:
8
10
12
14
18
24
Obviously, the more font sizes you use, the odder your captions look. The best approach is to use one size to avoid confusing the user any more than you have to.
To define the font size of your captions, follow these steps:
1. Click on the object whose font size you want to modify.
2. Open the Properties window (by clicking on the Properties Window icon on the toolbar).
3. Double-click on the Font property. Visual Basic displays a dialog box.
4. Choose the font size you want to use and click on OK.
* Changing the type style of an object's caption:
If changing the font and size of your captions isn't enough excitement for one day, Visual Basic also lets you change the type style of your captions. The number of available type styles depends on the font you're using for your caption.
For example, if you choose the MS Sans Serif font, Visual Basic gives you the following choices of type styles:
Regular
Italic
Bold
Bold Italic
Underline
Strikeout
If you want, you can even combine two or more type styles for extra emphasis.
To set different font styles, follow these steps:
1. Click on the object whose caption you want to modify.
2. Open the Properties window (by pressing F4).
3. Double-click on the Font property. Visual Basic displays a dialog box.
4. Click on the font style you want to use and click on OK. Visual Basic immediately changes the caption's appearance.
* Changing the background and foreground colors of captions:
Captions normally appear in boring black, white, and shades of gray. To make your captions stand out more colorfully, you can change the background and foreground colors. The BackColor property of an object represents the object's background color, and the ForeColor property represents the foreground color.
Unlike other types of objects, command buttons have only a BackColor property. The BackColor property simply changes the color that surrounds the caption when the command button is highlighted.
To change the color surrounding an object's caption, follow these steps:
1. Click on the object whose background color you want to change.
2. Open the Properties window (by pressing F4).
3. Double-click on the BackColor (or ForeColor) property and click on the Palette tab. Visual Basic displays a color palett
4. Click on the color you want. Visual Basic instantly obeys.
* Moving objects on the screen
Objects can appear anywhere on a form. Visual Basic provides two ways to define the position of an object on the form:
Use the mouse.
Change the Left and Top properties in the Properties window.
To change the position of an object using the mouse, follow these steps:
1. Click on the object you want to move so that black handles appear around the object.
2. Position the mouse over the object (not over one of the object's handles). Then hold down the left mouse button and move the mouse to where you want the object to appear.
3. Release the mouse button.
In case you haven't figured out by now what is happening, when you create and place an object for the first time, that position is where Visual Basic displays the object. Use the mouse whenever you want to move an object quickly without regard to exact placement on the screen.
For more precise measurements when moving an object, use the Properties window and type in values for the Left and Top properties.
For objects, the Left property measures the distance from the left edge of the form to the left edge of the object. The Top property measures the distance from the top of the form to the top of the object.
For forms, the Left property measures the distance from the left edge of the screen to the left edge of the form. The Top property measures the distance from the top of the screen to the top of the form.
To change the position of an object using the Properties window, follow these steps:
1. Click on the object you want to move so that black handles appear around the object.
(If you want to change the position of a form, click anywhere on the form so that handles appear around the edges of the form.)
2. Open the Properties window (by pressing F4).
3. Double-click on the Left property and type a new value.
4. Double-click on the Top property and type a new value.
* Deleting objects off the face of the earth
Sometimes you draw an object and then decide that you don't need it after all.
To delete an object, follow these steps:
1. Click on the object you want to delete.
2. Press Delete or choose Edit->Delete.
If you press Ctrl+Z right after you delete an object, you can undelete the object you previously deleted.
* Copying objects because you're too tired to draw new ones
After you draw an object that is the exact size you need, you may want to make a copy of the object rather than create a new one and go to the trouble of resizing it.
To copy an object, follow these steps:
1. Click on the object you want to copy.
2. Press Ctrl+C, choose Edit->Copy, or click on the Copy Icon.
3. Press Ctrl+V, choose Edit->Paste, or click on the Paste icon.
Visual Basic displays a dialog box that asks whether you want to create a control array. If you know what a control array is and want to create one, click on Yes; otherwise, click on No. Visual Basic displays a copy of your object in the upper-left corner of the form.
A control array lets you create two or more objects that share the same name. That way, two or more objects can share the same event procedure. If none of this makes any sense to you, just pretend this paragraph doesn't exist or refer to the latest edition of MORE Visual Basic For Dummies, which explains what control arrays are and why you might want to use them.
4. Move this copy of the object anywhere on your screen.
* Selecting more than one object to move, copy, or delete
Before you can move, copy, or delete any object, you have to select the object by clicking on the object. However, if you want to move, copy, or delete more than one object at the same time, you have two choices:
Use the mouse to select multiple objects.
Click on multiple objects while holding down Ctrl or Shift.
To use the mouse to select multiple objects, follow these steps:
1. Position the mouse at the upper-left corner of the group of objects you want to select. (But don't position the mouse directly over any of the objects you want to select.)
2. Hold down the left mouse button while you drag the mouse to the lower-right corner of the group of objects you want to select.
Visual Basic displays a dotted line around all the objects you select.
3. Release the mouse button.
Visual Basic displays a gray rectangle around all the objects you select.
To click on multiple objects while holding down Ctrl or Shift, follow these steps:
1. Click on the first object that you want to select.
Visual Basic displays black handles around the object.
2. Point to the second object that you want to select.
3. Press Ctrl or Shift while you click on the second object.
Visual Basic displays gray rectangles around this object and each of your previously selected objects.
4. Repeat Steps 2 and 3 until you select all the objects you want.
* Changing the size of objects
After creating an object, the next step is to define the object's size, which is a topic most men tend to exaggerate. Visual Basic provides two ways to change the size of an object:
Use the mouse.
Change the Height and Width properties in the Properties window.
To change the size of an object using the mouse, follow these steps:
1. Click on the object that you want to resize. Little black handles appear around the edges of the object.
2. Move the mouse to the edge of the object until the mouse pointer turns into a double-headed arrow.
3. Hold down the mouse button and drag the mouse. When the object is in the shape you want, release the mouse button.
To change the size of a form using the Properties window, follow these steps:
1. Click on the object you want to resize.
2. Open the Properties window (by pressing F4).
3. Double-click on the Height property, type a new value, and press Enter.
4. Double-click on the Width property, type a new value, and press Enter.
Use the mouse method when the exact size of your object isn't crucial. Change the Height and Width properties manually when you want absolute precision or when you feel like being picky about details that nobody else cares about.
* Defining the TabIndex property of your objects
The TabIndex property determines the order in which Visual Basic highlights buttons when the user presses either the Tab, up-arrow, down-arrow, right-arrow, left-arrow, or Shift+Tab keys.
The Tab, down-arrow, and right-arrow keys highlight the object with the next-highest TabIndex value.
The Shift+Tab, up-arrow, and left-arrow keys highlight the object with the next-lowest TabIndex value. To highlight option buttons, you can use only the up, down, right, and left arrow keys, but not the Tab or Shift+Tab keys.
The spacebar or Enter key selects a highlighted object.
Some objects, such as image boxes and menu objects, don't have a TabIndex property, so you can't highlight them by pressing any keys.
An object with a TabIndex property set to 0 appears highlighted as soon as your program runs. If the user presses Tab, the object with a TabIndex property of 1 is highlighted next, and so on.
The first object that you create has a TabIndex property of 0. The second object that you create has a TabIndex property of 1, and so on.
The only way to highlight an object stored inside a frame (see Chapter 6) is to press Tab or to click on the object with the mouse. After an object inside a frame is highlighted, pressing the up-, down-, left-, and right-arrow keys only highlights other command buttons or objects in that frame.
Although most users use a mouse to highlight and select objects, some users may not. For those rare instances, the keyboard is the only way these behind-the-times people can select the objects on your user interface.
To change the TabIndex property of an object, follow these steps:
1. Click on the object that you want to modify.
2. Open the Properties window (by pressing F4).
3. Double-click on the TabIndex property and type a number (such as 1 or 4).
Whenever you change the TabIndex property of a button, Visual Basic automatically renumbers the TabIndex of your other buttons. Thus, you can never assign two buttons identical TabIndex values.
If you have created lots of objects, you can set the TabIndex properties for them quickly and easily by following these steps:
1. Click on the object that you want highlighted last (the object that Is to have the highest TabIndex property value).
2. Open the Properties window (by pressing F4).
3. Click on the TabIndex property and type 0.
4. Click on the object that you want highlighted second to last.
5. Repeat Steps 2 through 4 until you set the TabIndex properties for all your objects to 0.
If you follow these steps, the last object you click on has a TabIndex of 0, the second-to-last object you click on has a TabIndex of 1, and so on.
For some reason if you don't want the user to be able to highlight an object by pressing the tab key, you can set that object's TabStop property to False.
* Dimming objects
If you don't want the user to press a particular object (such as a command button, a check box, or an image box), you can dim the object. A dimmed object tells the user, "Sometimes you can click on this object but not right now. So there."
To dim an object, follow these steps:
1. Click on the object that you want to dim.
2. Open the Properties window (by pressing F4).
3. Click on the Enabled property and set it to False.
A dimmed object doesn't do anything, so if you want to undim it during runtime events, you eventually have to use BASIC code.
To give you a sneak preview of the incredible power of BASIC code, here's how BASIC undims and dims a button. To undim a button, set the button's Enabled property to True. The following example undims a command button named cmdExit:
cmdExit.Enabled = True
To dim a button using BASIC code, set the button's Enabled property to False. The following example dims a command button named cmdExit:
CmdExit.Enabled = False
You can dim and undim buttons using BASIC code while your program is running. That way, you can dim and undim buttons in response to whatever the user is doing (typing, moving the mouse, pounding helplessly on the keyboard, and so on).
* Making objects invisible
Rather than dimming an object (which essentially taunts the user because the object is there but unavailable), you can make objects disappear completely.
To make an object disappear, follow these steps:
1. Click on the object you want to disappear.
2. Press F4 or select Properties from the View menu to open the Properties window.
3. Click on the Visible property and set it to False.
You can also make an object disappear using BASIC code. To do so, set the object's Visible property to False. The following example makes a command button named cmdNew disappear:
CmdNew.Visible = False
Like dimmed objects, invisible objects are useless unless you can make them visible once in a while. To make an object appear again, you have to use BASIC code to set the object's Visible property to True. The following example makes a command button named cmdNew appear:
CmdNew.Visible = True
* Adding Tooltips to an Object
Despite the standard user interface of Windows 95/98/NT, many people may still have no clue what the different objects of your program actually do. Rather than click on these unmarked objects at random (and risk wrecking their data), most people sit paralyzed with fear and wind up never using many parts of a program at all.
To overcome this hesitation, all of your objects can display tooltips. A tooltip is nothing more than brief text that explains what a particular object does. The tooltip remains invisible until the user moves the mouse cursor over the object and leaves the mouse hovering there for a few seconds. Then your Visual Basic program can spring to life and display the tooltip text.
To create tooltip text for your objects, follow these steps:
1. Click on any object on your form.
2. Open the Properties window (by pressing F4).
3. Double-click on the ToolTipText property.
4. Type the brief explanation that you want to appear whenever the user leaves the mouse cursor over the object.
* Doing Caption Changes for Yourself
The following sample program lets you change the caption on the form by typing a new caption in a text box and by clicking on the cmdCaption command button. Just create a user interface according to Table 5-3 and see for yourself the amazing power of Visual Basic.
TABLE 5-3
Properties to Change for CAPTION.VBP
Object - Form:
Property - Caption
Setting - The Incredible Changing Caption
Property - Height
Setting - 3885
Property - Width
Setting - 4680
Object - Lable1:
Property - Name
Setting - lblHeadline
Property - Caption
Setting - This caption can be changed by clicking on the command button below
Property - Height
Setting - 600
Property - Left
Setting - 240
Property - Top
Setting - 360
Property - Width
Setting - 4000
Object - Label2:
Property - Caption
Setting - Type a new caption here:
Property - Height
Setting - 300
Property - Left
Setting - 120
Property - Top
Setting - 1680
Property - Width
Setting - 1935
Object - Text1
Property - Name
Setting - txtCaption
Property - Height
Setting - 495
Property - Left
Setting - 2280
Property - Multiline
Setting - True
Property - Text
Setting - (Empty)
Property - Top
Setting - 1560
Property - Width
Setting - 1935
Object - Command1
Property - Name
Setting - cmdCaption
Property - Caption
Setting - Change Caption
Property - Height
Setting - 495
Property - Left
Setting - 1200
Property - ToolTipText
Setting - Click here to change the caption
Property - Top
Setting - 2640
Property - Width
Setting - 2175
For those of you who don't like typing, you can examine the CAPTION.VBP program on the enclosed CD-ROM.
Private Sub cmdCaption_Clock( )
LvlHeadline.Caption = txtCaption.Text
End Sub
When you run this program, just type a new caption in the text box and click on the command button labeled Change Caption. Visual Basic immediately displays your newly typed text in the top label.
++ CHAPTER 6
FORMS AND BUTTONS
In This Chapter:
Creating forms and drawing borders
Choosing the order in which forms are displayed
Creating buttons
The main part of a user interface is a window, which Visual Basic calls a form. A Visual Basic program needs at least one form, but most programs use two or more forms.
For example, a typical program uses one form to display a list of command buttons on which to click. If the user clicks on a command button, a second form appears displaying information such as names, addresses, and telephone numbers of people who owe you money.
The CD-ROM at the back of this book contains a simple Visual Basic program that shows how to change the background of a form. Feel free to experiment with this program and modify it so you can see how you can easily change the appearance of a form.
** Creating a Form
Visual Basic provides two ways to create a form for your program:
Create a blank form
Use a form template
Creating a blank form is handy when you want to design the appearance of the form all by yourself. As a quicker way of creating forms, you can use a form template, which provides pre-designed forms for displaying a Tip of the Day, Web Browser, or Splash Screen window in your program.
If you use a form template, you still have to customize the form for your program, but at least you won't have to waste time drawing objects on the form from scratch.
Form: Creates a blank form.
Dialog: Creates a skeleton of a dialog box with an OK and Cancel command button.
About Dialog: Displays information about your program.
Log in Dialog: Forces users to type a User ID and password before they can use your program.
Options Dialog: Displays a dialog box with tabs, allowing your program to offer options for customizing your program.
Splash Screen: Displays the name of your program along with a logo whenever your program loads.
Tip of the Day: Provides tips for using your program.
ODBC Log In: Displays options for connecting your program to a database.
Web Browser: Adds a Web browser to your program.
VB Data Form Wizard: Automatically creates a form to display and edit database information.
* To create a form for your program, follow these steps:
1. Choose Project->Add Form (or click on the Add Form Icon on the toolbar and click on form).
An Add Form dialog box appears.
2. Click on an icon in the Add Form dialog box, such as the Form (which adds a blank form), Web Browser, or Splash Screen, and click on Open.
* Saving Forms
After you create a form, you may want to save the form (if not for religious purposes, then at least for practical ones) so that you don't have to create the form all over again later. To save a form, Visual Basic gives you two choices:
Choose File->Save Form
Press Ctrl+S
If you have two or more forms displayed on the screen and you want to save changes to all your forms at once, choose File->Save Project or click on the Save Project icon. This command automatically saves every file (listed in the VBP project file) that makes up your entire Visual Basic program.
Get into the habit of periodically saving your forms. If your computer fails, the power goes out (but doesn't get your hard drive), or terrorists raid your home and riddle your computer with bullets, you lose only the changes that you've made since you last saved the form.
* Viewing different forms
Most Visual Basic programs that you create need two or more forms. To keep multiple forms from cluttering up your screen, Visual Basic kindly displays only one form at a time.
So if you want to switch to a different form to look at, follow these steps:
1. Choose one of the following to switch to the Project Explorer window:
Choose View->Project Explorer
Press CtrL+R
Click on the Project Explorer icon in the toolbar
2. Look for the Forms folder. If a plus sign appears to the left of the Forms folder, click on the plus sign.
Visual Basic displays a list of all the forms that make up your program.
3. Click on the form that you want to view and then click on the View Object icon in the Project Explorer window (or just double-click on the form name).
* Positioning a form with Form Layout window
The Form Layout window lets you decide where your forms initially appear on the computer screen when the program actually runs. In case you can't see the Form Layout window, choose one of the following:
View->Form Layout window
Click on the Form Layout window icon on the toolbar
The Form Layout window displays your forms within a tiny computer screen. To move a form's position, follow these steps:
1. In the Form Layout window, move the mouse cursor over the form that you want to move. The mouse cursor turns into a four-way pointing arrow.
2. Hold down the left mouse button and drag the mouse to move the form to its new position.
3. Release the left mouse button.
No matter where your form may appear on the screen while you're editing it, the Form Layout window controls the real position of your form when your program runs.
For a quick way to adjust the position of a form within the Form Layout window, right-click on a form, highlight Startup Position, and then choose an option from the pop-up menu. As another alternative, you can change the StartUpPosition property directly within the Properties window.
* Drawing borders around forms
Borders make forms look pretty, and they also give nations something to argue about. By changing the BorderStyle property, forms can have one of six types of borders.
In addition to making your forms look pretty, each border style also affects whether the user can move or resize the form.
The 0 - None style doesn't display a border around your form, but any objects your form contains can still be seen. A user cannot move, resize, or minimize this type of form.
The 1 - Fixed Single style displays a Control box, title bar, and Close box. Users can move, but not resize, this type of form.
The 2 - Sizable style is the default style. This style displays a Control box, a title bar, Minimize and Maximize buttons, and the Close box. Users can move, resize, and minimize or maximize this form.
The 3 - Fixed Diallog style displays a Control box, a title bar, and a Close box. Users can move this form but cannot resize, minimize, or maximize this form style.
The 4 - Fixed ToolWindow style displays a title bar and Close box. Users can move this form but cannot resize, minimize, or maximize this style of form.
The 5 - Sizable ToolWindow style displays a title bar and Close box. Users can move and resize this form.
To change the borders around your form using the Properties window, follow these steps:
1. Click anywhere on the form where there is not an object.
2. Open the Properties window (by pressing F4).
3. Click on the BorderStyle property.
4. Choose one of the following:
0 -None
1 - Fixed Single
2 - Sizable
3 - Fixed Dialog
4 - Fixed ToolWindow
5 - Sizable ToolWindow
* Minimizing and maximizing forms
Forms can cover part of the screen or the entire screen. Any form that hogs the whole screen is considered maximized. At the other extreme, forms can be shrunk and displayed on the screen as !cons. A shrunken form is considered minimized. Any form that just covers part of the screen is considered normal. Anyone who thinks that computer programmers invent too many definitions is also considered normal.
If you define a BorderStyle of 0 - None, 1 - Fixed Single, 3 - Fixed Dialog, 4 -Fixed ToolWindow, or 5 - Sizable ToolWindow, you cannot minimize or maximize a form.
To display a form as normal, minimized, or maximized while your program runs, follow these steps:
1. Click anywhere on the form. (Do not click on any objects on the form.)
2. Open the Properties window (by pressing F4).
3. Click on the WindowState property in the Properties window.
4. Choose one of the following:
0 - Normal
1 -Minimized
2 - Maximized
You can also give the user the option of minimizing or maximizing a form. To do so, your form needs to display Minimize and Maximize buttons.
To display Minimize and Maximize buttons on a form, follow these steps:
1. Click anywhere on the form where there are no objects.
2. Open the Properties window (by pressing F4).
3. Click on the MinButton (or the MaxButton) property in the Properties window and choose True or False. Depending on the border style you choose for your form, the default setting for the MinButton and MaxButton settings may be True or False.
* Removing forms
Occasionally, you may decide you don't want a particular form as part of your Visual Basic program after all. To remove a form from a Visual Basic project, follow these steps:
1. Choose one of the following to switch to the Project Explorer window:
Choose View->Project Explorer
Press Ctrl+R
Click on the Project Explorer Icon in the toolbar
2. Click on the form you want to remove.
3. Choose Project->Remove or right-click on the form name and choose Remove.
If you remove a form that you've previously saved, the form still exists on your hard disk; it just isn't part of your Visual Basic project anymore. To physically remove all traces of a form out of existence, use Windows Explorer and delete the form file.
** The Control Box
Users can move or resize a form by using the mouse. To minimize or maximize a form, simply click the Minimize or Maximize button. For those users who haven't caught up with current technology or refuse to use a mouse, each form also provides a control box, as well.
If you choose a BorderStyle of 0 - None, 4 - Fixed ToolWindow, or 5 - Sizable ToolWindow, you won't see a control box icon on your forms.
To activate the control box menu while a program is running, click on the control box icon with the mouse pointer or press Alt+spacebar. A menu appears, which lets the user move, resize, minimize, or maximize a form.
* Removing a control box
A control box appears on all forms. But if you think that the box looks really ugly and want to remove it, follow these steps:
1. Click anywhere on the form (except where you find objects on the form).
2. Open the Properties window (by pressing F4).
3. Click the ControlBox property in the Properties window.
4. Choose True or False.
Depending on the border style you choose for your form, the default value may be True or False.
* Displaying icons as the control bar on forms
Icons are special graphics symbols with the ICO file extension. Normally, Visual Basic displays the control box of a form with a default icon that looks like a sail flapping in the wind. If you don't like this icon, you can customize the icon. To change an icon with the Properties window, follow these steps:
1. Click anywhere on the form, but not on any objects on the form.
2. Open the Properties window (by pressing F4).
3. Double-click on the Icon property in the Properties window. Visual Basic displays a Load Icon dialog box.
4. Choose the Icon you want to use and click on Qpen.
* Choosing Which Form Visual Basic Displays First
When your program runs, the first form your program displays is generally the first form you've created. To make another form appear first, follow these steps:
1. Choose Project->ProjectName Properties (where ProjectName Is the name of your Visual Basic project). Visual Basic displays a ProSect Properties dialog box.
2. Click on the General tab. Visual Basic displays the Project Properties dialog box.
3. Click on the downward-pointing arrow of the Startup Object list box to display a list of all the forms for your project.
4. Choose the form you want to display first and click on OK.
** Defining Units of Measurement on a Form
The reason to have a form is so that you can have a place onto which you can put objects. You may notice, therefore, that every form always displays a grid to help you align objects.
By default, each form uses a unit of measurement called a twip, which sounds like something Elmer Fudd says. ("I'm tying stwing acwoss this path so that wabbit will twip ove wit. Eh, eh, eh, eh, eh.")
In case you actually care, 1,440 twips equal 1 inch. If you don't like using twips as your preferred unit of measurement, Visual Basic offers seven options:
Twips (1,440 twips = 1 inch)
Points (72 points = 1 inch)
Pixels (The number of pixels that equals 1 inch depends on your monitor's resolution.)
Characters (A character is 1/6 inch high and 1/12 inch wide.)
Inches (1 inch = 1 inch - amazing, don't you think?)
Millimeters (25.4 mm = 1 inch)
Centimeters (2.54 cm = 1 inch)
Hard-core programmers may be happy to know that Visual Basic even lets you create your own customized coordinate system. If you're thinking about doing that, you probably need to read a book with a title like Visual Basic For Hard-Core Programming Geniuses instead of this book. For the rest of us, the seven available units of measurement in Visual Basic are more than sufficient.
* To change your form's grid scale, follow these steps:
1. Click anywhere on the form. (Do not click any objects on the form.)
2. Open the Properties window (by pressing F4).
3. Click on the ScaleMode property.
4. Click on the downward-pointing arrow to display a list of all the different measurement units you can use.
5. Select the unit of measurement you want your form to use.
** Pushing Your Buttons
Pushing a button is a simple task that anyone can do. Even children can push buttons, which gives them the power to throw a hot dog in a microwave oven and shout with glee when the meat explodes before their eyes.
Everyone uses buttons. Your disk drive probably has a button that you press to eject a floppy disk. Your monitor has a button to turn the screen's power on and off. Even your mouse has a button (or two or three).
Because buttons are so familiar and easy to use, programs often display buttons on the screen that you can push with a mouse. Instead of forcing you to wade through various menus to find the right command, buttons conveniently display your options right before your eyes. All you have to do is figure out which button you want to press.
Buttons are a feature of nearly every program. Therefore, the rest of this chapter is all about making, modifying, and pushing your own buttons.
* Types of buttons
Essentially, a button is nothing more than an area on the screen that the user can click with the mouse. When pushed (clicked on), a command button rushes off and performs a command. (That's why they call them command buttons.) Visual Basic lets you create two types of buttons: command buttons and image buttons.
A command button displays a caption or an optional icon. This caption can be as unimaginative as OK, Cancel, or Quit. Or the caption can represent a particular command, such as Erase File, Next Screen, or Lose Mr. Johnson's Airline Reservation.
Command buttons often appear in dialog boxes where the program displays a message, such as, "Do you really want to erase your IRS tax files to avoid criminal prosecution?" The available choices may be Yes and No.
Unlike a command button that can display both captions and icons, an image button can display only a picture.
The advantage of image buttons is that they can be smaller than command buttons. The disadvantage is that unless the user knows which command each image button represents, the user has no idea how to use your image buttons.
Because image buttons aren't as self-explanatory as command buttons, a good idea is to type a brief description of the image button's purpose in the ToolTipText property (see Chapter 5).
For example, if the only way to exit your program is to click an image button that shows an open door, people may not understand the connection and may wind up turning off their computer to exit your program instead. (And then they probably aren't going to use your program again, either.)
* Creating buttons
To create a command button, follow these steps:
1. Click on the Command Button Icon In the Visual Basic Toolbox.
2. Move the mouse to the place on the form where you want to draw the command button.
3. Press and hold the left mouse button and drag the mouse to form a command button box.
4. Release the left mouse button to complete the operation. Visual Basic displays a boring label like Command1 in the command button.
Although command buttons are the easiest for users to see and click on, you may want to use an image button to give users the option of clicking on a picture to create multimedia programs. To create an image button, follow these steps:
1. Click on the Image Button !con in the Visual Basic Toolbox.
2. Drag the mouse to the place on the torm where you want to draw the image button.
3. Open the Properties window (by pressing F4).
4. Double-click on the Picture property. Visual Basic cheerfully displays a Load Picture dialog box.
5. Load any graphics file (bitmap, icon, or metafile). Visual Basic displays this graphic as your image button.
After you create a command or image button, you still have to write BASIC code to make the button do something when the user clicks on it.
* Displaying pretty icons on command buttons
Although most command buttons simply display a caption, such as OK, Cancel, Yes, No, or Blame It On Your Parents, you also can display icons on command buttons. Pictures can help clarify a command button's purpose. For example, rather than just having a command button display a caption that says "OK," you can also display a green traffic light picture on the command button.
To make a command button display an icon, you can define the following properties:
Style (Choose 1 - Graphical)
Picture (Defines the icon that appears on the command button)
DownPicture (Defines the icon that appears when the user clicks on the command button)
DisabledPicture (Defines the icon that appears when the command button is dimmed)
The DownPicture and DisabledPicture properties are optional if you want to display an icon on a command button.
To display a picture on a command button, follow these steps:
1. Click on the command button on the form that you want to display a picture.
2. Open the Properties window (by pressing F4).
3. Click on the Style property, click on the downward-pointing arrow, and choose 1 - Graphical.
4. Double-click on the Picture property. Visual Basic displays a dialog box.
5. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
6. Double-click on the DownPicture property to define a picture to appear when the user clicks on the command button.
7. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
8. Double-click on the DisabledPicture property to define a picture to appear If the command button appears dimmed.
9. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
* Changing the size of an image button
Visual Basic lets you freely draw an image box of any size that you want; however, if your graphics image is too big for your image button, the graphics image appears cut off.
To make your graphics image change size when you change the image button's size, you need to change the value of the image box Stretch property to True. (The default value of the Stretch property is False.)
For example, the image box in the upper-left corner has the Stretch property set to False, so no matter how large you make the image box, the graphics image remains the same size. The other three image boxes have their Stretch properties set to True; therefore, the graphics image adjusts in size when you change the size of the image box.
* Creating a default command button
The default command button is the one button that users can choose by pressing the Enter key right away as soon as the command buttons appears on the screen. A default button is the most likely choice for the user to choose.
For example, if the user gives a command to launch nuclear missiles at another nation, a dialog box may pop up asking, "Wouldn't you rather play a nice game of chess?" If the default button was Yes, the user can mindlessly hit Enter and save the world from nuclear destruction.
Only command buttons can be default command buttons. Image buttons can never be default command buttons because Visual Basic says so.
You can use two ways to create a default command button:
Set the command button's TabIndex property to zero. This highlights the command button designated as the default button.
Set the Default property on the default command button to True. This works only if no other object has a TabIndex value of zero.
To create a default command button using the TabIndex property, follow these steps:
1. Click the command button that you want to be the default button.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar).
3. Click on the TabIndex property and type 0.
If no other command buttons on your form have a TabIndex property of zero, you can create a default button by setting the Default property of a command button to True.
To create a default command button using the Default property, follow these steps:
1. Click on the command button that you want to be the default button.
2. Open the Properties window (by pressing F4).
3. Click the Default property and set the value to True.
4. Make sure that no other objects on the form have a TabIndex of zero.
What happens if one command button has a TabIndex of zero but another command button has a Default property that is set to True? The default command button is the one with the TabIndex of zero. So there.
* Defining the Cancel button
When users bang on Esc, they usually want to cancel their last command or exit out of the program. Any button that lets the user do this needs to be designated the Cancel button. Only one command button can be defined as the Cancel button. (Another good name for the Cancel button is the Panic button, but that destroys the image of programming as a fine science rather than an incoherent art.)
To create a Cancel command button, follow these steps:
1. Click the command button that you want to be the Cancel button.
2. Open the Properties window (by pressing F4).
3. Click the Cancel property, set the value to True, and change the button's caption to "cancel" or something similar.
* Grouping command buttons
Occasionally, you may want to group related command buttons on the screen to give the illusion of organization.
To create a group of command buttons, follow these steps:
1. Click on the Frame Icon in the Visual Basic Toolbox.
2. Move the mouse to where you want to draw the frame.
3. Hold down the mouse button and move the mouse to draw a frame.
4. Click on the Command Button icon in the Visual Basic Toolbox.
5. Move the mouse inside the frame to where you want to draw a command button.
6. Hold down the mouse button and move the mouse to draw your command button Inside the frame.
7. Repeat Steps 4 through 6 until you draw all the command buttons you need or until you decide that the time has come to do something else.
After you draw a command button inside a frame, the button remains forever trapped inside the frame. When you move the frame, all command buttons inside move along with their host frame.
You cannot create a command button outside a frame and then try to move the button inside a frame, so don't bother trying.
If a command button exists outside of a frame, click on it, choose the Copy or Cut command, click inside the frame so the frame's handles appear, then choose the Paste command. This lets you copy or cut a command button from outside a frame and put it inside the frame.
As a final modification for grouped command buttons, set the TabStop properties of all grouped buttons to False. Next, set the TabStop property of the first command button to True. That way, if a lame user presses Tab, only the first command button in a group becomes highlighted.
To turn off the TabStop properties for a group of buttons, follow these steps:
1. Click any command button within the frame, except for the first command button.
2. Open the Properties window (by pressing F4).
3. Click on the TabStop property and set the value to False.
4. Repeat Steps 1 through 3 until the TabStop property for all the command buttons (except the first command button) has been set to False.
5. Go to the kitchen and reward yourself with a Twinkle for your good deed.
* Test your newfound knowledge
1. Why do command buttons have names and captions?
a. So you have twice as many chances to call them a four-letter word.
b. The name is a bad word you can call the command button, and the caption is there so that you can write a funny punch line.
c. The name identifies the command button, and the caption is what actually appears on the screen.
d. Because Visual Basic says so, and any product sold by Bill Gates can't be wrong because he's a billionaire,
2. Why do you want to group buttons together in a frame?
a. To keep them from escaping into the wild.
b. So related commands are easy to find on the screen.
c. To make understanding what your program is supposed to do harder for anyone.
d. No reason, except to cause more confusion to people trying to learn how to program a computer for the first time.
++ CHAPTER 7
BOXES AND BUTTONS FOR MAKING CHOICES
In this chapter:
Creating check boxes and radio buttons
Creating list boxes and combo boxes
Sorting items in a list box or combo box
Making listed items look pretty
In school, multiple-choice tests were always easier than essay tests because you could substitute guessing for thinking and still get a decent grade. However, students aren't the only ones who don't want to think if they can avoid doing so. Most users are the same way - they want choices clearly laid out in front of their eyes. That way, they can make wild guesses and be on their merry way.
Visual Basic provides several ways to offer choices to users: check boxes, radio buttons (also called option buttons), list boxes, and combo boxes. Check boxes let users choose one or more options. Radio buttons let users choose only one option. List boxes and combo boxes offer users multiple choices.
The CD contains a Visual Basic program that shows how check boxes, radio buttons, list boxes, and combo boxes work. Feel free to modify the source code and see what happens.
** Creating Check Boxes and Radio Buttons
Check boxes get their name from those silly questionnaires that ask, "Check all that apply," as in:
Why do you want to work here? (Check all that apply.)
need the money.
want to participate in employee theft.
want a place where I can steal more office supplies.
need a safe place to hide from the police.
Radio buttons get their name from those old AM car radios that let you push a button to change stations quickly. Just as you can listen to only one radio station at a time, radio buttons let you choose only one option at a time. The following is an example of radio buttons.
What is your sex? (Choose only one.)
Male
Female
Ex-male (surgically a female)
Ex-fernale (surgically a male)
* Aligning your boxes and buttons
Check boxes and radio buttons are usually left-aligned, which means they look like the following:
This is left-aligned.
For some odd reason known only to those few programmers who actually use aligning options, you can also right-align check boxes and radio buttons.
This is right-aligned.
* To left-align or right-align a check box or radio button, follow these steps:
1. Click on the check box or radio button that you want to align.
2. Open the Properties window. (Press F4, choose View->PropertiesWindow, or click on the Properties Window Icon on the toolbar.)
3. Click on the Alignment property and set the value to 0 - Left Justify or 1 - Right Justify.
* Grouping check boxes
Check boxes rarely appear by themselves. Usually, two or more check boxes huddle together like frightened farm animals. The best way to isolate groups of check boxes is to use a frame. Frames visually separate different groups of check boxes. To create a group of check boxes, follow these steps:
1. Click on the Frame Icon In the Visual Basic Toolbox.
2. Move the mouse to where you want to draw the frame.
3. Hold down the left mouse button and move the mouse to draw a frame.
4. Click on the Check Box icon In the Visual Basic Toolbox.
5. Inside the frame, move the mouse to where you want to draw a check box.
6. Hold down the left mouse button and move the mouse to draw your check box.
7. Repeat Steps 4 through 6 until you've drawn all the check boxes you want inside your frame or until you decide the time for a break has arrived.
As a final modification to a group of check boxes, set the TabStop properties of all grouped check boxes to False. Next, set the TabStop property of the first check box to True. That way, when someone presses the Tab key, only the first check box in that frame becomes highlighted.
To turn off the TabStop properties for a group of check boxes, follow these steps:
1. Click on any check box or radio button within the frame, except the first check box.
2. Open the Properties window. (In other words, press F4, choose View->PropertiesWindow, or click on the Properties Window icon on the toolbar.)
3. Click on the TabStop property and set the value to False.
4. Repeat Steps 1 through 3 until the TabStop property has been set to False for all but one of the check boxes.
* Grouping radio buttons
If the radio buttons on a form are not grouped inside a frame, Visual Basic assumes that all radio buttons appearing on the same form belong to the same group. Thus, even if two radio buttons have nothing in common with each other but they appear on the same form, only one of the radio buttons can be chosen at any time.
If you need to display two or more groups of radio buttons, you have to group them within a frame. Otherwise, Visual Basic lumps all the radio buttons in a single group, which means that only one radio button can be chosen at any given time.
* To create a group of radio buttons in a frame, follow these steps:
1. Click on the Frame icon in the Visual Basic Toolbox.
2. Move the mouse to where you want to draw the frame.
3. Hold down the left mouse button and move the mouse to draw a frame.
4. Click on the Radio Button icon in the Visual Basic Toolbox.
5. Inside the frame, move the mouse to where you want to draw a radio button.
6. Hold down the left mouse button and move the mouse to draw your radio button.
7. Repeat Steps 4 through 6 until you've drawn all the radio buttons you want inside your frame or until you have to go to the bathroom.
After you draw a radio button inside a frame, the button remains trapped inside that frame forever. When you move the frame, all radio buttons inside move along with their host frame.
* Adding icons to check boxes and radio buttons
Normally, check boxes and radio buttons display only a caption; but if you want to get fancy, you also can display icons on your check boxes and radio buttons, too.
* To make a check box or radio button display an icon, you must change the following properties as explained below:
Style: Choose 1 - Graphical
Picture: Defines the icon that appears on the check box or radio button
DownPicture: Defines the icon that appears when the user clicks on the check box or radio button
DisabledPicture: Defines the icon that appears when the check box or radio button is dimmed
For some odd reason, Visual Basic makes check boxes and radio buttons with pictures appear like command buttons. Make sure that you take this strange appearance into account, or else both you and your users may mistake a check box or radio button for a command button instead.
* To display a picture on a check box or radio button, follow these steps:
1. Click on the check box or radio button on the form that you want to display a picture.
2. Open the Properties window. (Press F4, choose Viewt->PropertiesWindow, or click on the Properties Window icon on the toolbar.)
3. Click the Style property, click the downward-pointing arrow, and choose 1 - Graphical.
4. Double-click on the Picture property. Visual Basic displays a dialog box.
5. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
6. Double-click on the DownPicture property to define a picture to appear when the user clicks on the command button.
7. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
8. Double-click on the DisabledPicture property to define a picture to appear if the command button appears dimmed.
9. Click on the Picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
** Offering More Choices with List Boxes and Combo Boxes
When you have only a few choices, check boxes and radio buttons work nicely. If you have ten or more choices, however, bombarding the user with a screen full of check boxes or radio buttons can be intimidating and ugly. To present many choices to the user in a clear and helpful manner, Visual Basic provides two alternatives to check boxes and radio buttons: list boxes and combo boxes.
List boxes display long lists of options from which users can choose. If users want to choose something that isn't on the list, too bad. They can't.
Combo boxes also display long lists of options for the user to choose. The difference is that combo boxes also let the user type a choice if the selection the user wants cannot be found on the list. Figure 7-4 shows an example of a list box and a combo box. Notice that the combo box displays items only if you click on the down arrow; the list box always displays items.
is a combo box.
* Creating list boxes and combo boxes
List boxes are like fast-food menus. You can choose only what's on the menu because the folks working there don't know how to handle special requests. Combo boxes are like fancy restaurants where you have a choice of ordering off the menu or saying, "I know this is a vegetarian restaurant, but I want the cook to grill me a steak anyway."
* To create a list box, follow these steps:
1. Click on the List Box !con in the Visual Basic Toolbox.
2. Move the mouse to the place on the form where you want to draw the list box.
3. Hold down the left mouse button and move the mouse to draw the list box. Visual Basic displays one list box with a dull caption such as List3.
4. Repeat Steps 1 through 3 until you've drawn all the list boxes you need.
* To create a combo box, follow these steps:
1. Click on the Combo Box icon In the Visual Basic Toolbox.
2. Move the mouse to the place on the form where you want to draw the combo box.
3. Hold down the left mouse button and move the mouse to draw the combo box. Visual Basic displays one combo box with a dull caption such as Combo1.
4. Repeat Steps 1 through 3 until you've drawn all the combo boxes you need.
* Combo box styles
A combo box lets you type a choice or select one from the displayed list. For added variety, three styles of combo boxes are available:
Drop-down combo box (Style 0, the default)
Simple combo box (Style 1)
Drop-down list box (Style 2)
The drop-down combo box lets users type an item. If users have no idea what to type, they can click the downward-pointing arrow to the right of the combo box, and the combo box politely displays a list of possible choices. Visual Basic always creates this type of combo box unless you change the Visual Basic Style property.
The simple combo box always displays the list on the screen but also gives users the choice of typing an item.
Unlike the other two combo box styles, you must draw the full size of a simple combo box style so the user can see the items stored in the combo box.
The drop-down list box is actually a list box and always displays a range of choices, but you can't type anything of your own.
If you don't want the user to type anything into a combo box, set the combo box's Style property to 2 - Dropdown List.
At this point, you may be asking, "Wait a minute. Why do I want to create a combo box and then turn it into a stupid list box?" Unlike ordinary list boxes, a drop-down list box doesn't display the list on the screen until the user clicks on the arrow to the right of the box. This type of list box is useful when you need to conserve screen space.
* To define the style for a combo box, follow these steps:
1. Click on the combo box that you want to change. (This assumes that you've already created the box.)
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Click on the Style property.
4. Click on the arrow in the Settings Box to display your list of choices. (Hey, what do you know? The Settings Box is an example of a dropdown list box!)
5. Click on the combo box style you want.
** Test your newfound knowledge
1. What is the main difference between a check box and a radio button?
a.You can choose one or more check boxes but only one radio button.
b. Radio buttons tune in to your favorite radio station, but check boxes are places where you save canceled checks.
c. I don't know. Aren't you supposed to be the teacher with all the answers?
d. Everything is one, man. Like, the answer is all in your point of view.
2. What is the major difference between a list box and a combo box?
a. A combo box gives you a choice of typing an item or choosing one from a displayed list. A list box forces you to choose an item from a displayed list.
b. A list box is spelled L-I-S-T, but a combo box is spelled C-O-M-B-O.
c. Combo boxes are cooler than list boxes because a combo box tends to be more confusing to the average user.
d. No difference. In fact, two out of three French chefs think that they both taste exactly like butter.
** Adding Items to List Boxes and Combo Boxes
After you create your list box or combo box, you have to fill up your box with items. (Otherwise, you have no real point in creating a list box or combo box, now do you?) Visual Basic gives you two ways to add items to a list box or combo box:
Use the List property in the Properties window.
Use BASIC code.
** To add items to a list box or a combo box using the List property, follow these steps:
1. Click on the list box or combo box that you want to add items to.
2. Open the Properties window. (Press F4, choose View->PropertiesWindow, or click on the Properties Window Icon on the toolbar.)
3. Double-click on the List property. A drop-down text box appears.
4. Type the first item you want to appear in your list box or combo box. Next, press Ctrl+Enter. Repeat this step for each item you want to add.
5. Press Enter.
When typing items in the List property, make sure that you press CtrL+Enter in between each item. The moment you press Enter, Visual Basic assumes you're done typing items in the List property.
If you want to use BASIC code to add items to a list box or combo box, the secret BASIC command to use is AddItem. So if you want to add the item "Pick me" to a list box named 1stCommands, here is the magic BASIC code that you use to do so:
1stCommands.AddItem "Pick me"
You can add items to a list box or combo box any time your program is running, but the most common time is when the form containing the list box or combo box first loads.
To add items to a list when a form loads, follow these steps:
1. Click on the form in the Project Explorer window.
2. Click on the View Code !con.
3. Click in the Object list box and choose the form name. Visual Basic displays the following procedure:
Private Sub Form_Load()
End Sub
4. For each item you want to display in a list box or combo box, use the AddItem secret command.
For example, if you had a list box named 1stToDo and a combo box named cboHideln, the Sub Form-Load () procedure may look like the following:
Private Sub Form_Load()
1stToDo.AddItem "Call stockbroker"
1stToDo.AddItem "Make airline reservations"
1stToDo.AddItem "Act normally until noon"
1stToDo.AddItem "Steal $250,000"
1stToDo.AddItem "Fake headache"
1stToDo.AddItem "Leave work early"
1stToDo.AddItem "Go to airport"
cboHideln.AddItem "Acapulco"
cboHideIn.AddItem "Rio de Janeiro"
cboHideIn.AddItem "Paris"
cboHideIn.AddItem "Tokyo"
cboHideIn.AddItem "New York"
cboHideIn.AddItem "Bangkok"
End Sub
This procedure adds these items to the 1stToDo list box and to the cboHideln combo box whenever the first form of your program loads.
** Highlighting default items
The purpose of list boxes and combo boxes is to provide users with choices. To make choosing items even more mindless and thus more efficient from the user's point of view, combo boxes can display default items. (With list boxes, the first item is the default item.) A default item is the item that the computer assumes the user wants unless instructed otherwise. (For a default item to be the least likely choice doesn't really make much sense.)
** To create a default item for a combo box, follow these steps:
1. Click on the combo box for which you want to assign a default Item.
2. Open the Properties window. (Press F4, choose View->PropertiesWindow, or click on the Properties Window icon on the toolbar.)
3. Click on the Text property.
4. Type the item that you want to appear as the default item in this combo box.
If you set a combo box's Style property to 2 - Dropdown List, you can define a default item by using BASIC code to set the ListIndex property to a value such as 0 (to make the first item the default item):
CboHideIn.ListIndex = 2
When a user clicks on the combo box, the default item is highlighted.
If you don't define a default item, Visual Basic displays the combo box's generic name (Combo1, Combo2, and so on) as the default item. Because this looks very ugly and amateurish, you should always define a default item for your combo boxes.
** Sorting Items In a list box or combo box
The order that you add items to a list box or combo box is the order in which the items appear. For a little variety, Visual Basic lets you sort items in two ways:
Alphabetically
Any way you want
When Visual Basic sorts a list alphabetically, the list is sorted without regard to whether items are capitalized or not. For example, Visual Basic considers "Your Momma" and "YOUR MOMMA" to be identical.
To sort items in a list box or combo box alphabetically, follow these steps:
1. Click on the list box or combo box in which you want to display items alphabetically.
2. Open the Properties window. (Press F4, choose View->PropertiesWindow, or click on the Properties Window icon on the toolbar.)
3. Click on the Sorted property and set the property to True.
Visual Basic always sorts items with the A's on top and the Z's at the bottom. You cannot sort items in descending order, with the Z's on top and the A's at the bottom (unless, of course, you flip your monitor upside down).
If alphabetic sorting isn't what you want, you have to sort items one-by-one yourself. Visual Basic assigns an index number (which is just an ordinary number such as 1 or 3) to each item in a list.
The first item in a list is assigned an index number of 0, the second item is assigned an index number of 1, the third item is assigned an index number of 2, and so on. (If you've ever been in a European elevator where the ground floor is labeled 1, the first floor is labeled 2, and the second floor is labeled 3, you are going to recognize the confusing way that Visual Basic assigns index numbers.)
To put an item at the top of a list, you still have to use the magic Add Item BASIC command in the following way:
cboHideIn.AddItem "Harare", 0
If you omit the index number, Visual Basic adds items to a list in one of two ways:
If the Sorted property of the list box or combo box is False, the item goes to the bottom of the list.
If the Sorted property of the list box or combo box is True, the item goes in the correct alphabetic order.
If you set the Sorted property of a list box or combo box to True and add items by using index numbers, Visual Basic adds the item according to the index number and does not sort the newly added items alphabetically.
** Removing items from a list box or combo box
Adding items and sorting them may make your lists look nice, but wiping out an item to satisfy that destructive urge that everyone experiences once in a while is more fun.
Visual Basic gives you two ways to remove an item from a list:
Use the RemoveItem BASIC command to remove items one at a time.
Use the Clear BASIC command to wipe out an entire list at once.
To use the Remove Item BASIC command, you have to know the index number of the item you want to remove. For example, to remove the item with an index number of 5 that's located in a list box named 1stToDo, use the following BASIC command:
1st ToDo.Removeltem 5
To use the Clear BASIC command to wipe out an entire list in a single blow, you need the name of the list box or combo box that contains the list you want to kill. To wipe out the entire contents of a combo box named cboHideln, use the following BASIC command:
CboHideIn.Clear
Before using the Clear BASIC command, make sure that you really want to wipe out an entire list.
** Displaying check boxes in your list boxes
When you display items in a list box, you can also display them as check boxes. By using check boxes, you can check off items in a list box such as to-do lists, grocery lists, or lists of New Year's resolutions that you never plan to complete.
* To display check boxes in a list box, follow these steps:
1. Click on the list box where you want check boxes to appear.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Click on the Style property and choose 1 - Checkbox.
To figure out which item a user may have chosen in a checked list box, you have to use BASIC code. Remember that items in a list box are assigned a number. The top item is given an index number of 0, the second from the top a number of 1, the third from the top 2, and so on. Knowing this, you have to write an event procedure that tells you which item was just checked (or unchecked). To do this, you have to use the event procedure attached to your list box such as:
Private Sub List1-ItemCheck(Item As Integer)
GlobalVariable = Item
' This will print the item checked (or unchecked)
Print Item
End Sub
The moment someone checks (or unchecks) an item in a checked list box, the ItemCheck event procedure runs and the Item variable tells you which item was checked (or unchecked). So if the top item is checked (or unchecked), the ItemCheck event procedure assigns the number 0 to Item.
Then you have to assign your own variable to Item so you can tell the other parts of your program which list box item got checked (or unchecked). From there, you have to write BASIC code to figure out what to do if someone checks or unchecks an item.
** Creating Multiple Column List Boxes
For aesthetic purposes - or just because you're bored and want to goof around - you can dispIay multiple columns in a list box. Visual Basic provides three types of multiple columns, depending on the value defined by the Column property:
Value 0: A single column list with vertical scrolling (the default appearance of list boxes)
Value 1: A single column list with horizontal scrolling (but no vertical scrolling)
Any value greater than 1 in number: A multiple column list of two or more columns with horizontal scrolling (but no vertical scrolling)
* To change the way a list box displays items in columns, follow these steps:
1. Click on the list box that you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Click on the Columns property and type 0, 1, or any number larger than 1.
** Making Listed Items Look Pretty
To spice up your lists and make them look a little less like boring shopping lists, Visual Basic lets you change the font, type style, and size of your list's items.
Fonts are different ways to display text. Normally, Visual Basic uses the MS Sans Serif font, but you can use any font stored in the memory of your computer. (MS Sans Serif is similar to the Helvetica font, and the Visual Basic MS Serif font is similar to the Times Roman font.)
* To change the font of items that appear in a list box or combo box, follow these steps:
1. Click on the list box or combo box whose font you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Double-click on the Font property. Visual Basic displays a Font dialog box.
4. Click on the font you want and click on OK. Visual Basic immediately changes the font in the list box or combo box.
Be careful when you use fonts. Novices often get carried away and use so many bizarre fonts that all semblance of normality is lost. Unless you have a really good reason to use different fonts, let Visual Basic use its default font of MS Sans Serif.
You can also change the size of your items, making them smaller or larger. However, the larger the type size is, the larger your list box or combo box must be to show the entire item at once.
The larger the font size, the more the items in your list boxes and combo boxes stand out. Usually, the best method is to use one size for all your list boxes or combo boxes to avoid confusing the user any more than you have to.
* To define the font size, follow these steps:
1. Click on the list box or combo box whose font size you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Double-click on the Font property. Visual Basic displays a Font dialog box.
4. Click on the font size you want and click on OK.
Visual Basic immediately changes the font size of the list box or combo box.
Besides changing the font and size, you can also change the font style and display text in bold, italics, underline, or strikeout.
* To set any one or more font styles, follow these steps:
1. Click on the list box or combo box whose fonts you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar).
3. Double-click on the Font property. Visual Basic displays a Font dialog box.
4. Click on the font style you want and click on OK.
Visual Basic immediately changes the font style of the list box or combo box.
The more attractive you make your list boxes and combo boxes, the more likely the user is going to notice the boxes, at least (if not use them). Just remember that you want to make your program easy to use, not a work of art. If you want to get creative, take up finger painting. If you want to create useful programs and make millions of dollars, make your programs easy, fun, and simple to use.
++ CHAPTER 8
TEXT BOXES FOR TYPING AND SHOWING WORDS
In This Chapter:
Creating text boxes
Filling text boxes and hiding passwords
Using different fonts, sizes, type styles, and colors
Despite the growing acceptance of icons and graphical user interfaces, not all choices can always be offered through command buttons, radio buttons, or combo boxes. Sometimes your program may need to display a word, sentence, paragraph, or entire novel on the screen. And sometimes the user may want to type in a good word or two as well.
So what's the solution? Combo boxes work with words or short phrases, but if your program needs to display a chunk of text or if the user needs to type in a substantial amount of information, a text box can make your job a whole lot easier.
Text boxes have two purposes in life:
To show text on the screen
To let the user type text into the program
Text boxes are among the most flexible programming objects because you can display instructions in a text box and the user can type a reply using ordinary words. If you use enough text boxes in your programs, you may help increase literacy among our population today.
The Visual Basic program enclosed on the CD-ROM provides a simple example of using a text box for accepting a password. Dig into the source code and play around. At the very least, you may mess up the program but still learn something in the process.
** Creating a Text Box
Text boxes are like miniature word processors but can display only one font, one size, and one type style (such as bold or italics). So if you want to display multiple fonts in a text box, give up that thought right now because you can't.
When a user types text in a text box, the following keys work:
Delete: Erases the character to the right of the cursor
Backspace: Erases the character to the left of the cursor
Shift+Arrow: Highlights a block of text
Ctrl+Left arrow: Moves the cursor one word to the left
Ctrl+Right arrow: Moves the cursor one word to the right
Home (or Ctrl+Home): Moves the cursor to the beginning of the line
End (or Ctrl+End): Moves the cursor to the end of the line
Shift+any movement key (such as Home): Highlights text
F11 or F12: Doesn't do a thing and is about as useful on your keyboard as wisdom teeth in your mouth
To create a text box, follow these steps:
1. Click on the Text Box Icon in the Visual Basic Toolbox.
2. On the form, move the mouse where you want to draw the text box.
3. Hold down the left mouse button and draw the text box. Visual Basic displays your text box with default text inside, such as Text1.
4. Repeat Steps 1 through 3 until you've drawn all the text boxes you need or until you find something else to do.
** Putting pretty borders around text boxes
Normally, Visual Basic displays a single line around a text box, defining the boundaries of that text box. If you want to keep your users guessing where a text box is, you can remove this border.
* To change the borders around a text box, follow these steps:
1. Click on the text box whose border you want to change.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Click on the BorderStyle property and choose one of the following:
0 -None
1 - Fixed Single
** Displaying wordes in a text box
After you create a text box, the next step is to put some text in the box. By default, Visual Basic displays the text box's name in the text box, such as Text1.
Changing the Text property of a text box does not affect the Name property of the text box. So if you want your text box to display something more exciting than Text1, you have to change the Text property.
The Text property can contain anything from a blank line (which means your text box appears empty) to ordinary text, to a mass of incomprehensible text that resembles a typical computer manual.
* To change the Text property of a text box, follow these steps:
1. Click on the text box whose Text property you want to change.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Double-click on the Text property (or just click once on the Text label) and type whatever text you want to appear in your text box.
If you want to change the contents of your text box while your program is running, you have to use BASIC code such as:
txtMessage.Text = "This will now appear in the text box."
Whenever you change the Text property of a text box, the new contents of the text box completely wipe out the previous contents of the text box.
** Aligning text in a text box:
To make a text box look nice and organized, Visual Basic can align the text as left-justified, right-justified, or centered.
* To align text in a text box, follow these steps:
1. Click on the text box whose text you want to align.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Click on the MultiLine property and set the value to True. If the MultiLine property is set to False, Visual Basic ignores any changes you make to the Alignment property.
4. Click on the Alignment property and choose one of the following:
0 - Left Justify
1 - Right Justify
2 - Center
Changing the Alignment property doesn't align the text until you first change the text box's MultiLine property to True.
** Word-wrapping text boxes:
In addition to displaying text that the programmer typed into the Text property, a text box also lets users type in their own text. By default, a text box is pretty stupid at handling text. If you type text into a text box, the text box cheerfully displays the text as one huge line that scrolls endlessly out of sight. To make a text box wrap words within the box's boundaries like in a word-processor program, you have to set the text box's MultiLine property to True.
* To set the MultiLine property to True for a text box, follow these steps:
1. Click on the text box where you want to use word-wrapping.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Click on the MultiLine property and set the value to True.
When a text box has a MultiLine property of True, the text box word-wraps text within the boundaries of the text box. If you change the width of a text box while your program is running, the text box automatically word-wraps the text within the new size of the text box. Now, don't you think that what computers can do nowadays is amazing?
** Adding horizontal and vertical scroll bars in text boxes
Word-wrapping is a fine way to display text within a text box, but if the text box isn't tall enough, the text box can't display all the text you may type in. To solve this problem, you may also have to add horizontal or vertical scroll bars.
Be careful! If you add a horizontal scroll bar to a text box, that turns off all word-wrapping. When a text box uses horizontal scroll bar, the only way a user can type on the next line is to press Enter.
Adding a vertical scroll bar lets users type and display more text than the text box can display. With vertical scroll bars, users can press PageUp or PageDown to display text in a text box.
* To add scroll bars to a text box, follow these steps:
1. Click on the text box in which you want to add scroll bars.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Click on the ScrollBars property and choose one of the following:
0 - None
1 -Horizontal
2 - Vertical
3 - Both (horizontal and vertical scroll bars)
Vertical and horizontal scroll bars work only if you have set the text box's MultiLine property to True. (Otherwise, there's no point in having scroll bars remain if you can't display more than one line of text.)
** Making a Password Text Box
In case you work for the CIA, FBI, NSA, DIA, IRS, or any organization that spends lots of money, buries itself in secrecy, and hides behind a three-letter acronym, you may be interested in the ability of Visual Basic to create special password text boxes.
Rather than display ordinary text, password text boxes mask any text you type into a text box with a single character, such as an asterisk (*).
To create a password text box, you need to define the character that the text box displays when someone types in text.
* To create a password text box, you need to follow these steps:
1. Click on the text box that you want to turn Into a password text box.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Click on the MultiLine property and set the value to False.
Always set the MultiLine property of a password text box to False, otherwise the password text box can't mask any typed characters. This is the Visual Basic way of saying that passwords can't be so long that they require two or more lines to write.
4. Click on the PasswordChar property and type the masking character, such as an asterisk.
The masking character can only be a single character.
** Limiting the Length of Text
To prevent people from getting too wordy, you can set the maximum length of text for a text box. This way, people can't type rambling essays about what they did last summer in your text boxes.
To define the maximum number of characters that a text box can accept from the user, change the MaxLength property. If the user tries to type any characters beyond the MaxLength limit, Visual Basic beeps and accepts no more.
(Alas, Visual Basic doesn't have a minimum length property. For ordinary text boxes, this isn't a problem, but if you're creating a password text box, you can bet that at least one bozo is going to choose a one-letter password that some hacker can easily guess.)
* To define the maximum length of characters that a text box accepts, follow these steps:
1. Click on the text box whose maximum character length you want to define.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Double-click on the MaxLength property and type any number greater than zero.
A value of zero effectively means that no limit to the number of characters a user can type in a text box exists.
** Changing Fonts, Sizes, and Type Styles
Visual Basic normally displays text in a text box using the MS Sans Serif font, but if you want to exercise your creativity, you can choose any font stored in the memory of your computer. (The MS Sans Serif font is similar to the Helvetica font, and the Visual Basic MS Serif font is similar to the Times Roman font.)
* To change the font used in a text box, follow these steps:
1. Click on the text box whose font you want to change.
2. Open the Properties window.
3. Double-click on the Font property. Visual Basic displays a font dialog box.
4. Click on the font you want and click on OK. Visual Basic immediately changes the font in the text box.
Be careful when using fonts. Beginners often get carried away and choose really bizarre fonts that confuse more than they clarify. Unless you have a really good reason to use a different font, just use the default font, MS Sans Serif, for most of your work.
You can also change the size of your text, making your text smaller or larger. Of course, the larger the type size of your text, the larger your text box must be to show the entire text. The larger the font size, the less text you can display. To avoid confusing the user any more than necessary, use one font size for all your text boxes.
* To define the font size for a text box, follow these steps:
1. Click on the text box whose font size you want to modify.
2. Open the Properties window.
3. Double-click on the Font property. Visual Basic displays a font dialog box.
4. Click on the font size you want and then click on OK. Visual Basic immediately changes the font size of the text displayed in your chosen text box.
Changing the font and type size of text can be fun so Visual Basic also gives you additional ways to change the appearance of your text to display bold, italic, or strikeout.
* To choose a font style, follow these steps:
1. Click on the text box whose font that you want to modify.
2. Open the Properties window.
3. Double-click on the Font property. Visual Basic displays a font dialog box.
4. Click on the font style you want and click on OK. Visual Basic immediately changes the font style of your chosen text box.
** Test your newfound knowledge
1. Give two uses for text boxes.
a.To store letters from your Scrabble game and to contain words that may win you a million dollars on Wheel of Fortune.
b. To display text on-screen and to let users type text into a program.
c. To store all the computer books that you buy but never read, and to make cardboard forts that your children can hide in.
d.To use as a litter box and to give your cat something to read.
2. If a text box has the PasswordChar property set to * (asterisk) and the MaxLength property set to 10. what happens?
a. I have to flip back through the pages of this book to find the answer, so wait while I do that.
b. I'm not sure, but whatever happens must be important because this question is listed here.
c. This defines the secret password that is needed to break into the Pentagon's computers.
d. The text box accepts a maximum of 10 characters and displays an asterisk in place of an actual typed character.
** Coloring Text Boxes
If you loved the idea of writing in different colors with crayons when you were a kid, then you are going to love the idea of coloring your text boxes using Visual Basic.
Normally, Visual Basic displays text in black against a white background. For more creativity, you can change the foreground and background colors of your text boxes.
The color inside the text box (background color) is defined by the BackColor property. The color of the text itself (foreground color) is defined by the ForeColor property.
To change the background or foreground color of a text box, follow these steps:
1. Click on the text box whose background or foreground color you want to modify.
2. Open the Properties window.
3. Double-click on the BackColor or ForeColor property and click on the Palette tab. Visual Basic displays a color palette.
4. Click on the color you want. Visual Basic immediately changes the color of your chosen text box.
By changing the color of your text boxes, you can highlight certain information and make it easier for the user to see (or ignore). Just remember that too many colors can be distracting and that some people may be colorblind, which means they won't experience the full effect of your program if they can't see the text displayed in certain colors. Remember, use colors sparingly.
++ CHAPTER 9
SCROLL BARS AND LABELS
In This Chapter:
Creating scroll bars
Moving the scroll box
Making labels
Aligning and word-wrapping text
Not all choices in life can be divided into neat categories like check boxes, radio buttons, or list boxes. Sometimes users may need to make choices that require a wide range of gradual adjustments.
Think of adjusting the volume on a stereo. If the only three choices you have are soft, medium, and loud, you can't adjust the volume to your taste. That's why most stereos let you turn a knob or press a button that gradually adjusts the volume higher or lower.
For minute measurements, or for moving through long lists of information, use scroll bars. Although text boxes, forms, and list boxes have built-in scroll bars, you can create separate scroll bars on your own.
The CD-ROM contains a Visual Basic program showing how a vertical scroll bar works. Study the source code and modify it to see what happens.
** Creating Scroll Bars
Visual Basic lets you create two types of scroll bars: horizontal scroll bars and vertical scroll bars. Horizontal scroll bars point left and right, just like the fast-forward and rewind buttons on your VCR or tape player. Vertical scroll bars point up and down, just like the volume control on some stereos.
To create a scroll bar, follow these steps:
1. Click the Horizontal or Vertical Scroll Bar Icon In the Visual Basic Toolbox.
2. Move the mouse cursor on the form to where you want to draw the scroll bar.
3. Hold down the left mouse button and move the mouse to draw the scroll bar, then release the left mouse button.
4. Repeat Steps 1 through 3 until you draw all the scroll bars you feel like making.
If you double-click on the Horizontal or Vertical Scroll Bar icon in the Visual Basic Toolbox, Visual Basic creates a scroll bar on a form right away.
* Setting scroll bar maximum and minimum values
Scroll bars are actually graphical representations of numeric values. The value of a scroll bar can range from -32,768 to 32,767. These numerical values can represent anything you want, such as measurements or quantities.
By default, Visual Basic sets the maximum value to 32,767 and the minimum value to 0. On horizontal scroll bars, the maximum value is represented when the scroll box is at the rightmost position on the scroll bar. The minimum value is represented when the scroll box is at the leftmost position on the scroll bar.
On vertical scroll bars, the maximum value is represented when the scroll box is at the bottommost position on the scroll bar. The minimum value is represented when the scroll box is at the topmost position on the scroll bar.
Obviously, the default values of 32,767 and 0 may be too extreme for most programs. To define a smaller range of values, you have to change the scroll bar's Max and Min settings.
To change the Max and Min settings for a scroll bar, follow these steps:
1. Click on the scroll bar whose Max and Min values you want to change.
2. Open the Properties window.
3. Double-click (or click once) on the Max property and type a new value.
4. Double-click (or click once) on the Min property and type a new value.
If the Min value is larger than the Max value, the scroll bar acts topsy-turvy In this case, the scroll bar represents the maximum value at the leftmost or topmost position and the minimum value at the rightmost or bottommost position.
* Where does the scroll box appear in my scroll bars?
By default, Visual Basic assigns scroll bars the value defined by the Min property (such as 0). This means that if your Max and Min values are positive, the scroll box always appears in the topmost position in a vertical scroll bar and the leftmost position in a horizontal scroll bar.
By default, the scroll box represents the minimum value of the scroll bar. However, if you want your scroll bars to display a default value of something other than the minimum value, you have to change the scroll bar's Value property.
To change the scroll bar's value, follow these steps:
1. Click on the scroll bar whose value you want to change.
2. Open the Properties window.
3. Click on the Value property and type a new value. Visual Basic dutifully changes the scroll bar's value while you watch.
* Moving the scroll box
The scroll box represents the current value of the scroll bar. To move the scroll box, users can do any of the following:
Drag the scroll box within the scroll bar.
Click on the scroll arrows at each end of the scroll bar.
Click in the area between the scroll box and each scroll arrow.
Each time the user clicks the scroll arrows, the scroll box moves a certain distance. By default, this distance is 1. Therefore, if your Min value is 0 and your Max value is 12, you have to click on the scroll arrow 12 times to move the scroll box from one end of the scroll bar to the other.
To modify the distance the scroll box moves when the user clicks on a scroll arrow, follow these steps:
1. Click on the scroll bar that you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Double-click (or click once) on the SmallChange property and type a new value.
Likewise, each time the user clicks on the scroll bar shaft (in the area between the scroll box and the scroll area), the scroll box moves a certain distance. By default, this distance is 1, which means that if the Min value is 0 and the Max value is 5, you have to click five times to move the scroll box from one end of the scroll bar to the other.
To modify the distance the scroll box moves when the user clicks on the scroll shaft, follow these steps:
1. Click on the scroll bar that you want to modify.
2. Open the Properties window.
3. Double-click (or click once) on the LargeChange property and type a new value.
The values for LargeChange and SmallChange can vary between 1 and 32,767. The smaller the value, the smaller the distance the scroll box moves. The larger the value, the larger the distance the scroll box moves.
Here's some information you may never use: You can set LargeChange or SmallChange to values greater than the Max value. This just means that when the user clicks your scroll bar, the scroll box immediately jumps to one end of the scroll bar.
** Test your new found knowledge
1. When can you use a scroll bar?
a.When you want to give the illusion of complexity.
b. When the user needs to choose a range of values.
c.When you need to rewrap toilet paper that your cat unrolled onto the floor.
d.When nothing else seems to work, and you've run out of ideas on how to make your program easier to use,
2. What are the three ways to move the scroll box within a scroll bar?
a. Press the arrow keys, flip the mouse upside down, or unplug the computer.
b. Telekinesis, verbal threats, or pushing the scroll box with your finger.
c. The scroll box never moves. This is a trick question, right?
d. Drag the scroll box with the mouse, click the scroll arrows, or click the scroll bar shaft inside the scroll bar,
** Creating Labels
For pure decoration, you can sprinkle labels on any of your Visual Basic forms. Labels simply identify the objects on your form. In real life, you see labels all the time, such as the label MEN or WOMEN on a restroom door, FIRE EXTINGUISHER over a fire extinguisher in a public building, or POWER next to your monitor's on-and-off button. Labels simply call your attention to something you may otherwise overlook.
Both text boxes (see Chapter 8) and labels can display text on-screen. The main difference is that the user can modify the text inside a text box but can't modify text inside a label.
Psst, although users can't modify text in a label, BASIC code can modify the text inside a label. In this way, your labels can display changing messages to the user, such as "Sorry, that option isn't available at this time," "Now printing page 2 of 9," or "What are you, stupid or something?".
* To create a label, follow these steps:
1. Click on the Label icon in the Visual Basic Toolbox.
2. Move the mouse cursor on the form to where you want to draw the label.
3. Hold down the left mouse button and move the mouse to draw the label. Release the left mouse button.
Visual Basic draws a label with a boring caption, such as Label2.
4. Repeat Steps 1 through 3 as often as necessary for each label you want to draw.
If you double-click on the Label icon in the Visual Basic Toolbox, Visual Basic creates a label on a form right away.
* Putting pretty borders around labels
Normally, labels don't display any visible borders. However, you may want to put a boundary around a label to make the label easier to see. Visual Basic gives you two choices for label borders: a fixed single line or nothing at all.
To create a border around a label, follow these steps:
1. Click on the label whose border you want to change.
2. Open the Properties window.
3. Click on the BorderStyle property and choose one of the following:
0 -None
1 - Fixed Single
* Changing the size of labels
The size of a label on the screen determines the length of the caption (text) that the label can display. If a label is too small, part of the caption is cut off.
Note that the label caption (the text) doesn't change in size when you change the size of a label. To change the size of a label's caption, you have to change the Font property.
You may not know how much room your captions need, but to keep adjusting the width and height of your labels can be a real pain. Because computers are good at doing things that people don't want to do, Visual Basic can take care of this mundane task.
Visual Basic automatically can adjust the size of a label to fit any caption you stick inside of the label. Such automatic adjusting labels are perfect for displaying messages whose length may vary.
An automatic adjusting label grows or shrinks horizontally to match the leength of the label's caption. So if you have a really long caption, the label cheerfully expands in size and disappears off the right side of the screen. To make an automatic adjusting label word-wrap text, you have to set the label's WordWrap property to True.
To create an automatic adjusting label, follow these steps:
1. Click on the label that you want to automatically adjust to the size of its caption.
2. Open the Properties window.
3. Click on the AutoSize property and choose True.
The advantage of automatic adjusting labels is that you can use BASIC code to give a label captions of various sizes without ever worrying that a particular caption isn't going to fit. The disadvantage is that you don't have control over the label's maximum size. If you're not careful, a label can get too big and cover other parts of your user interface.
* Aligning text within a label
To make your label captions look nice and organized, Visual Basic offers three options for aligning captions.
Left-justified
Right-justified
Centered
To align a caption in a label, follow these steps:
1. Click on the label whose caption you want to align.
2. Open the Properties window.
3. Click on the Alignment property and choose one of the following:
0 - Left Justify
1 - Right Justify
2 - Center
* Word-wrapping labels
If you set a label's AutoSize property to True, the label expands horizontally as long as you keep stuffing it with text. However, if you want a label to expand vertically instead, you have to set both the label's AutoSize and WordWrap properties to True.
Setting both the AutoSize property and the WordWrap property to False means that long captions may be cut off at the bottom if your label isn't tall enough.
Setting AutoSize to False and WordWrap to True has the same effect as setting both AutoSize and WordWrap to False.
Setting AutoSize to True and WordWrap to False means that the label expands horizontally to fit an entire caption. However, the label shows only one line of the caption.
Setting both AutoSize and WordWrap to True means that the label grows or shrinks vertically to fit a caption.
To set the WordWrap property for a label to True, follow these steps:
1. Click on the label in which you want to use word-wrapping.
2. Open the Properties window.
3. Click on the WardWrap property and set the value to True.
When creating labels that adjust to the size of a caption, be careful that your labels don't accidentally grow over and cover up other parts of your user interface. Otherwise, you may really confuse someone who is trying to use your program.
++ CHAPTER 10
PRETTY PICTURES AND OBJECTS FROM GEOMETRY
In This Chapter:
Creating picture boxes and image boxes
Making the picture boxes and image boxes fit their contents
Creating geometric shapes
Altering the appearance of lines, circles, and other shapes
You can sprinkle labels and pictures on any of your Visual Basic forms. AIthough pictures can make your forms look nice, they also can be an actual part of your program.
For example, a picture on the front of a road map showing a smiling gas station attendant with the label "Always Trust Your Car to the Man Who Wears the Star" is superfluous and decorative. However, a road map using pictures to show city highways and major side streets can be integral and necessary.
If you sprinkle plenty of labels and pictures in your user interface, your program is going to be easier to use and understand. After all, that's the purpose of creating a user interface in the first place.
** Creating Pictures
Visual Basic provides two ways to display pictures on the screen:
In a picture box
In an image box
Use a picture box to display graphics or to group buttons together. Use an image box to display graphics or to create image buttons. To create a picture box or image box, follow these steps:
1. Click on the Picture Box or Image Box icon in the Visual Basic Toolbox.
2. Using the mouse, hold down the left mouse button on the form and drag to where you want to draw the picture box or Image box, and draw the box by dragging the mouse.
3. Repeat Steps 1 and 2 until you draw all the picture boxes or Image boxes you need.
If you double-click on the Picture Box or Image Box icon in the Visual Basic Toolbox, Visual Basic draws the picture box or image box on the form for you automatically. After drawing a picture box or image box, its size will likely change depending on the size of the graphic image you want to put in it.
* Displaying pictures in picture boxes or image boxes
After you create a picture box or an image box, putting a picture in the box is only natural. (Why else do you create the box?) Picture boxes and image boxes can display three types of graphics images:
Bitmap files: (BMP or DIB file extensions) Consist of patterns of dots, or pixels, which are the types of files created by paint programs, such as Microsoft Paint. If you enlarge a bitmap image, the image tends to look grainy and ugly.
Icon files: (ICO file extensions) Special kinds of bitmap files with a maximum size of 32 x 32 pixels.
Metafiles: (WMF file extensions) Images created by lines and geometric shapes that most people have forgotten about since high-school geometry. These types of files are created by draw programs, such as CorelDRAW.
To load a picture in a picture box or image box, follow these steps:
1. Click on the picture box or image box into which you want to load a graphics file.
(This assumes that you've already drawn the picture box or image box on a form. Otherwise, draw the picture box or image box as explained in the Creating Pictures section.)
2. Open the Properties window.
3. Double-click on the Picture property in the Properties window. Visual Basic displays the Load Picture dialog box.
4. Click on the picture file you want to load. Visual Basic instantly loads the picture in the picture box or image box.
In addition to loading pictures by using the Properties window, you also can load and remove pictures while your program is running (during runtime). To load a picture into a picture box or image box, use the following LoadPicture command:
ImgGreeting.Picture = LoadPicture("c:\graphics\martian.bmp")
The LoadPicture command specif ies the exact drive, directory, and file to store in the Picture property of a picture box or an image box.
To remove a picture from a picture box or image box during run time, use the LoadPicture command as follows:
ImgGreeting.Picture = LoadPicture("")
This statement essentially loads a blank image into the Picture property of a picture box or image box.
* Putting nifty borders around picture boxes and image boxes
To define the edge of your picture box or image box, Visual Basic can display a border. By default, neither type of box displays a border. To change the border around a picture box or image box, follow these steps:
1. Click on the picture box or image box whose border you want to change.
2. Open the Properties window.
3. Click on the BorderStyle property and choose one of the following:
0 - None
1- Fixed Single
** Changing the Size of Picture Boxes or Image Boxes
Generally, the size of picture boxes or image boxes has no effect on the size of the graphics image that the picture box or image box displays, with two exceptions:
Metafile graphics always change size to fit within a picture box or an image box.
If an image box's Stretch property is set to True, bitmap and !con graphics change size to fit within the image box.
* Changing the size of graphics images
Bitmap and icon graphics appear in their original size no matter what the size of the picture box or image box (unless the Stretch property is set to True). Therefore, if you create a huge picture box but load in a tiny bitmap graphics image, all you see is a tiny bitmap graphics image with lots of empty space around the image.
Unlike bitmap or icon graphics, metafiles expand or shrink to fill an entire picture box or expand to their original size to fill an entire image box. To change the size of a metafile, just change the size of the picture box or image box holding the metafile.
If you use a picture box, you can never (and I mean never) change the size of bitmap or icon graphics. On the other hand, if you use an image box, you can change the size of bitmap or icon graphics by changing the image box's Stretch property. To change the size of bitmap or icon graphics in an image box, follow these steps:
1. Click on the image box whose Stretch property you want to change.
2. Open the Properties window.
3. Click on the Stretch property and set the value to True.
After an image box has the Stretch property set to True, you can adjust the size of bitmap or icon graphics just by changing the size of the image box. (Isn't it amazing what $2,000 computers can do?)
* Automatically changing the size of picture boxes
If you're too busy to bother creating and adjusting the size of your picture boxes, let Visual Basic do such formatting automatically. All you need to do is set the AutoSize property to True. The moment you load the bitmap or icon graphics image into a picture box, the picture box immediately shrinks or expands to fit tightly around the graphics image, just like shrink-wrap around a box of floppy disks. To make a picture box automatically adjust its size around a graphics image, follow these steps:
1. Click on the picture box whose AutoSize property you want to change.
2. Open the Properties window.
3. Click on the AutoSize property and set the value to True.
** Coloring Picture Boxes
Visual Basic usually displays a plain gray background in picture boxes. If your graphics images fill up the entire picture box, the background color is irrelevant. But if the graphics image isn't big enough, the background color can be seen.
The background color is defined by the BackColor property. With a tasteful background color, you can highlight your graphics and make them more colorful.
To change the background color of a picture box, follow these steps:
1. Click on the picture box whose background color you want to change.
2. Open the Properties window.
3. Double-click on the BackColor property in the Properties window and then click on the Palette tab.
4. Click on the color you want. Visual Basic instantly obeys.
** Test your newfound knowledge
1. Which lets you change the size of a bitmap or icon graphic: a picture box or an image box?
a. Neither. Bitmaps and icon graphics are perfect just the way they are, and to believe that you can change anything for the better is blasphemous.
b. A picture box, because I don't have the slightest idea what the difference is.
c. An image box, but only if you set the Stretch property to True.
d. This question is too hard. I need a nap.
2. What are the three types of graphics files you can load in a picture box or image box?
a. Bitmaps, icons, and metafiles.
b, Bitmaps, graffiti, and forgeries of famous paintings.
c. Bitmaps, centerfolds, and home videos.
d. Bigfoot, UFOs, and grainy pictures of Elvis walking through downtown Manhattan.
** Lines. Circles, and Other Nightmares from Geometry
What performs absolutely no useful function except a decorative one? If you answered, "The vice president of the United States," you're close, but the real answer is the parts of a user interface that make the interface look more attractive.
The prettier something looks, the friendlier people feel toward the pretty something, which explains why physically attractive people can go through life without having to pay their own bills. So if you make your user interface pretty, the likely consequence is that more people are actually going to try to use your user interface.
Visual Basic provides seven objects for adding visual makeup to your user interface. These objects are as follows:
Lines
Squares
Rectangles
Ovals
Circles
Rounded rectangles
Rounded squares
* Creating lines
Lines are useful for underlining or separating items on the screen. To create a line, follow these steps:
1. Click on the Line Icon in the Visual Basic Toolbox.
2. On the form, move the mouse to where you want the line to start.
3. Hold down the mouse button and move the mouse to where you want the line to end.
4. Release the mouse button.
If you double-click on the Line icon in the Visual Basic Toolbox, Visual Basic draws a line on the form for you automatically.
* Creatinig circles and rectangles
Circles and rectangles can enclose and separate items on the screen. Or they can be an excuse for doodling on company time when the boss thinks that you're really writing a program. To create a circle or rectangle, follow these steps:
1. Click on the Shape icon in the Visual Basic Toolbox.
2. On the form, move the mouse to where you want the top-left corner of the circle or rectangle to appear.
3. Hold down the left mouse button and move the mouse to where you want the bottom-right corner of the circle or rectangle to end.
4. Release the mouse button. At this point, Visual Basic displays a rectangle on the screen. If that's what you want, stop right here; otherwise, continue to Step 5.
5. Open the Properties window.
6. Click on the Shape property and click on the arrow in the Settings Box. Visual Basic displays a list of shapes to choose from:
0 - Rectangle
1 - Square
2 - Oval
3 - Circle
4 - Rounded Rectangle
5 - Rounded Square
7. Click on the shape you want to create.
* Changing the color of lines and other shapes
Visual Basic usually draws lines, circles, and rectangles by using a solid black line. Although boring ol' black is okay for most purposes, sometimes a little color can spice up your user interface. You define the color of a line by using the BorderColor property. To change the line color of a line or shape, follow these steps:
1. Click on the line or shape whose line color you want to change.
2. Open the Properties window.
3. Double-click on the BorderColor property in the Properties window and click on the Palette tab.
4. Click on the color you want. Visual Basic instantly obeys.
* Changing the thickness of titles
Lines can be from 1 to 8,192 in thickness. (The numbers are relative and not related to an actual unit of measurement.) Any line thicker than 100, however, tends to look like a fat sausage on the screen. To change the thickness of a line, follow these steps:
1. Click on the line or shape whose line thickness you want to change.
2. Open the Properties window.
3. Double-click on the BorderWidth property and type a new value. Visual Basic immediately changes the thickness of your line.
4. Marvel at the wonder of technology and how one day you are going to be able to tell your children, "When I was going to school, we had to draw lines using an Etch-A-Sketch. You kids have line drawing so easy with computers and everything."
* Changing the appearance of lines, circles, and rectangles
Visual Basic usually draws lines, circles, and rectangles with a solid line. Although a solid line is easier to see, you may want to create special effects that look like perforations on a page or Morse code. Visual Basic provides the following seven line styles.
Transparent
Solid (the default)
Dash
Dot
Dash-Dot
Dash-Dot-Dot
Inside Solid
If the thickness of a line is greater than 1, the only BorderStyle settings you can use are 1 (Solid) and 6 (Inside Solid). If you use a different BorderStyle setting, nothing happens and you may think that Visual Basic is broken.
To confuse matters even more, the appearance of a line is determined by the BorderStyle property. Normally, you think of a border as something surrounding an object. But in the Visual Basic world of twisted logic, the BorderStyle defines the appearance of a line. To change the appearance of lines by themselves or lines that make up your circles or rectangles, follow these steps:
1. Click on the line, circle, or rectangle whose appearance you want to change.
2. Open the Properties window.
3. Click on the BorderStyle property and choose one of the following:
0 - Transparent
1 - Solid
2 - Dash
3 - Dot
4 - Dash-Dot
5 - Dash-Dot-Dot
6 - Inside Solid
If you choose any style from 2 to 5, set the BorderWidth property to 1. Otherwise, Visual Basic displays the BorderStyle you selected as a solid line.
* Changing the size and position of lines
When you create a line, try to draw the line as the exact size you need. (What's the point of drawing a long line when you know that you really need a short one?) Visual Basic provides two ways to change the size and position of a line:
You can use the mouse.
You can change the X1, X2, Y1, and Y2 properties in the Properties window.
The mouse is the quickest and sloppiest way to change the size and position of a line. But if you insist on using the mouse, follow these steps:
1. Click on the line that you want to change.
Visual Basic displays a blue rectangle at each end of the line. These rectangles are called handles. (Because clicking on a single line can be an exercise in frustration, you can also move the mouse above the line, hold down the mouse button, move the mouse below the line, and release the mouse button.)
2. Move the mouse over one of these handles until the mouse pointer turns into a crosshair.
3. Hold down the mouse button and move the mouse to adjust the line. When the line takes the shape you want, release the mouse button.
If you prefer not to soil your hands by touching the mouse, you can use a more refined method favored by people of distinction everywhere: Use the Properties window. To change the size of a line using the Properties window, follow these steps:
1. Click on the line you want to change.
2. Open the Properties window.
3. Double-click on the X1 property and type a new value.
4. Double-click on the Y1 property and type a new value.
5. Double-click on the X2 property and type a new value.
6. Double-click on the Y2 property and type a new value.
* Changing the size of circles, rectangles, and other shapes
Happily, changing the size of circles, rectangles, and other shapes is much easier than changing the size of a line. You can use the mouse or the Properties window to change the size of a shape. To change the size of a shape using the mouse, follow these steps:
1. Click on the shape you want to change. Visual Basic displays black handles around the shape.
2. Move the mouse over one of these handles until the mouse pointer turns into a double arrow.
3. Hold down the mouse button and move the mouse. When the object is the shape you want, release the mouse button.
For those who prefer to use a keyboard at the expense of ease and convenience, you can change the size of a shape also by using the Properties window. To change the size of a shape by using the Properties window, follow these steps:
1. Click on the shape you want to change.
2. Open the Properties window.
3. Double-click on the Height property and type a new value.
4. Double-click on the Width property and type a new value.
** Filling Shapes with Colours and Pretty Patterns
The inside of a shape is usually empty, blank, and boring. For more excitement than most people's hearts can handle, you can change the color and pattern of the inside of a shape. Visual Basic provides eight patterns that you can use to fill the inside of a shape. The pattern is defined by the FillStyle property. The color of the pattern is defined by the FillColor property.
To define the fill pattern of a shape, follow these steps:
1. Click on the shape whose inside pattern you want to change.
2. Open the Properties window.
3. Click on the FillStyle property and choose one of the following:
0 - Solid
1 - Transparent
2 - Horizontal Line
3 - Vertical Line
4 - Upward Diagonal
5 - Downward Diagonal
6 - Cross
7 - Diagonal Cross
To change the color of a shape's fill pattern, follow these steps:
1. Click on the shape whose pattern color you want to change.
2. Open the Properties window.
3. Double-click on the FillColor property in the Properties window and click on the Palette tab.
4. Click on the color you want. Visual Basic obeys instantly.
** Changing the Background Color of Shapes
In addition to changing the color of the fill pattern inside a shape (fill color) and the line color that makes up a shape (border color), you can also change a shape's background color (back color). Confused?
Before you can change the background color of a shape, you must set the shape's BackStyle property to Opaque. (The default is Transparent, which means that the shape is invisible. If the shape is invisible, changing the color isn't going to do a thing.)
To change the background color of a shape, follow these steps:
1. Click on the shape whose background color you want to change.
2. Open the Properties window.
3. Click on the BackStyle property and set the value to Opaque.
4. Double-click on the BackColor property in the Properties window and click on the Palette tab.
5. Click on the color you want. Visual Basic instantly obeys.
** Try Changing an Object's Size for Yourself
The following sample program lets you change a circle's thickness by using the horizontal scroll bar. To see for yourself, create three objects with the following property settings.
If you don't feel like creating the program listed below, just load and run the SHAPE.VBP file off the enclosed CD-ROM.
Object - Form
Property - Caption
Setting - The Shrinking/Growing Circle
Object - Shape1:
Property - Name
Setting - shpCircle
Property - Height
Setting - 1455
Property - Left
Setting - 1440
Property - Shape
Setting - 3 (Circle)
Property - Top
Setting - 1080
Property - Width
Setting - 1695
Object - HScroll1:
Property - Name
Setting - hsbCircle
Property - Height
Setting - 255
Property - Left
Setting - 720
Property - Max
Setting - 20
Property - Min
Setting - 1
Property - Top
Setting - 360
Property - Width
Setting - 3255
Double-click on the horizontal scroll bar and type the following in the Code window:
Private Sub hsbCircle-Change()
shpCircle.BorderWidth = hsbCircle.Value
End Sub
To run the program, press F5. Then click on the horizontal scroll bar and watch the circle grow before your eyes. Amazing! Astound your friends! Be the hit of your next cocktail party! Visual Basic reveals it all!
++ PART III
MAKING MENUS
In this part ....
Pull-down menus are a fancy way to organize all the options available in your program. That way, if users want to do something with your program, they just have to choose the right pull-down menu and pick the appropriate command.
This part of the book shows you how to make pull-down menus in your own programs. Believe it nor not, making your own menus is actually simple. (The hard part is making your program actually work the way you want it to, which is something that even eludes the grasp of the Microsoft programmers).
++ CHAPTER 11
CREATING AND EDITING PULL-DOWN MENUS
In This Chapter:
Creating menus and menu titles
Adding separator bars
Using shortcut keys and check marks
Dimming or making menu commands disappear
Generally, every menu bar contains the following menu titles: File, Edit, Window, and Help. The File menu appears on the far left, the Edit menu appears next, the Window menu appears next to last, and the Help menu appears last. In between the Edit and Window menus are menu titles unique to a particular program.
Every menu consists not only of menu titles but also menu commands. The menu titles appear at the top of the screen in a menu bar, and the menu commands appear in pull-down menus.
** The Basic Elements of a Menu Bar
Before creating menus, decide how many menu titles your program needs and where each command belongs in your menu titles.
Visual Basic can create menus for you if you use the VB Application Wizard to help you create your program (as I explain in Chapter 3).
The File menu needs to contain commands directly related to file operations, such as opening, closing, saving, and printing files, as well as quitting the program so that you can go to the kitchen and get something to eat.
The Edit menu, needs to contain commands related to editing (duh), such as Undo (and Redo), Cut, Copy, Paste, Clear, and Select All.
The Window menu needs to contain commands related to opening, closing, arranging, and switching among different windows.
The Help menu needs to contain commands for getting help from the program. Typical help commands include a table of contents to the help system, an alphabetical index, propaganda about product support, and a useless About command that displays information the programmers think looks cute on the screen.
Any other menus you sandwich between the Edit and the Window menu titles need to clearly organize the type of commands hidden underneath.
For example, many word-processing programs have a Tools menu title that displays commands for grammar checking, hyphenation, macro creation, and other commands that 99 percent of the working population of America is never going to use.
If your menu titles are unique to your particular program (in other words, they're not the standard Edit or Window menu titles found on other programs), try to make your menu titles descriptive - that way users will have a better idea where to find a specific command.
** Making Menus for Your User Interface
To create and edit menus, you have to open the Menu Editor window. (If you use the VB Application Wizard, Visual Basic can create standard pull-down menus for you automatically, but you still need to use the Menu Editor window to edit your menus.)
In the name of freedom and confusion, Visual Basic provides three ways to display the Menu Editor window:
Press CtrI+E.
Choose Tools->Menu Editor.
Click on the Menu Editor icon on the toolbar.
You can create one set of pull-down menus for each form. So if your program contains two forms, you can have a completely different menu for each form. Of course, having multiple menus may confuse users, but if you're a typical programmer who generally doesn't care what users think, this isn't going to bother you one bit.
The Menu Editor window is where you define everything to create your menus. The first two things you have to define for all your menu titles are their names and their captions.
* Naming menus
Every menu titile and menu command has a caption and a name. The caption is what appears on the screen. The name never appears on the screen; you use the name to identify which menu command the user chooses.
Captions can be up to 40 characters long, including numbers, spaces, punctuation, and the underscore character (_). Of course, the longer your caption is, the more space the caption is going to gobble up on the screen.
Because captions appear on the screen, you can use an ampersand (&) in your captions, such as &File or T&able. Why do you want to do such a silly thing? An ampersand in front of any letter makes that letter underlined in the caption.
When a letter is underlined in a menu title, users can pull down that menu by pressing and holding down the Alt key and pressing whatever letter is underlined. If a menu title is named &Window, the caption appears on the screen as Window, and users can pull the menu down by pressing Alt+W. Offering this option can help users choose menu titles by using these keyboard shortcuts rather than using the mouse.
When the ampersand underlines a letter in a menu command caption, users can choose the caption simply by typing the underlined letter without pressing Alt. So if a menu command caption is named &New, the caption appears on the screen as New. Users can choose that caption by first pulling down the menu and then pressing N.
Names, like captions, can be up to 40 characters long, including numbers and the underscore character. Because names never appear on the screen, you can make them as long as you want until you reach the magic number of 40.
Unlike captions, names cannot include spaces, punctuation, or words that the editors at IDG Books Worldwide, Inc., deem offensive and, hence, may harm sales of this book.
For menu names, Visual Basic recommends that the name begins with mnu, as in the following examples:
mnuFile
mnuWindow
mnuFileOpen
Visual Basic doesn't care whether you use uppercase or lowercase consistently. If you really want to, you can use the following names for menus:
MNufiLEmNuwINDow
MNUfileOPEN
Such names are not only hard to read, but the scattered casing also makes you look illiterate. So for consistency (and to protect your image), the best method is to adopt Microsoft's style and stick with this style whenever you use Visual Basic.
To identify menu commands that appear under certain menu titles, include the menu title as part of a menu command's name. For example, if the menu title File is named mnuFile, menu commands (such as the Open, Save, and Exit commands) that appear in the File menu should have names like mnuFileOpen, mnuFileSave, and mnuFileExit.
* Making menu tittes
Creating pull-down menus for your Visual Basic programs is a two-step process:
First you create the menu titles that appear in the menu bar.
Next you create the menu commands that appear under each menu title.
To create menu titles that appear in the menu bar at the top of a form, follow these steps:
1. Click on the form to which you want to add menu titles.
2. Open the Menu Editor window by pressing CtrI+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. In the Caption text box of the Menu Editor window, type the menu title that you want to have appear on the screen, including any ampersands.
As you type, Visual Basic displays your caption in the Menu control list box.
4. Press Tab to move the cursor to the Name text box.
5. Type your menu name, beginning with mnu followed by the menu caption itself, such as mnuFile or mnuFiIePrint.
You can mix uppercase and lowercase, but for consistency with Visual Basic programmers around the world, stick with the style mnuFileExit where you use uppercase letters to identify separate words such as File and Exit.
6. Press Enter or click on Next to create the next menu title.
7. Repeat Steps 3 through 6 until you create all the menu titles that you want to have appear at the top of the screen in the menu bar.
8. Click on OK.
Visual Basic displays your menus at the top of the form.
* Adding and deleting menu tittes and commands
Creating menu titles is fairly straightforward. Unfortunately, nothing in life is permanent, and that can include your menu titles. For that reason, Visual Basic gives you the option of adding or deleting menu titles.
To add another menu title to a form, follow these steps:
1. Click on the form to which you want to add another menu title.
2. Open the Menu Editor window by pressing CtrI+E, choosing Tools->Menu. Editor or clicking on the Menu Editor icon on the toolbar.
3. Click on the menu title that you want to appear to the right of your new menu title.
4. Click on Insert.
Visual Basic pushes the previously highlighted menu title down and highlights a blank line.
5. Click on the Caption text box and type your new menu title caption, such as &Tools or Forma&t.
6. Press Tab to move the cursor to the Name text box and type your new menu name (such as mnuTools or mnuFormat) and then press Enter.
7. Click on OK.
To delete a menu title, follow these steps:
1. Click on the form from which you want to delete a menu title.
2. Open the Menu Editor window by pressing CtrI+E, choosing Tools->Menu Editor, or clicking on the Menu Editor Icon on the toolbar.
3. Click on the menu title you want to delete.
4. Click on Delete. Visual Basic deletes the highlighted menu title.
5. Click on OK.
When you delete a menu title or menu command, any BASIC code you've written for that particular menu command still exists, so you'll have to delete this code as well.
** Creating Menu Commands under Menu Titles
After you create the menu titles that appear in the menu bar at the top of a form, the next step is to create the commands to appear underneath each menu title.
In the Menu control list box, all flush left items are menu titles that appear in the menu bar. Indented items are menu commands that appear below a menu title.
To create menu commands, follow these steps:
1. Click on the form to which you want to add menu commands.
2. Open the Menu Editor window by pressing CtrI+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
Visual Basic obediently opens the Menu Editor window.
3. Click underneath the menu title where you want to display the menu commands. For example, if you want to put menu commands underneath the File menu title, click underneath the File menu title.
4. Click on Insert.
5. Click on the Caption text box and type the menu command's caption, such as &Save or &Print.
6. Press Tab to move the cursor to the Name text box.
7. Type the menu command's name, such as mnuFiIeSave or mnuFilePrint.
8. Click on the right-arrow button to indent the menu command. This indentation shows you that an item is a menu command and not a menu title. (How's that for similar but confusing terms?)
9. Click on OK.
** Moving Menu Titles and Commands
When you create pull-down menus, you can always edit and change them at a later time. Visual Basic gives you four ways to move your menu titles and commands around:
Up
Down
Indent right
Indent left
In the Menu Editor window, Visual Basic provides four arrow buttons that enable you to move items up, down, right, or left. Moving an item up or down in the Menu control list box simply rearranges that item's position on the menu bar or in a pull-down menu.
To move an item up or down in the Menu control list box, follow these steps:
1. Click on the form containing the menu titles or commands that you want to rearrange.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. Click on the item in the Menu Editor window that you want to move up or down.
4. Click on the up-arrow button to move the item up, or click on the down-arrow button to move the item down.
5. Click on OK when you're done goofing around
When within the Menu Editor window, indenting an item to the right turns a menu title into a menu command. Likewise, while within this window, indenting an item to the left turns a menu command into a menu title.
To indent an item left or right in the Menu control list box, follow these steps:
1. Click on the form containing the menu titles or commands that you want to rearrange.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor Icon on the toolbar. Visual Basic obediently opens the Menu Editor window.
3. Click on the item that you want to indent left or right.
4. Click on the right-arrow button to indent the Item to the right, or click on the left-arrow button to indent the item to the left.
5. Click on OK
** Making Menus Pretty
Pull-down menus conveniently list commands where users can (hopefully) find them. To make your menus even easier to use, Visual Basic also lets you separate menu commands with separator bars, display check marks next to currently used menu commands, add shortcut keys so users don't have to use your pull-down menus at all, and dim or remove menu items altogether.
* Putting separator bars in menus
Separator bars are lines in a pull-down menu that divide groups of commands. Generally, separator bars group related items so that users can find the command they want.
To create a separator bar, follow these steps:
1. Click on the form containing the menus to which you want to add separator bars.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. Click on the item in the Menu Editor window that you want to appear directly below the separator bar.
4. Click on Insert so that Visual Basic displays an empty line.
If necessary, you may have to click on the right- or left-arrow buttons to make the separator bar appear on the same level as the items the bar is dividing.
5. Click on the Caption text box, type a hyphen (-), and press Tab to move the cursor to the Name text box.
6. Type any name you want to identify the separator bar.
Ideally, the name should include part of the menu title, such as mnuFileBar1 or mnuEditBar3.
7. Click on OK to close the Menu Editor window.
* Test your newfound knowledge
1. Why are pull-down menus so usefull?
a. They hide commands so that users can't find them.
b.They make programs easier to use by organizing related commands in easy-to-find menus.
c. Pull-down menus are useful for confusing people while giving them the illusion that it's their fault for not knowing how to use your program in the first place.
d. If pull-down menus are so useful, how come people still need to buy 400-page books to teach them how to use these stupid programs?
2. How can you create and edit pull-down menus in a Visual Basic program?
a. Press Ctrl+E, choose Tools->Menu Editor, or click on the Menu Editor icon on the toolbar to open the Menu Editor.
b. Copy someone else's program and hope that they don't notice.
c. To create pull-down menus, you need to earn a four-year degree studying C++ and Java.
d.Visual Basic can create pull-down menus?
* Assigning shortcut keys
After a while, you are likely to get tired of using pull-down menus every time you want to pick a command. For commonly used commands, a good idea is to assign these commands to shortcut keys, such as Ctrl+S to choose the Save command or Ctrl+X to choose the Cut command. Such a shortcut key lets the user give a command without wading through multiple pull-down menus.
Shortcut keys appear on menus next to the commands they represent. In this way, users can quickly discover the shortcut keys for all your menu commands.
To assign a shortcut key to a menu command, you have to use the Menu Editor window again. Although you may want to make up your own shortcut keys, Visual Basic lets you choose from only a limited list of possible keys.
Visual Basic doesn't let you assign the same shortcut keys to different commands. If you try to, Visual Basic scolds you with an Error dialog box.
To assign shortcut keys to menu commands, follow these steps:
1. Click on the form containing the menus to which you want to add shortcut keys.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor Icon on the toolbar. Visual Basic displays the Menu Editor window.
3. Click on the menu command for which you want to assign a shortcut key.
4. Click on the down-arrow button in the Shortcut list box.
Visual Basic displays a list of possible keystroke combinations you can use.
5. Scroll through this list until you find the right keystroke combination.
Ideally, you want to choose keystroke combinations that are easy to remember, such as Ctrl+S for the Save command or Ctrl+X for the Cut command. Visual Basic displays your choice in the Menu control list box.
6. Click on OK.
Now when you click on your pull-down menus, shortcut keys appear next to some of the commands. Because you haven't written any BASIC code to tell these commands what to do, nothing happens if you press any of the shortcut keys.
* Putting check marks next to menu commands
Check marks, which appear next to items on a menu, visually show that the items already have been selected. These check marks are often useful in identifying which font, type style, or size is currentIy in use.
If you want to make any default choices in your pull-down menus, you can have check marks appear when your program runs.
Check marks can appear next to menu commands only and riot menu titles. If you try to put a check mark next to a menu title, Visual Basic screams and displays the error message - Can't put check mark here.
To add check marks to menu commands, follow these steps:
1. Click on the form containing the menus to which you want to add check marks.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. Click on the menu command from the Menu Editor window that you want a check mark to appear next to.
4. Click on the Checked check box.
5. Click on OK.
If you put check marks next to your menu commands, you're going to want to remove the check marks eventually. To do this, you have to use (gasp!) BASIC code.
To remove a check mark that's next to a menu command, just set the command's Checked property to False. The following example removes a check mark from a menu command named mnuFont12:
mnuFont12.Checked = False
To add a check mark using BASIC code, just set the menu command's Checked property to True. The following example adds a check mark next to a menu command named mnuFontHelvetica:
MnuFontHelvetica.Checked = True
* Dimming menu commands
Sometimes using certain commands doesn't make sense. For example, until you select a block of text, having the Cut or Copy commands as options is pointless. To prevent users from choosing menu commands that aren't available, you can dim the commands. That way, the commands still appear in the menus, but the user can't choose them.
To dim a menu item, follow these steps:
1. Click on the form containing the menu commands that you want to dim.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor lcon on the toolbar. Visual Basic cheerfully displays the Menu Editor window.
3. Highlight the menu item that you want to dim.
4. Click on the Enabled check box to remove the check mark.
5. Click on OK.
If you dim a menu command, eventually, you're going to want to undim the command. To do so, you have to use BASIC code. To undim a menu command, set the command's Enabled property to True. The following example undims a menu command named mnuEditCut:
MnuEditCut.Enabled = True
To dim a menu command while your program is running, use BASIC code. Just set the menu command's property to False. The following example dims a menu command named mnuEditCopy:
MnuEditCopy.Enabled = False
* Making menu commands disappear
Rather than dim a menu command, you can make the command disappear. For example, some programs remove all menu titles except File and Help from the menu bar until the user opens or creates a file. (After all, displaying an Edit menu when you have nothing to edit is pointless.)
To remove a menu item, follow these steps:
1. Click on the form containing the menu commands you want to make invisible.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. From the Menu Editor window, highlight the menu item that you want to make invisible.
4. Click on the Visible check box to remove the cheek mark.
5. Click on OK.
After you make a menu command invisible, eventually you're going to have to make the command visible. To do so, you have to use BASIC code. To make a menu command visible, set the command's Visible property to True. The following example makes a menu title named mnuEdit visible:
mnuEdit.Visible = True
To make a menu command invisible while your program is running, use BASIC code and set the menu item's property to False. The following example makes a menu title named mnuTools disappear:
mnuTools.Visible = False
Just remember that all these fine points of beautifying your menus make your program easier to use and give your program that professional look. As any professional programmer can tell you, if a program looks good, users assume that any error they come across must be their fault. And that's the real reason programmers spend so much time creating a user interface - so that users aren't going to blame the programmers when the program fails catastrophically.
++ CHAPTER 12
SUBMENUS, GROWING MENUS, AND POP-UP MENUS
In This Chapter:
Creating submenus
Dynamically growing menus
Creating pop-up menus
A typical menu bar displays a list of menu titles at the top of the screen. Selecting one of the menu titles displays a pull-down menu.
Unfortunately, a menu bar can hold only a limited number of menu titles, and a pull-down menu can hold only as many commands as can appear on the screen simultaneously. So what happens if you write a killer application that requires more commands than can possibly appear on the menu bar or in multiple pull-down menus? The solution is to use submenus (or to redesign your program).
** Creating Submenus
Submenus are often used to bury a command several layers deep within a series of pull-down menus. If organized properly, submenus clearly show the relationship between various topics. If organized improperly, your program is going to look just as confusing as the most popular commercial programs that millions of people are forced to use every day.
For example, many programs have a Format menu title. Under this Format menu may be commands such as TypeStyle, Font, and Size. Choosing Font often displays a submenu listing all the possible fonts available.
Visual Basic lets you create up to four levels of submenus. Although this number of submenus can be handy, most programs use only one level of submenus to avoid burying commands so deeply that no one can find them again. Rumor has it that the Watergate tapes, Jimmy Hoffa's body, and the location of the Holy Grail reside somewhere in a well-known program's submenus.
Whenever a menu item displays an arrowhead symbol, the symbol indicates that a submenu exists for that item. When you create submenus, Visual Basic displays this arrowhead symbol automatically.
The Menu Editor window is the only place where you can define submenus. Any item that appears flush left appears as a menu title in the menu bar. Items indented once appear on the pull-down menus, items indented twice appear on the first submenu level, items indented three times appear on the second submenu level, items indented four times appear on the third submenu level, and items indented five times appear on the fourth and last submenu level.
To create submenus, follow these steps:
1. Click on the form on which you want to create submenus.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. In the Menu Control list box, highlight the menu item that you want to make into a submenu.
4. Click on the right-arrow button to indent the item.
5. Click on OK.
Each level of indentation (submenu level) is represented by four dots in the Menu Control list box.
If you want to move submenus up a level (such as from submenu level 3 to submenu level 2), you can. Just follow these steps:
1. Click on the form containing the submenus that you want to modify.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. In the Menu Control list, highlight the menu item box that you want to move up a level.
4. Click on the left-arrow button to indent the item to the left.
5. Click on OK when you finish playing around.
Rather than using multiple levels of submenus, most of the really cool programs use dialog boxes. (See Chapter 13 to find out all about dialog boxes.) A dialog box lets users make multiple choices all at once instead of making choices one at a time through many submenu levels. While submenus are fine for choosing a few options, dialog boxes are better for choosing lots of options.
Then again, what's the point of offering submenus if even Microsoft recommends against doing so? This is just one of the many ways that Microsoft gives you the freedom to write hard-to-use user interfaces so that your programs can never pose a real threat to Microsoft's own programs.
* Changing Menu Captions While Your Program Is Running
In certain cases, changing the caption of a menu command while the program is running is necessary. The most common menu command that changes is the Undo command in the Edit menu. After choosing the Undo command, some programs may toggle Undo to display the Redo command.
To change a menu caption, you have to use BASIC code. Just find the name of the menu item that you want to change and set the item's Caption property to a new caption. The following example changes the mnuEditUndo caption to Redo:
mnuEditUndo.Caption = "Redo"
The following example changes the mnuEditUndo caption back to Undo:
mnuEditUndo.Caption = "Undo"
When changing menu captions, you can use the ampersand (&) symbol to display a menu command hot key. (You can read more about hot keys in Chapter 11.) For example, the following code changes the mnuEditUndo caption to Undo where the capital letter U appears underlined:
mnuEditUndo.Caption = "&Undo"
** Designing Dynamically Growing Menus
If you use many Windows-based programs (such as Microsoft Word), you may notice an odd feature: Each time you load a program, the File menu displays a list of the last four or five files you worked on. If you ever open two or more windows in the same program, you may notice that the Window menu also lists the names of the files currently open.
To create a dynamically growing menu, you have to create empty spaces in your menu. To do this, use the Menu Editor window and create BASIC code to add items to make the items visible.
To create a dynamically growing menu, follow these steps:
1. Click on the form where you want to create a dynamically growing menu.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. Click below the last menu command under the menu title to which you want to add new items.
For example, if you want to add new items under the File menu, click below the last menu command under the File menu title.
4. Click on Insert to add the item.
5. Click on the right-arrow button to indent the empty lines so that they appear as menu commands below the menu title.
6. Leave the Caption text box empty and press Tab.
7. In the Name text box, type the same name for each of these empty lines, such as mnuFileMRU.
8. Type 0 in the Index text box for the first empty line.
Each time you add another empty line, increase the number in the Index text box by 1. You want the first empty line Index text box to contain 0, the second to contain 1, the third to contain 2, and so on.
9. Click in the Visible check box to clear the check box.
10. Press Enter.
11. Repeat Steps 3 through 10 until you create four or five empty lines below the last menu title in the Menu Editor Control list box. Make sure that each empty line has a different number in its Index text box.
12. Click on OK.
The preceding steps create a dynamically growing menu, but to actually add items to this menu, you have to use BASIC code. The following example shows how to add an item to give the illusion of a dynamically growing menu:
Private Sub Form_Load()
mnuFileMRU(O).Caption = "&1 C:\VB\HELLO.VBP"
mnuFileMRU(O).Visible = True
End Sub
The second line of the preceding Form_Load event procedure sets the caption of the first empty line to 1 C:\VB\HELLO.VBP. The third line makes this item visible on the pull-down menu.
** Creating Pop-Up Menus
Pop-up menus (also called context menus) are often used to quickly display a list of commands on the screen. Any menu or submenu can appear as a popup menu. Pop-up menus are usually programmed to appear when the user presses the right mouse button.
To create a pop-up menu, you have to use the BASIC command PopupMenu. The following example displays the mnuEdit menu as a pop-up menu when the user clicks the right mouse button:
Private Sub Form_MouseDown(Button As Integer, Shift As _ Integer, X As Single, Y As Single)
If Button = 2 Then ' Right mouse button pressed _
PopupMenu mnuEdit ' Pops up the mnuEdit menu
End If
End Sub
Instead of using the number 1 to represent the left mouse button and the number 2 to represent the right mouse button, you can use the variables vbPopupMenuLeftButton and vbPopupMenuRightButton, respectively.
To create a pop-up menu, follow these steps:
1. Click on the form containing the menu that you want to turn into a pop-up menu.
2. Press F7, choose View->Code, or click on the View Code Icon In the Project Explorer window.
The Code Editor window appears.
3. Click in the Object list box and choose Form.
4. Click in the Procedure list box and choose MouseUp. Visual Basic displays an empty Private Form_MouseUp procedure.
5. Type the following code below the Private Sub Form_MouseDown statement and above the End Sub statement:
If Button = vbPopupMenuRightButton Then
PopUpMenu (type the menu name here such as mnuEdit)
End If
If for some odd reason you want to make a menu pop up by pressing the left mouse button, substitute the vbPopupMenuRightButton variable with the vbPopupMenuLeftButton variable instead.
* Defining the location of pop-up menus
Normally, the pop-up menu appears wherever the mouse pointer happens to be. However, if you want pop-up menus to appear in a specific location on the screen, you can specify the exact coordinates.
For example, the following PopupMenu BASIC command displays a pop-up menu named mnuTools at X-coordinate 500 and Y-coordinate 650:
PopupMenu mnuTools, 500, 650
For those people who are finicky about where the pop-up menu appears relative to the mouse pointer, you can specify whether the menu appears to the left, to the right, or dead center of the mouse. Use one of the following commands:
PopupMenu mnuEdit, 0 ' Left aligned
PopupMenu mnuEdit, 4 ' Center aligned
PopupMenu mnuEdit, 8 ' Right aligned
Using numbers to specify right or left alignment can be easy to type but ultimately confusing to understand what the code actually does. To simplify matters, Visual Basic lets you use honest-to-goodness English phrases that represent the above numbers, such as the following:
PopupMenu mnuEdit, vbPopupMenuLeftAlign ' Left aligned
PopupMenu mnuEdit, vbPopupMenuCenterAlign ' Center aligned
PopupMenu mnuEdit, vbPopupMenuRightAlign ' Right aligned
Essentially, these statements tell Visual Basic, "Hey stupid, whenever you see the words vbPopupMenuRightAlign, substitute the number 8 instead."
Using the old method in a Visual Basic procedure looks like the following:
Private Sub Form_MouseDown(Button As Integer, Shift As _ Integer, X As Single, Y As Single)
If Button = 2 Then ' Right mouse button pressed
PopupMenu mnuEdit, 4 ' Old, confusing way
End If
End Sub
Now here's what the modern, easy-to-read method using English looks like:
Private Sub Form_MouseDown(Button As Integer, Shift As _ Integer, X As Single, Y As Single)
If Button = vbPopupMenuRightButton Then
PopupMenu mnuEdit, vbPopupMenuCenterAlign
End If
End Sub
In case your mind is racing ahead already to new possibilities, you can left-, center-, or right-align a menu around a specific X-coordinate. For example, if you want to center-align the pop-up menu around a specific X-coordinate, do the following:
PopupMenu mnuTools, 4, 500, 650
or
PopupMenu mnuTools, vbPopupMenuCenterAlign, 500, 650
This example displays the pop-up menu at the X- and Y-coordinates of 500 twips and 650 twips, respectively.
If you change the units of measurement on a form from twips to inches or centimeters, you may have to change the X- and Y-coordinate values as well to correspond to your new scale. (See Chapter 6 in case you want to define different units of measurement for your form.)
* Defining the right mouse button to work with pop-up menus
Normally, pop-up menus work like ordinary pull-down menus. To choose a command, you just click on the command with the left mouse button. However, because the right mouse button is used about as often as you use your wisdom teeth, you may want to give users the capability to use either the left or right mouse button.
The following code example activates the right mouse button:
PopupMenu mnuEdit, 2
To replace numbers with English, you can substitute the following:
PopupMenu mnuEdit, vbPopupManuRightButton
By default, Visual Basic assumes that you always want to use the left mouse button with any pop-up menu.
Trick question: How do you define both center-alignment and right-button activation? The answer is to use the Or operator, as shown in the following:
PopupMenu mnuEdit, 2 Or 4
To use English instead of numbers, you can use the following:
PopupMenu mnuEdit, vbPopupMenuRightButton Or vbPopupMenuCenterAlign
Most popular programs (such as WordPerfect, Excel, and Paradox) use submenus, dynamically growing menus, and pop-up menus. But for less complicated programs, such as games, you probably aren't going to need all these different menu features.
Fortunately, most users are already familiar with all these menu features, so when a pop-up menu or submenu appears on the screen, the user is not going to be shocked. The secret is to use these features only if you have to. Remember, the more fancy features you add to your program, the more programming concerns you have to worry about. (Now aren't you glad you decided to find out how to program your computer?)
* Test your newfound knowledge
1. Explain why you may want your menu captions to change while your program is running.
a. To confuse your users so that they think they're doing something wrong.
b. To move important commands every five minutes to keep users on their toes.
c. To toggle menu captions like Undo and Redo.
d. Because you aren't considered a real programmer unless you use every possible feature of a programming language to make your programs more complicated to use.
2. Explain what the following BASIC code does:
Private Sub
Form_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
If Button = vbPopupMenuRightButton Then PopupMenu mnuFont
End If
End Sub
a. When the user presses and releases the right mouse button, the program displays the mnuFont menu as a popup menu.
b. When the user presses the right mouse button, World War Ill is declared.
c. When the user presses any button, the mouse self-destructs with a loud popI
++ CHAPTER 13
DIALOG BOXES
In This Chapter:
Creating dialog boxes
Adding icons and buttons to dialog boxes
Using common dialog boxes
Pull-down menus certainly make life easier for users (provided, of course, that the users know how to use the menus). In addition to pulldown menus, nearly every program also uses dialog boxes.
Dialog boxes are those tiny windows that pop up on the screen. Most of the time, the computer uses dialog boxes to let the user know what the computer is doing, such as "Now printing page 4 of 67"' or "Windows 98 just crashed again and here's an application error number that you won't understand anyway."
However, dialog boxes also let the computer ask questions of users, such as "Cancel printing?" or "Do you really want to exit out of Windows?" A fancy dialog box may be crammed full of options so that the user can make multiple choices all at one time. Just as most Windows-based programs use similar pull-down menus (File, Edit, Help), these programs also use similar dialog boxes.
** Creating a Simple Dialog Box
A dialog box displays a brief message on the screen along with one or more command buttons. Dialog boxes typically contain the following four parts.
A title bar
A message
An eye-catching icon
One or more command buttons
The title bar identifies the purpose of the dialog box, such as About This Program. The message contains text that appears in the dialog box, such as "Are you sure that you want to start World War Ill?" The icon provides visual information about the dialog boxs importance. The number and type of command buttons can vary from one to three.
The simplest dialog box is one that displays a message on the screen and provides an OK command button so that the user can make the dialog box go away.
The following BASIC code creates the simple dialog box.
Private Sub Form_Load ( )
MsgBox "Get a Macintosh", ,"Another Windows Crash"
End Sub
This simple dialog box does nothing more than appear on the screen and then disappear when the user clicks on OK.
* Adding icons to a dialog box
Icons can help grab a user's attention to your dialog box. Visual Basic uses four icons.
Critical Message: Alerts the user to an extremely important question, such as "If you continue, you are going to erase all the files on your hard disk. Are you sure that you want to do this?"
Warning Query. (A question mark.) Highlights less-threatening questions, such as "Do you really want to exit from Microsoft Word?"
Warning Message: (An exclamation mark.) Emphasizes warnings that the user needs to know about, such as "You are about to replace all 79 pages of your document with a period!"
Information Message: Makes otherwise drab and boring messages look interesting, such as "Printing all 3,049 pages of your document may take a long time. Click on OK if you want to go through with this."
To add an icon to a dialog box, just add the numeric value of the icon between the dialog box message and the title bar text, as follows:
Private Sub Form_Load( )
MsgBox "It crashed again!", 16, "Windows Error Message'
End Sub
Normally, Visual Basic lets you display only one of four possible icons in a dialog box from scratch. To do this, create a separate form, set the form's BorderStyle property to Fixed Dialog, and draw command buttons and an image box directly on this form. You can then draw an image box on the form and load any type of icon you want.
Just remember that creating a dialog box by using a separate form requires you to draw the command buttons, label, and image box, and to write BASIC code to make the whole dialog box work. If you just want to create a dialog box quickly and easily, use the MsgBox command instead.
* Defining the number and type fo command buttons in a dialog box
Dialog boxes can contain from one to three command buttons. A numerical value represents each command button. Table 13-1 lists the six command button combinations.
||
TABLE 13-1
Command Button Combinations Available in Visual Basic
Displays - OK button
Value - 0
Visual Basic Constant - vbOKOnly
Displays - OK and Cancel buttons
Value - 1
Visual Basic Constant - vbOKCancel
Displays - Abort, Retry, and Ignore Buttons
Value - 2
Visual Basic Constant - vbAbortRetryIgnore
Displays - Yes, No, and Cancel buttons
Value - 3
Visual Basic Constant - vbYesNoCancel
Displays - Yes and No button
Value - 4
Visual Basic Constant - vbYesNo
Display - Retry and Cancel buttons
Value - 5
Visual Basic Constant - vbRetryCancel
To define a command button combination, choose the combination you want and type the numerical value of the combination or the Visual Basic constant between the dialog box's message text and title bar text, such as
Private Sub Form_Load( )
MsgBox "File not found", 2, "Error Message"
End Sub
Private Sub Form_Load( )
MsgBox "File not found", vbAbortRetryIgnore, "Error Message"
End Sub
* Which command button did the user select in a dialog box?
If a dialog box just displays an OK command button, clicking on that OK command button usually makes that dialog box go away. However, dialog boxes with two or more command buttons give users a choice. When dealing with multiple command buttons on a dialog box, you have to write BASIC code to figure out:
Which button the user chose
What your program should do depending on which button the user chose
The seven possible buttons a user can choose are represented by the numerical balues in TABLE 13-2.
To make your program determien which command button a user chose, you have to set a variable equal to the MsgBox BASIC code, as shown in the following line:
Reply = MsgBox("File not foune", 2, "Error Message")
This code displays a dialog box with the Abort, Retry, Ignore command buttons. If the user clicks on Abort, the value of Reply is 3. If the user clicks on Retry, the value of Reply is 4. If the user clicks on Ignore, the value of Reply is 5.
Note.. Whenever you assign a variable to represent the value chosen from a dialog box, you must use parentheses to enclose the dialog box parameters.
||
TABLE 13-2
Command Buttons a User Can Choose
Button Selected - OK
Numerical Value - 1
Visual Basic Constant - vbOK
Button Selected - Cancel
Numerical Value - 2
Visual Basic Constant - vbCancel
Button Selected - Abort
Numerical Value - 3
Visual Basic Constant - vbAbort
Button Selected - Retry
Numerical Value - 4
Visual Basic Constant - vbRetry
Button Selected - Ignore
Numerical Value - 5
Visual Basic Constant - vbIgnore
Button Selected - Yes
Numerical Value - 6
Visual Basic Constant - vbYes
Button Selected - No
Numerical Value - 7
Visual Basic Constant - vbNo
** Commonly Used Dialog Boxes
While a simple dialog box may be sufficient occasionally, you may want to use a more complicated, yet more common, dialog box such as:
Open
Save As
Color
Font
Print
Before you can use one of these common dialog boxes, you may have to load the Common Dialog Box icon into the Visual Basic Toolbox. To do this, follow these steps:
1. Choose Project->Components or press Ctrl+T. A Components dialog box appears.
2. Click on the Controls tab.
3. Make sure that a check mark appears in the Microsoft Common Dialog Control 6.0 check box. If a check mark does not appear, click on the check box.
4. Click on OK. The Visual Basic Toolbox displays the Common Dialog Box icon.
To display one of the five commonly used dialog boxes in your program, follow these steps:
1. Click on the Common Dialog Box Icon in the Visual Basic Toolbox.
2. Move the mouse anywhere on the form. Hold down the mouse button and move the mouse down and to the right. Let go of the mouse button.
This draws the Common Dialog Box icon on the form.
If you double-click on the Common Dialog Box icon in the Visual Basic Toolbox, Visual Basic draws the Common Dialog Box icon on the form for you automatically.
Where you place the Common Dialog Box icon on a form is irrelevant because the icon is always invisible when your program runs. Putting the Common Dialog Box icon on a form essentially tells Visual Basic, "Okay, this icon gives you the magical power to display the Open, Save As, Print, Color, or Font dialog box whenever I tell you to."
Because you need only one Common Dialog Box icon per form to display the different types of common dialog boxes, you don't have to change the icon's name. Just use the default name that Visual Basic gives the icon, which is CommonDialog1.
* Test Your Newfound Knowledge
1. Why do you want do display an icon in a dialog box?
a. In case an illiterate computer user wants to use your program.
b. To catch the eye of the user and provide a visual cue. For example, a Critical Message icon can warn users that something terrible is about to happen if they don't do something immediatley.
c. To see if the user is smart enough to realize that the dialog box has nothing important to say.
2. To use the predefined Open, Save As, Color, Font, or Print dialog box, what must you do first?
a. Buy the Visual Basic manuals that used to come with the program for free
b. Create a new form, draw three command buttons, two check boxes, one list box, and a partridge in a pear tree.
c. Save your file and exit Visual Basic
d. Make sure that you have drawn the Common Dialog Box icon on your form.
* Displaying the Open dialog box
The Open dialog box lets users choose a drive, directory, and file to open. The user also has the choice of displaying only specific file types, such as those matching the *.TXT or *.EXE criteria.
To display the Open dialog box, you need only one magic BASIC command that looks like this:
CommonDialog1.ShowOpen
If you want to define the list of files that the Open dialog box displays, you have to use something technical called a filter. A filter tells Visual Basic what types of files to display, such as all those with the TXT or BAT file extension.
A filter consists of two parts: the label that appears in the list box and the filter itself. Table 13-3 lists some examples of labels and filters. For added clarity, labels usually include the filter they use.
Text files, for example, usually have the file extension TXT, but sometimes they have the file extension ASC. So the label "Text Files (*.TXT)" lets you know that the dialog box shows only text files with the TXT file extension (and not text flies with the ASC file extension).
To define your labels and filters, use BASIC code, as in the following example:
CommonDialog1.Filter = "All Files (*.*) |*.*|Text Files - (*.TXT) |*.TXT|Batch Files (*.BAT) - |*.BAT| Executable Files
(*.EXE) |*.EXE"
This code also establishes the filter order, which you can utilize with the corresponding filter index number. In the above example, the All Files filter is first, and so has a filter index number of 1. After you define the filter, you have to tell Visual Basic which filter to display by default. Again, you use BASIC code:
CommonDialog1.FilterIndex = 1
This code line displays "All Files (*.*)" in the List Files of Type list box. Or you can choose the following:
CommonDialog1.FilterIndex = 4
This line displays "Executable Files (*.EXE)" in the List Files of Type list box.
The order in which you define your filter (using the Common Dialog1.Filter command) determines the FilterIndex number. For example, if you change the filter to
CommonDialog1.Filter = "Text Files (*.TXT) - |*.TXT|All Files(*.*)|*.*"
the following code displays "Text Files (*.TXT)" in the List Files of Type list box:
CommonDialog1.FilterIndex = 1
||
TABLE 13-3
Labels and Filters
Label - All Files (*.*)
Filter - *.*
Label - Text Files (*.TXT)
Filter - *.TXT
Label - Batch Files (*.BAT)
Filter - *.BAT
Label - Executable Files (*.EXE)
Filter - *.EXE
* How to create an Open dialog box
Obviously, if your program is going to store data in a file, the program is going to have to open a file at some point. Because nearly every program needs to open files, make your life easy and use the Open dialog box.
To actually display an Open dialog box, you need to write BASIC code, such as:
Private Sub mnuFileOpen_Click( )
CommonDialog1.Filter "Text Files (*.TXT) |*.TXT |AII Files (*.*)l*.*"
CommonDialog1.Filterlndex = 1
CommonDialog1.ShowOpen
End Sub
This event procedure tells Visual Basic:
1. When the user clicks on the Open command underneath the File pulldown menu, follow the instructions sandwiched in between the first and last lines of the Private Sub mnuFileOpen_Click( ) event procedure.
2. The second line tells Visual Basic what types of files to display in the Open dialog box.
3. The third line tells Visual Basic to display files matching the first list of files def ined by the Filter property. In this case, the third line tells the Open dialog box to display *. TXT text files.
4. The fourth line tells Visual Basic to display the Open dialog box on the screen, displaying only files with the TXT file extension.
At this point, the Open dialog box looks nice and seems to work, but because you haven't written any BASIC code to tell the dialog box what to do, the box doesn't do a thing but look pretty.
* Which file did the user choose from an Open dialog box?
After you display an Open dialog box, the next big question is to find out which file the user chose. When the user clicks on a file displayed by the Open dialog box, Visual Basic stores the filename in the Filename property of the Common dialog box. So if you want to retrieve the filename that the user clicked on, you need to set a variable to store the filename property, such as:
WhatFile = CommonDialog1.filename
So the complete event procedure may look like this:
Private Sub mnuFileOpen_Click( )
Dim WhatFile As String
CommonDialog1.Filter = "Text Files (*.TXT) |*.TXT|All_Files (*.*)|*.*"
CommonDialog1.FilterIndex = 1
CommonDialog1.ShowOpen
WhatFile = commonDialog1.filename
End Sub
The CommonDialog1.file name property contains both the filename and the directory that the file is stored in, such as C:\MyDocuments\Secrets\Resume.txt. If the user clicks the Cancel command button in the Open dialog box, the filename property is set to " " (no text).
* Displaying a Save As dialog box
A Save As dialog box is nearly identical to the Open dialog box. However, the text of the title bar is not the same. (The Open dialog box's title bar says "Open" and the Save As dialog box's title bar says "Save As".)
The only BASIC command you need to use to display a Save As dialog box is:
CommonDialog1.ShowSave
However, you may also want to use filters to display certain types of files, such as:
Private Sub mnuFileSaveAs_Click( )
CommonDialog1.Filter = "Text Files (*.TXT) | .TXT | All_Files (*.*) |*.*"
CommonDialog1.FilterIndex = 1
CommonDialog1.ShowSave
End Sub
* Which file did the user choose from a Save As dialog box?
Like the Open dialog box, the Save As dialog box stores the filename in the Filename property of the Common dialog box. So if you want to retrieve the filename on which the user clicked, you need to set a variable to store the filename property, such as:
WhatFile = CommonDialog1.filename
So the complete event procedure may look like this:
Private Sub mnuFileSaveAs_Click( )
Dim WhatFile As String
CommonDialog1.Filter - "Text Files (*.TXT) |*.TXT |All_Files (*.*) |*.*"
CommonDialog1.FilterIndex = 1
CommonDialog1.ShowSave
WhatFile = CommonDialog1.fiIename
End Sub
The Save As dialog box doesn't actually save files on its own. To save a file under a different name, you have to write additional BASIC code that tells your computer to save a file onto a disk.
Rather than use Visual Basic's Open and Save As dialog boxes, try experimenting with a free File dialog box control that offers more flexibility and features while being smaller to use. You can download this FileDialog control from the CCRP Web site at www.mvps.org/ccrp. (Note that an independent group of programmers created this FileDialog box control, so if it doesn't work, don't blame Microsoft.)
* Displaying a Color dialog box
The Color dialog box lets users choose colors or mix their own.
To display a Color dialog box, you have to use two magic BASIC commands:
CommonDialog1.Flags = cdlCCRGBInit
CommonDialog1.ShowColor
The first line tells Visual Basic that the Common dialog box, defined by CommonDialog1, can be used to retrieve a color from the Color dialog box.
The second line tells Visual Basic to display the Color dialog box.
For example:
Private Sub Command1_Click( )
CommonDialog1.Flags = cdlCCRGBInit
CommonDialog1.ShowColor
End Sub
* Which color did the user choose from the Color dialog box?
The Color dialog box stores the color the user chose in the Color property of the Common dialog box. So if you want to retrieve the color (which Visual Basic stores as a number) that the user clicked on, you need to set a variable to store the Color property, such as:
WhatColor = CommonDialog1.Color
So the complete event procedure may look like this:
Private Sub Command1_Click( )
Dim WhatColor as Long
CommonDialog1.Flags = cdICCRGBInit
CommonDialog1.ShowColor
WhatColor = CommonDialog1.Color
End Sub
* Displaying a Font dialog box
The Font dialog box lets users choose different fonts, font styles, and point sizes. Each time the user chooses an option, this dialog box displays a sample so that the user can see whether the font, font style, or point size looks okay.
To display a Font dialog box, you have to use two magic BASIC commands:
CommonDialog1.Flags = cdlCFEffects Or cdlCFBoth
CommonDialog1.ShowFont
The first line tells Visual Basic that the Common dialog box, defined by CommonDialog1, can be used to retrieve a font value from the Font dialog box.
The second line tells Visual Basic to display the Font dialog box. For example:
Private Sub Command1_Click ( )
CommonDialog1.Flags = cdlCFEffects Or cdlCFBoth
CommonDialog1.ShowFont
End Sub
* Which captions did the user choose from the Font dialog box?
The Font dialog box lets the user choose a variety of options, as Table 13-4 shows:
TABLE 13-4
Properties That Store Values from the Font Dialog Box
Property - Color
What Information It Contains - The selected color. To use this property, you must first set the Flags property to cdlCFEffects.
Property - FontBold
What Information It Contains - Whether bold was selected.
Property - Property
What Information It Contains - What information it contains.
Property - FontItalic
What Information It Contains - Whether italic was selected.
Property - FontStrikethru
What Information It Contains - Whether strikethru was selected. To use this property, you must first set the Flags property to cdlEFEffects.
Property - FontUnderline
What Information It Contains - Whether underline was selected. To use this property, you must first set the Flags property to cdlCFEffects.
Property - FontName
What Information It Contains - The selected font name
Property - FontSize
What Information It Contains - The selected font size.
So the complete event procedure may look like this:
Private Sub mnuFormatFont_Click( )
Dim TextColor As Long
Dim Bold As Boolean
Dim Italic As Boolean
Dim Underline As Boolean
Dim Strikethru As Boolean
Dim Font As String
Dim Size As Integer
CommonDialog1.Flags = cdlCFEffects Or cdlCFBoth
CommonDialog1.ShowFont
TextColor = CommonDialog1.Color
Bold = CommonDialog1.FontBold
Italic = CommonDialog1.FontItalic
Underline = CommonDialog1.FontUnderline
Strikethru = CommonDialog1.FontStrikethru
Font = CommonDialog1.FontName
Size = CommonDialog1.FontSize
End Sub
* Displaying a Print dialog box
A Print dialog box lets users choose the printer, the print range, and the print quantity.
To display a Print dialog box, use the following BASIC command:
CommonDialog1.ShowPrinter
For example:
Private Sub mnuFilePrint_Click( )
CommonDialog1.ShowPrinter
End Sub
To set the default value for the number of copies to print, you have to use the following BASIC commands:
CommonDialog1.Copies = 1
CommonDialog1.ShowPrinter
For example:
Private Sub Command1_Click( )
CommonDialog1.Copies = 1
CommonDialog1.ShowPrinter
End Sub
Like the Open, Save As, Color, and Font dialog boxes, the Print dialog box looks like it works but really doesn't do anything until you write BASIC code to tell the box how to work after the user clicks on OK.
Using the common dialog boxes that I discuss in this chapter can give your programs that all-important professional look and feel that people have come to expect from software. As any professional programmer can tell you, the more your program looks to be in working order, the more likely that people are going to believe that the program does work.
++ PART IV
THE BASICS OF WRITING CODE
In this part.....
Hurray! Here's the first chapter where you actually find out how to write your own BASIC code to make your computer do something worthwhile. Until now, you may have only drawn the parts that make up a user interface (with an occasional BASIC command thrown in). But everyone knows that looks aren't everything (unless you're a centerfold, looking to marry a multimillionaire who will die within the year). What matters is not only that your user interface looks good, but that your user interface also responds to the user.
Although the thought of writing BASIC code may seem intimidating, it's not. BASIC code is nothing more than a set of step-by-step instructions that tell the computer exactly what to do. So get ready to start coding (a programmers' term for writing computer commands). You'll find that programming can really be fun, easy, and almost as addictive as drawing your user interface or playing a computer game.
++ CHAPTER 14
EVENT PROCEDURES
In This Chapter:
Creating event procedures
Viewing different event procedures
Editing in the code window
Whenever the user takes any action, such as clicking the mouse, pressing a key, passing out on the keyboard, or putting a bullet through the monitor, the action is called an event. The moment an event occurs, Visual Basic looks for BASIC code to tell the program what to do. The BASIC code that responds to a specific event is called an event procedure.
A single Visual Basic program can consist of several thousand event procedures. If you have that many, however, you either have a tremendously complicated program or you're an incredibly incompetent programmer.
With so many possible events and so many possible event procedures in a single program, how does Visual Basic know which event procedure to use?
The answer is easy. When an event occurs, this event is usually directed at some part of your program's user interface. For example, most users click the mouse button only when the mouse is pointing at an object, such as a command button, check box, or menu command on the screen.
Every object can have one or more event procedures, and each event procedure responds to one specific event, such as clicking the mouse or pressing a key.
** Types of Events
Events can be classified into three categories:
Keyboard events occur when the user presses a certain key, such as Tab, or a certain keystroke combination, such as Ctrl+P.
Mouse events occur when the user moves the mouse, clicks or doubleclicks the mouse button, or drags the mouse across the screen.
Program events occur when a Visual Basic program does something on its own, such as loading, opening, or closing a form. Whereas keyboard and mouse events occur when the user does something, program events occur when BASIC code does something.
Although Visual Basic can respond to a multitude of events, you generally want your user interface to respond only to a few events, such as clicking the mouse or pressing a certain key. As soon as Visual Basic detects an event, your program immediately looks to see what part of the user interface needs to respond.
When the user clicks the mouse, for example, Visual Basic first identifies the event. ("Okay, that was a mouse click.") Next, Visual Basic looks to see where the user clicked the mouse. ("The user clicked the mouse on the OK command button.")
Visual Basic then finds that particular command button's event procedure, which contains BASIC code that tells your program what to do when the user clicks the mouse button.
** Creating event procedures
One object can respond to one or more events. For example, a command button can respond to the user's clicking the mouse button or pressing Enter.
Two or more objects can respond to the same event. For example, both a command button and a check box can respond to a mouse click, but they may have different instructions that tell Visual Basic what to do next.
To write an event procedure, you have to perform the following tasks:
Identify the part of your user interface that is going to respond
Open the Code window.
Identify the event to which Visual Basic is to respond.
Write BASIC code to process the event.
Make sure that all the objects of your user interface have names before creating any event procedures. If you create an event procedure for an object and later change that object's name, you will have to rewrite your event procedures.
The following three parts of a user interface can have events associated with them:
Forms
Objects (command buttons, check boxes, and so on)
Pull-down menus
* To create an event procedure for a form, follow these steps:
1. Click anywhere on the form, but not on any objects on the form.
2. Open the Code window by pressing F7, choosing View->Code, or double-clicking anywhere on the form (but not on any objects on the form).
Visual Basic displays the Code window on the screen along with an empty event procedure.
* To create an event procedure for an object, such as a command button or check box, follow these steps:
1. Click on the object so that little black rectangles (handles) appear around it.
2. Open the Code window by pressing F7, choosing View->Code, or by double-clicking on the object.
Visual Basic displays the Code window on the screen along with an empty event procedure. You may still have to click on the Procedure list box to choose a specific event to respond to, such as Click or KeyPress.
If you double-click on an object (such as a command button), Visual Basic displays the Code window right away.
* To create an event procedure for a pull-down menu, follow these steps:
1. Click on the pull-down menu title containing the menu command you want.
2. Click on the menu command you want to write BASIC code for.
Visual Basic displays the Code window on the screen along with an empty event procedure.
** Getting to know the parts of event procedures
When you create an event procedure for the first time, Visual Basic displays an empty event procedure in the Code window. All empty event procedures consist of two lines, such as:
Private Sub cmdExit_Click( )
End Sub
The first line of any event procedure contains five parts:
Private Sub: Identifies the procedure as a subroutine.
The object's name: In this example, the object is a command button named cmdExit.
An underscore
The event name: In this example, the event is a mouse click.
A pair of parentheses, containing any data that the subroutine may need to work: In this example, the parentheses are empty, indicating that no additional data is necessary.
The preceding event procedure says to the computer, "Here are the instructions to follow whenever the user clicks the mouse on the command button named cmdExit. Now leave me alone."
Because this example contains no instructions to follow, this event procedure does absolutely nothing, much like many co-workers you may know.
Any time you change the name of an object, make sure that you change the name of all event procedures connected to the newly named object as well. Otherwise, Visual Basic doesn't know which event procedures belong to which objects on your user interface.
** Splitting the Code window in half
After you start to write lots of event procedures, the Code window may not be able to display all your event procedures at the same time. If you want to view two or more event procedures on the screen at the same time, you can split the Code window in half horizontally. You can divide the Code window only in half (not in thirds, quarters, and so on).
* To split the Code window in half, follow these steps:
1. Move the mouse pointer to the Split bar.
The Split bar appears at the top of the vertical scroll bar. As soon as the mouse pointer appears over the Split bar, the mouse pointer turns into two horizontal parallel lines with arrows pointing up and down.
2. Hold down the left mouse button and drag the mouse down.
When the Split bar divides the Code window the way you want, let go of the mouse button.
* To display the Code window as a single window again, follow these steps:
1. Move the mouse pointer over the Split bar that divides the Code window in half.
The mouse pointer turns into two horizontal parallel lines with arrows pointing up and down.
2. Hold down the left mouse button and drag the mouse all the way up to the top (or bottom) of the Code window, then let go of the mouse button.
** Editing in the Code Window
The Code window works like a simple word processor. Table 14-1 lists the different keystroke commands you can use to edit your event procedures.
TABLE 14-1
Common Editing Keys
Keystroke - Delete
What Happens - Deletes the character to the right of the cursor
Keystroke - Backspace
What Happens - Deletes the character to the left of the cursor
Keystroke - Ctrl+Y
What Happens - Deletes the line that the cursor is on
Keystroke - Home
What Happens - Moves the cursor to the front of the line that the cursor is on
Keystroke - End
What Happens - Moves the cursor to the end of the line that the cursor is on
Keystroke - Ctrl+Home
What Happens - Moves the cursor to the first event procedure
Keystroke - Ctrl+End
What Happens - Moves the cursor to the last event procedure
Keystroke - Ctrl+Down arrow
What Happens - Displays the next procedure
Keystroke - Ctrl+Up Arrow
What Happens - Displays the previous procedure
Keystroke - Ctrl+Page Down
What Happens - Goes to the first line of the next procedure
Keystroke - Ctrl+Page Up
What Happens - Goes to the first line of the current or previous procedure
Keystroke - Shift+F2
What Happens - If the cursor appears in a procedure name, this command displays the BASIC code of that procedure
Keystroke - Ctrl+Right arrow
What Happens - Goes one word to the right
Keystroke - Ctrl+Left arrow
What Happens - Goes one word to the left
Keystroke - Page Down
What Happens - Displays the next page down in the Code window
Keystroke - Page Up
What Happens - Displays the next page up in the Code window
Keystroke - Insert
What Happens - Toggles the Insert mode on or off
Keystroke - Ctrl+X
What Happens - Cuts a selected block of text
Keystroke - Ctrl+C
What Happens - Copies a selected block of text
Keystroke - Ctrl+V
What Happens - Pastes a previously Cut or Copied block of text
Keystroke - Ctrl+Z
What Happens - Undoes the last thing you did (typed a letter, erased a sentence, and so on)
Keystroke - Ctrl+F
What Happens - Finds a word that you specify
Keystroke - F1
What Happens - Displays the Visual Basic help system
Keystroke - F3
What Happens - Finds the next word that you specified previously using the Ctrl+F command
Keystroke - Shift+F3
What Happens - Finds the last occurrence of the word that you specified previously using the Ctrl+F command
Keystroke - F6
What Happens - Switches between Code window panes (if the Code window is split)
Keystroke - Ctrl+H
What Happens - Searches for a word and replaces it with something else
Keystroke - Ctrl+P
What Happens - Displays the Print Dialog box
To help you write BASIC code, the Code window also automatically high-lights BASIC reserved keywords in color. This way you can see which commands are BASIC reserved keywords and which are commands you've created on your own.
To delete an entire event procedure, highlight the procedure by using the mouse or the cursor keys and press Delete.
** Viewing Different Event Procedures
A typical Visual Basic program consists of event procedures stored in FRM form files. To help you find a particular event procedure to examine or edit, you have two choices:
Choose an object name from the Object list box and then choose an event from the Procedure list box in the Code window.
Use the Object Browser window.
** Choosing an event procedure with the Object and Procedure list boxes
To use the Object and Procedure list boxes to find an event procedure, you must know the FRM form file where Visual Basic has stored the event procedure. The Object list box contains all the objects stored on a form. If you click on the Object list box, you can find the object containing the event procedure that you want to see.
The Procedure list box contains all the events that an object can respond to. Each time you select a different event, Visual Basic displays a different event procedure in the Code window.
* To display an event procedure by using the Object and Procedure list boxes, follow these steps:
1. Choose one of the following to switch to the Project Explorer window:
* Click on the Project Explorer window with the mouse
* Press CtrL+R
* Choose View->Project Explorer
2. Click on the form file that contains the event procedure that you want.
3. Click on the View Code icon in the Project Explorer window (or choose View->Code).
4. Click on the downward-pointing arrow of the Object list box and click on an object name.
5. Click on the downward-pointing arrow of the Procedure list box and click on an event name.
Visual Basic displays your chosen event procedure.
If an event procedure contains BASIC code, the Procedure list box displays the event name in bold-face type. If an event name appears in normal type, then that event procedure is empty.
** Choosing an event procedure with the Object Browser window
The Object Browser is most useful when you want to view event procedures stored in different files.
* To display an event procedure with the Object Browser, follow these steps:
1. Choose View->Object Browser, or press F2 to display the Object Browser Window.
2. Click on the downward-pointing arrow in the Project/Library list box and click on the Project that contains the event procedure you want to see.
3. Click on a Form file name that appears in the Classes left pane.
All event procedures stored in that Form file appear in bold-face in the right pane.
4. Double-click on the event procedure that appears in the right pane.
Table 14-2 lists the most common events to which objects can respond.
** TABLE 14-2
Common Events
Event - Activate
Occurs When - A form becomes the active window
Event - Change
Occurs When - The contents of a combo box, directory list box, drive list box, scroll bar, label, picture box, or text box change.
Event - Click
Occurs When - The user clicks the mouse button once on the object
Event - DblClick
Occurs When - The user clicks the mouse button twice in rapid succession on an object
Event - Deactivate
Occurs When - A form changes from being an active window to an inactive window
Event - DragDrop
Occurs When - The user holds down the mouse button on an object, moves the mouse, and releases the mouse button
Event - DragOver
Occurs When - The user holds down the mouse button on an object and moves the mouse.
Event - DropDown
Occurs When - The list portion of a combo box drops down to display a list of choices
Event - GotFocus
Occurs When - An object becomes highlighted when the user presses Tab or clicks on an object, or if a form loads.
Event - KeyDown
Occurs When - The user presses a key
Event - KeyPress
Occurs When - The user presses and releases an ANSI key, such as a keyboard character, Ctrl key combination, Enter, or backspace key. (Basically, an ANSI key can be any letter, number, or oddball keystroke combination that you press).
Event - KeyUp
Occurs When - The user releases a key
Event - LostFocus
Occurs When - An object is no longer highlighted because the user pressed Tab or clicked on another object, or if a form has unloaded.
Event - MouseDown
Occurs When - The user presses a mouse button
Event - MouseMove
Occurs When - The user moves the mouse
Event - MouseUp
Occurs When - The user releases a mouse button
The combination of the object name and the event name defines the name for an event procedure. Because object names must always be unique, no two event procedures on the same form can have the same name.
Although no two event procedures on the same form can share the same name, event procedures on different forms can have the same name. For example, you might have a command button named cmdExit that appears on two different forms. If this happens, you can have the following event procedure stored on both forms:
Private Sub cmdExit_Click( )
End Sub
Two or more objects can share the same name if you make them into a control array. No need to memorize this term right now. Just make a note of this term and move on. If you really want to know about control arrays, pick up a copy of the latest edition of More Visual Basic For Dummies.
** The Event Procedure That Every Program Needs
The simplest and most important event procedure that every program needs is one that stops your program. The following event procedure tells Visual Basic to stop running your program the moment the user clicks on a command button named cmdExit:
Private Sub cmdExit_Click( )
Unload Me
End Sub
If you don't include an event procedure to stop your program, the only way a user can stop your program is by rebooting the computer or turning the whole system off. Because this isn't the best way to exit a program, always make sure that your program contains at least one (or more) ways for the user to exit your program at any given moment.
The old way to stop a Visual Basic program was to use the End keyword, such as:
Private Sub cmdExit_Click( )
End
End Sub
However, Microsoft recommends that instead of using the End keyword, you should use the Unload Me command instead. However, the Unload Me command stops a Visual Basic program only if all forms of your program have been unloaded using the Unload command. If even one form of your program is not unloaded, the Unload Me command will not stop a Visual Basic program from running.
** Test your newfound knowledge
1. What is an event, and what are the three types of events?
a. An event is something that you must get tickets for, such as a concert, a sports event, or the circus.
b. Events are things that happen to your computer, such as having a drink spilled on the keyboard, having all your files
erased by mistake, and having the dog eat a floppy disk.
c. Events occur when the user presses a key or mouse button or when the program changes appearance. The three types are keyboard, mouse, and program events.
d. An event is a holiday or celebration that lets you take the day off from work. The three types of events are legal holidays,
reunions, and funerals for non-existent relatives.
2. What do the Object list box and Procedure list box do in the Code window?
a. They list all the possible reasons why you need to write your program in C++ or Java rather than in Visual Basic.
b. The Object list box lets you choose an object for which you can write an event procedure. The Procedure list box lets you choose all the possible events to which an object can respond.
c. The Object list box contains a list of all the blunt objects you can use to hit your computer. The Procedure list box lists all the events that you can attend instead of staring at your computer screen.
d. Neither list box does anything worth remembering, so don't bother to ask me this question again.
++ CHAPTER 15
USING VARIABLES
In This Chapter....
Using Variables
Assigning numbers and strings to variables
Declaring data types
After you know what you want your program to do, you can start writing BASIC code. The first code you need to write is inside your event procedures.
At the simplest level, an event procedure tells the computer what to do. An event procedure for exiting a program, for example, gives the computer a single Unload Me instruction:
Private Sub cmdExit_Click( )
Unload Me
End Sub
This event procedure requires absolutely no information from the user beyond the simple event of the user's clicking on the cmdExit command button.
What happens, however, when a user types a name, an address, or a telephone number into a program? Obviously, the program must read this information from the user interface and then do something with the information.
The CD-ROM contains a simple program to show you how to declare, use, and display a string variable in a text box on two different forms.
** Reading Data
Any information that a program gets from outside the computer is data. Nearly all but the simplest programs receive data, do something to the data, and spit the data out again.
A word processor receives data as words, which the word processor formats to look pretty and then prints neatly on paper. A database receives data as names, addresses, and phone numbers. The database stores this information someplace and then displays the data in a way that you think is useful. A nuclear-missile guidance system receives data as target coordinates. The missile system uses this data to guide a warhead to a target and wipe entire cities off the planet in the name of peace.
Every useful program in the world follows these three basic steps:
1. Get data.
2. Do something to the data.
3. Spit the data back out again.
Every useless program in the world has these four characteristics:
Is too hard to understand and use
Costs a great deal of money
Claims to be user-friendly
Doesn't work
The whole purpose of a program is to turn computers into electronic sausage grinders. Stick information in one end and out comes the information on the other end. No matter what kind of program you examine - word processor, spreadsheet, database, or game - all programs manipulate the following:
Numbers
Strings
Numbers can be positive or negative, whole numbers or fractions, or just about any other type of number you can think of (including telephone numbers to hot dates, numbers that form a combination to a safe containing wads of money, and imaginary numbers that no one except mathematicians truly understand).
Strings are characters strung together. A character is anything you can type from the keyboard, including letters, punctuation marks, and (don't get confused now) numbers.
Depending on how the program decides to treat them, numbers can be considered as numbers or as a string. For example, most programs treat your telephone number or street address as a string but treat your age or weight as a number.
A single letter is considered a string. An entire sentence is also a string. You can even consider the first chapter of War and Peace a string. Strings can be any collection of letters, spaces, and numbers grouped together.
** Values and Variables
When you type a number or a string into a program, how does the computer keep track of that number or string? After all, you may know that 555-1234 represents a phone number, but to the computer, the phone number is just another number or string.
To store data, programs use variables, a time-tested concept from algebra. When you write a program, you have to tell the program, "Okay, when someone types 555-1234, give this number a name, such as PhoneNumber, and store the number someplace where you can find it again."
When your program needs to retrieve or manipulate this data, the program says, "Okay, where did I put this information? Oh, that's right, I stored the information in a place (variable) called PhoneNumber." The computer obediently rushes to the PhoneNumber variable and yanks out whatever number or word the computer stored there.
Variables can hold a wide variety of data (which is why they're called variables, a more scientific-sounding name than flaky, wishy-washy, or schizophrenic). The information stored in a variable is called a value because a value represents either a string or a number.
** Using variabtes
Two types of variables exist:
Those you make up
Those already defined as the properties of every object on a form
Every time you draw an object to make your user interface, Visual Basic automatically creates a whole bunch of variables (called properties) set with default values. To look at the values of an object's properties, you have to use the Properties window. (Press F4 or choose View->Properties Window.)
Property values can represent numbers (such as defining the width and height of an object), True or False (such as defining whether an object is visible), or strings (such as captions on a command button). Properties simply define the appearance of an object on the screen.
Variables are names that can represent any type of value. Properties are special names for variables that affect the appearance of an object.
To create a variable on your own, just give the variable a name. After you type a name for a variable, the variable magically springs into existence. You have two ways to create a variable:
Use the Dim statement to declare the variable.
Just name the variable and assign a value to it.
The only place where you can type (and create) a variable name is in the Code window. The only place in the Code window where you can type a variable name is sandwiched between the first and last lines of a procedure.
** Declaring variables:
Creating a variable is as simple as typing a name and assigning a value to the variable, as shown in the following example:
Private Sub Form1-Load( )
PetName "Bo the cat"
Age = 6
End Sub
The preceding event procedure says the following:
1. When a form named Form loads, follow these instructions.
2. First, create a variable called PetName and set the value to the string "Bo the cat".
3. Second, create a variable called Age and set the value to the number 6.
Although creating a variable out of thin air in the middle of a procedure is perfectly acceptable, this is not considered good programming practice. Unless you examine an event procedure line by line, you have no idea how many variables the procedure may be using.
A better programming practice is to declare your variables at the beginning of each event procedure. To declare a variable, use the Dim command, as shown in the following example:
Dim VariableName1, VariableName2
Type as many variable names as you want in the preceding command.
Rewriting the preceding event procedure causes the procedure to look like the following:
Private Sub Form1_Load( )
Dim PetName, Age
PetName = "Bo the cat"
Age = 6
End Sub
You can also declare variables explicitly, as in the following:
Dim PetName as String, Age as Integer
I give you the details on this technique in this chapter's section called "Data types."
Although declaring a variable can add an extra line or two to an event procedure, do you see how easy you can find the names of all the variables used? Instead of examining an event procedure line by line, you can just glance at the first few lines and see a list of all the variable names used.
Using the Dim statement simply helps you, the programmer, understand what the procedure does. As far as the computer is concerned, the computer doesn't care whether you use the Dim statement or not.
Naming variables:
You can name your variables anything you want, and you can store anything you want in them.
However, naming a variable PhoneNumber and then stuffing somebody's address in the variable is pretty foolish.
To make your life easier, give your variables names that represent the data you're going to store in them. For example, naming a variable PhoneNumber makes sense if you're going to store phone numbers in the variable. Likewise, you want a variable named BusinessName to hold only the words that make up business names.
When naming your variables, you must adhere to some unbreakable rules, which follow; otherwise, Visual Basic throws a tantrum. All variables must:
Begin with a letter
Be a maximum of 255 characters in length (with an obvious minimum of one character in length).
Contain only letters, numbers, and the underscore character (_); spaces and punctuation marks are not allowed.
Be any word except a Visual Basic reserved word, such as End or Sub.
If your variable names meet these criteria, all is going to be well. (Of course, that doesn't mean your program is going to work, but at least Visual Basic is going to be happy.) The following are examples of Visual Basic-approved variable names:
Phone
Here_is_Your_Name
Route66
The following are some no-nos for variable names, which Visual Basic refuses to use:
123Surprise (This name begins with a number)
Just Work (This name contains a space)
Sub (This name is a Visual Basic reserved keyword).
* Assigning numbers to variables
Now that you know how to create variables by naming them, how do you assign a value to a variable and get the value back out again? Easy - you use something mysterious called an equal sign (=).
To assign a value to a variable, you have to write a BASIC command, as in the following example:
VariableName = Value
Rather than telling the computer, "Hey, stupid. Assign the number 36 to a variable named Age," you can just write:
Age = 36
Variables can hold only one value at a time. If a variable already holds a value and you assign another one to the variable, the variable cheerfully tosses out the old value and accepts the new one. You can give two commands, as in the following example:
Age = 36
Age = 49
Visual Basic first says, "Okay, let my variable named Age hold the number 36." Then Visual Basic looks at the second line and says, "Okay, let my variable named Age hold the number 49, and forget that the number 36 ever existed."
Because the properties of an object are variables, you can assign values to an object's property in the same way. For example, suppose that you wanted to change the Height property of a text box named txtPassword to 1200. Here's how you do this:
txtPassword.Height = 1200
This tells Visual Basic, "Find the object named txtPassword and change the Height property to 1200"
If you want to be more specific, you can even do this:
frmSecret!txtPassword.Height = 1200
This tells Visual Basic, "On the form named frmSecret, find the object named txt Password and change the Height property to 1200"
If you don't include the form's name, Visual Basic assumes that the object you want is located on the form containing your BASIC code. To retrieve the value from an object's property (such as finding out the height of a text box), assign a variable to that object's property such as:
DimButtonHeight As Integer
ButtonHeight = txtEatThis.Height
1. Create a variable called ButtonHeight as an integer variable.
2. Assign the variable called ButtonHeight to the value stored in the Height property of a text box named txtEatThis.
** Assigning strings to variables
Assigning strings to variables is similar to assigning numbers to variables. The only difference is that you have to surround a string with quotation marks so that Visual Basic knows where the string begins and ends.
For example, you can assign a variable with a single-word string:
Name = "John"
Or you can assign a variable with a string consisting of two or more words:
Name = "John Doe".
Or
Name = "John Smith Doe the Third and proud of it"
Not all strings consist of letters. Sometimes you may want to assign a variable with a phone number or social security number, as follows:
PhoneNumber = "555-1234"
SocialSecurity = "123-45-6789"
What happens if you don't include the quotation marks and just type the following?
PhoneNumber = 155-1234
SocialSecurity = 123-45-6789
Without the quotation marks, Visual Basic thinks the hyphen is a subtraction symbol and that you want the program to calculate a new result. Instead of storing 555-1234 in the PhoneNumber variable, Visual Basic stores the number -679. Instead of storing 123-45-6789 in the Social Security variable, Visual Basic stores -6711.
The golden rule of assigning variables is this: When you assign a variable with letters or numbers that you want treated as a string, put quotation marks around the letters or numbers.
** Modifying properties
Assigning a variable with numbers or strings isn't some dry, academic exercise that has little relation to anything in real life. For example, if you want to display a message on the screen, you need to modify the properties of a label or text box. If you want to create animation, you need to constantly change the Left and Top properties that define an object's position on the screen. Because the properties of an object are variables, you can modify an object by assigning new values to the object's properties.
Suppose that the two command buttons and the text boxes, shown in Figures 15-2 and 15-3, have the following properties:
Object - Text Box
Property - Name
Setting - txtMessage
Property - Text
Setting - (Empty)
Object - Top Command button
Property - Name
Setting - cmdHello
Property - Caption
Setting - &Hello
Object - Bottom command button
Property - Name
Setting - cmdBye
Property - Caption
Setting - &Good-bye
Now, suppose that the first command button has the following event procedure:
Private Sub cmdHello_Click( )
txtMessage.Text = "Hello, world!"
End Sub
And the second command button has the following event procedure:
Private Sub cmdBye_Click( )
txtMessage.Text - "Good-bye, cruel world!"
End Sub
When you click on the Hello command button, this is what happens:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdHello.
2. Visual Basic quickly finds the event procedure named cmdHello-Click( ) and looks for further instructions.
3. The cmdHello-Click( ) event procedure tells Visual Basic, "Find a text box named txt Message and replace the Text property with the string "Hello, world!' "
4. The string "Hello, world!" pops up inside the text box named txtMessage.
When you click on the Good-bye command button, the following process happens:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdBye.
2. Visual Basic quickly finds the event procedure named cmdBye-Click and looks for further instructions.
3. The cmdBye_Click( ) event procedure tells Visual Basic, "Find a text box named txtMessage and replace the Text property with the string "Good-bye, cruel world!" "
4. The string "Good - bye, cruel world! " pops up inside the text box named txtMessage .
You can use BASIC code to modify the properties of any object that appears on a form. By modifying the properties of other objects, you can display messages and information to the user.
The only property that BASIC code cannot change is the Name property of any object. The only way to change the Name property of an object is through the Properties window.
** Assigning variables to other variables
Besides assigning numbers or strings to a variable, you can also assign the value of one variable to another variable. To do this, you have to write a BASIC command like the following:
FirstVariableName = SecondVariableName
For example, consider adding a second text box with the following properties.
Object - Second text box
Property - Name
Setting - txtCopyCat
Property - Text
Setting - (Empty)
Consider the following modifications to the cmdHello_Click event procedure:
Private Sub cmdHello_Click( )
txtMessage.Text = "Hello, world!"
txtCopyCat.Text = txtMessage.Text
End Sub
And to the cmdBye-Click event procedure:
Private Sub cmdBye_Click( )
txtMessage.Text = "Good-bye, cruel world!"
txtCopyCat.Text = txtMessage.Text
End Sub
Now when you click on the Hello command button, this is what happens:
1 . Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdHello.
2. Visual Basic quickly finds the event procedure named cmdHello_Click( ) and looks for further instructions.
3. The cmdHello_Click( ) event procedure tells Visual Basic, "Find a text box named txtMessage and replace the Text property with the string "Hello, world!" "
4. Visual Basic sees the second instruction that says, 'find a text box named txtCopyCat and replace its Text property with whatever is stored in the txtMessage. Text property."
5. The string "Hello, world! " pops up inside the text box named txtMessage and the text box named txtCopyCat.
And when you click on the Good-bye command button, the following happens:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdBye.
2. Visual Basic quickly finds the event procedure named cmdBye_Click( ) and looks for further instructions.
3. The cmdBye_Click( ) event procedure tells Visual Basic, "Find a text box named txtMessage and replace its Text property with the string "Good-bye. cruel world!" "
4. Visual Basic sees the second instruction that says, "Find a text box named txtCopyCat and replace its Text property with whatever is stored in the txtMessage. Text property."
5. The string "Good-bye, cruel world!" pops up inside the text box named txtMessage and the text box named txtCopyCat.
** Assigning Values to Objects Stored in Other Forms
To assign a value into an object's property, use this simple command:
ObjectName.PropertyName = Value
ObjectName is the name of the object. PropertyName is the property you want to change. Value is the number or string you want to assign to the property that is to affect the object named ObjectName.
If you want to change the properties on a form that hasn't been loaded yet, you have to specify the form name such as:
FormName!Ob,jectN,ame.PropertyName = Value
For example, to change the Text property of the txtMessage text box, you have to type the following:
txtMessage.Text = "Hello, world!"
So how can you change the property of an object stored on another form? The solution is easy. You just have to specify the name of the form on which the object is stored.
Figure 15-5, for example, shows two forms. Form 1 contains two text boxes, named txtMessage and txtCopyCat, and two command buttons, named cmdHello and cmdBye. Form 2 contains one text box with the following properties:
Object - Form
Property - Name
Setting - A Second Form
Object - Text Box
Property - Name
Setting - txtNewBox
Property - Text
Setting - (Empty)
So how can the event procedure stored in Form 1 modify the Text property of an object stored on another form? You can simply use the following command:
FormName!ObjectName.PropertyName = Value
FormName specifies the name of the form that contains the object you want to modify. ObjectName is the name of the object. PropertyName is the property you want to change. Value is the number or string you want to assign to the property.
If you want the text box named txtNewBox to display the same message that the text boxes txtMessage and txtCopyCat display, you add the following command to the cmdHello_Click and the cmd Bye_Click event procedures:
Form2!txtNewBox.Text = txtMessage.Text
The cmdHello-Click event procedure now looks like this:
Private Sub cmdHello_Click( )
txtMessage.Text = "Hello, world!"
txtCopyCat.Text = txtMessage.Text
Form2.Show
Form2!txtNewBox.Text = txtMessage.Text
End Sub
And the cmdBye-Click event procedure now looks like this:
Private Sub cmdBye_Click( )
txtMessage.Text = "Good-bye, cruel world!"
txtCopyCat.Text = txtMessage.Text
Form2.Show
Form2!txtNewBox.Text = txtMessage.Text
End Sub
The following explains what happens when you click on the Hello command button:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdHello.
2. Visual Basic quickly finds the event procedure named cmdHello_Click and looks for further instructions.
3. The cmdHello_Click( ) event procedure tells Visual Basic, "Find a text box named txtMessage and replace the Text property with the string "Hello, world!""
4. Visual Basic sees the second instruction that says, "Find a text box named txtCopyCat and replace its Text property with whatever is stored in the txtMessage.Text property."
5. The Form2.Show command tells Visual Basic, "Find a form named Form2 and display the form on the screen."
6. The Form2!txtNewBox.Text = txtMessage.Text command tells Visual Basic, "On the form named Form2, look for a text box named txtNewBox and stuff its Text property with the value stored in the text box named txtMessage."
7. The string "Hello, world! " pops up inside all three text boxes, named txtMessage, txtCopyCat, and txtNewBox.
When you click the mouse on the Good-bye command button, the following happens:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdBye.
2. Visual Basic quickly finds the event procedure named cmdBye_Click and looks for further instructions.
3. The cmdBye_Click event procedure tells Visual Basic, "Find a text box named txtMessage and replace its Text property with the string "Good-bye, cruel world!""
4. Visual Basic sees the second instruction that says, "Find a text box named txtCopyCat and replace its Text property with whatever is stored in the txtMessage.Text property."
5. The Form2.Show command tells Visual Basic, "Find a form named Form2 and display this form on the screen."
6. The Form2! txt NewBox.Text = txtMessage.Text command tells Visual Basic, "On the form named Form2, look for a text box named txtNewBox and stuff the Text property with the value stored in the text boxnamed txtMessage."
7. The string "Good-bye, cruel world! " pops up inside all three text boxes, named txtMessage, txtCopyCat, and txtNewBox.
** Data Types
Variables can hold numbers and strings. However, you may want a variable called FirstName to contain nothing but strings. If a variable called FirstName winds up holding the number 56, that variable can cause an error if the computer expects a string but gets a number instead. To restrict the type of information a variable can hold, you can declare a variable to hold a specific data type. Data types tell Visual Basic, "See this variable? This variable can hold only strings or certain types of numbers, so there!"
Here are three primary reasons to use data types:
So that you can easily see the type of data each variable can hold.
To prevent variables from accidentally storing the wrong type of data and causing an error.
To use memory more efficiently because some data types (such as double) require more memory to use than other data types (such as byte).
If you write a BASIC command that attempts to assign a string into a data type that accepts only numbers, Visual Basic squawks and displays an error message. This squawking helps you catch possible errors in your program long before you finish - and distribute - your program to others.
Visual Basic provides the ten data types shown in Table 15-1.
** TABLE 15-1
The Visual Basic Data Types
Data Type - Byte
Accepts Numbers That Range From - 0 to 255
Data Type - Boolean
Accepts Numbers That Range From - True or False
Data Type - Currency
Accepts Numbers That Range From - -922337203685477.5808 to 922337203685477.5807
Data Type - Date
Accepts Numbers That Range From - Dates between January 1,100 and December 31, 9999
Data Type - Double
Accepts Numbers That Range From - -1.79769313486232E308 to -4.94065645841247E-324 and 4.94065645841247E-324 to 1.79769313486232E308
Data Type - Integar
Accepts Numbers That Range From - -32,768 to 32,767
Data Type - Long
Accepts Numbers That Range From - -2,147,483,648 to 2,147,483,647
Data Type - Numbers
Accepts Numbers That Range From - Same range as Double
Data Type - Single
Accepts Numbers That Range From - -3.402823E38 to -1.401298E-45 and 1.401298E-45 to 3.402823E38
Data Type - String
Accepts Numbers That Range From - 0 to 65,5000 characters
Data Type - Variant
Accepts Numbers That Range From - (When storing numbers, same range as Double. When storing strings, same range as String).
Visual Basic actually gives you two ways to declare a variable:
Dim MyString As String
or
DimMyString$
The first method can be verbose but clear. The second method uses something called type declaration characters. This makes declaring variables easier but makes your commands harder to read and understand at first glance.
If you want to be clear and don't mind typing a lot of extra words like "As String" or "As Integer," use the first method. If you want to save time and don't care to make your code readable, use the second method. Here's a short table listing all the type declaration characters you can use to declare variables as different data types:
Data Type - Currency
Character - @
Example - Dim Loot@
Equivalent To - Dim Loot As Currency
Data Type - Double
Character - #
Example - Dim Average#
Equivalent To - Dim Average As Double
Data Type - Integar
Character - %
Example - Dim Age%
Equivalent To - Dim Age As Integer
Data Type - Long
Character - &
Example - Dim Huge&
Equivalent To - Dim Huge As Long
Data Type - Single
Character - !
Example - Dim Tiny!
Equivalent To - Dim Tiny As Single
Data Type - String
Character - $
Example - Dim Name$
Equivalent To - Dim Name As String
** Declaring objects as data types:
To store whole numbers, use the Integer data type. If you need to store really small or really large numbers, use the Long data type. If you just need to store numbers no smaller than 0 or larger than 255, use the Byte data type.
To store numbers with decimal points, use the Single data type. If you need to store really small or really large numbers with decimal points, use the Double or Numbers data type.
To store numbers representing currency (that's money, in non-technical terms), use the Currency data type.
To store words and letters, use the String data type.
To store dates, use the Date data type.
To store True or False values, use the Boolean data type.
You can also use the Variant data type to store numbers or strings.
By default, Visual Basic assigns all variables as a Variant data type unless you specifically tell Visual Basic otherwise. The only reason to specifically declare a variable as a Variant data type is for clarity in reading your code. As far as Visual Basic is concerned, declaring a variable as a Variant data type is redundant, such as telling people that you drive a Ford Mustang automobile.
To declare a variable as a particular data type, use the following command:
Dim VariableName As DataType
For example, to declare a variable named MyName as a string data type, you want to use the following command:
Dim MyName As String
When Visual Basic sees this statement, Visual Basic thinks, "Okay, this is a variable named MyName, and the programmer defined the variable as a String data type so that this variable can hold only strings."
When Visual Basic sees the statement:
Dim MyName As Variant
Visual Basic thinks, "Okay, this is a variable named MyName, and the programmer defined this variable as a Variant data type so that it can hold numbers or strings."
And when Visual Basic sees the statement:
Dim MyName
Visual Basic thinks, "Okay, this is a variable named MyName; because the programmer is too lame to define the data type, I can automatically assume that this variable is a Variant data type."
If you want to declare multiple variables on a single line, you must explicitly declare each variable, such as:
Dim MyName As String, PetName As String
Suppose you declare your variables like this:
Dim MyName, PetName As String
Visual Basic assumes that the MyName variable is a Variant data type and PetName is a String data type.
An everyday event procedure:
To study an actual, honest-to-goodness event procedure that declares variables as specific data types, consider the following:
Private Sub Form1_Load( )
Dim PetName As String, Age As Integer
PetName = "Bo the cat"
Age = 6
End Sub
The variable declaration statement Dim PetName As String, Age As Integer tells Visual Basic, "Okay, create a variable named PetName and make sure that you use this variable to hold any strings."
Visual Basic continues, "Then create a variable named Age and make sure that the variable holds only numbers greater than or equal to -32,768 but less than or equal to 32,767."
The next command tells Visual Basic, "Assign the string "Bo the cat" to the variable PetName." Faster than a speeding bullet, Visual Basic checks to make sure that the PetName variable really can hold string values. Thankfully, the statement Dim PetName As String defined PetName to hold only string values, so everything is a-okay.
Finally, Visual Basic says, "Assign the number 6 to the variable Age." Quickly, Visual Basic checks that the variable Age really can hold a number as massive as 6. Because 6 falls within the declared range of an integer (between -32,768 and 32,767), Visual Basic cheerfully allows this statement to pass as valid.
Always use the smallest data type possible. For example, if you know the variable Age is never going to hold a number larger than 32,767, declare Age as a Byte data type. If you need larger or smaller numbers, choose the Long data type. By choosing the appropriate data type, you can optimize the use of your computer's memory.
* String data types
In case you want to restrict the length of the strings a variable can hold, you can define the maximum length by using the following command:
Dim VariableName As String * Size
The value of Size can vary from 1 to 65,500. So if you want to keep a variable from storing more than ten characters, use the following command to specify Size as 10:
Dim VariableName As String * 10
For example, suppose that you declare the following:
Dim FirstName As String * 5
The following are valid strings that you can assign to the FirstName variable:
" 12345 "
"Bo"
"Jacob"
"Will"
If you try to assign to the FirstName variable a string that is longer than five characters, however, this is how Visual Basic actually stores a string:
String Assignment What Really Happens
FirstName = Marilyn FirstName = Maril
FirstName = Bobcat FirstName = Bobca
FirstName = King Edwards FirstName = King
If a string is too long for the declared string length of a variable, Visual Basic ruthlessly chops off the string. If you don't define a maximum string length, or if you define the variable as a Variant data type, the variable can hold up to 65,500 characters.
* Scope of variables
The scope of a variable determines the accessibility of a variable within a Visual Basic program. Visual Basic lets you declare the scope of variables in three ways:
Local
Module
Public
A local variable exists only within the procedure in which the variable is created, and a local variable can be used only within the procedure in which the variable is declared. The purpose of a local variable is to isolate a specific variable in the single procedure in which the variable is being used. That way, if a variable is screwing up and storing the wrong value, you can easily isolate the problem and fix it.
To declare a local variable, declare the variable within an event procedure, as follows:
Private Sub Command1_Click( )
Dim FulIName As String
End Sub
Remember, a local variable can be used only in the one event procedure in which the variable is declared. But what if you want to create a variable that two or more event procedures can share? In that case, you have to create a module variable.
Module variables are variables that can be used only by other procedures stored in the same file.
To declare a module variable, follow these steps:
1. Open the Code window by pressing F7, choosing View->Code, or double-clicking anywhere on the form (but not on any objects on the form).
2. Click on the Object list box in the Code window and choose (General).
Visual Basic displays (Declarations) in the Procedure list box.
3. Type your variable declaration using the Dim command.
Module variables are useful for sharing a variable amoung procedures stored in the same file, such as the same Form (FRM) file. If you want to use a variable that can be used by any event procedure, no matter what file the variable may be stored in, you need to create a public variable.
Public variables are variables that can be the most convenient to use, because every event procedure in your Visual Basic program can access them. But be careful! Most programmers avoid using public variables because if your program stores the wrong value in a public variable, you have to search your entire program to find the part that is messing up.
By comparison, if your program messes up a module variable, you can isolate the problem in the file in which you declared the module variable. Likewise, if your program messes up a local variable, the only possible place the problem can occur is in the procedure in which you declared the local variable.
To declare a public variable, follow these steps:
1. Choose Project->Add Module to create a BAS module file. An Add Module dialog box appears.
2. Click on Module and click on Open. Visual Basic displays the Code window with (General) displayed in the Object list box and (Declarations) in the Procedure list box.
3. Declare your public variable by using the Public command. For example:
Public FullName As String
To make sure that programs are easy to understand and modify later, create local, module, or public variables. If you want to throw caution aside, you can create variables as you need them in the middle of your programs, but doing so will make your programs harder to read and understand later.
If you plan to write large programs, always declare your variables. Like eating your vegetables, this is something that may seem distasteful at first but, later on, can actually be useful.
To force yourself to declare your variables, just add the following command in the General area (like a module variable) in every file (Form or Module file) of your program:
Option Explicit
When you use the Option Explicit command, Visual Basic screams every time you try to use a variable without declaring it first.
++ CHAPTER 16
RESPONDING TO THE USER
In This Chapter .....
Getting data from text boxes
Determining which radio button and check box the user chose
Retrieving data from list boxes and scroll bars
A user interface makes your program look nice and pretty. Unfortunately, a nice and pretty user interface can be as useless as an attractive person without any brains (think of some of your co-workers). If you want your program to have more substance than an empty-headed fashion model, you have to make sure that your program can respond intelligently to the user.
To make your user interface responsive, your program must:
Get information from the user interface
Calculate a result
Display that result back on the user interface
For example, when the user chooses an item from a list box, the program has no idea which item the user chose. If you look at the screen, you may be tempted to say, "Hey, stupid computer. If I can see which item the user has chosen, why can't you?"
But what you see on the user interface isn't what the computer sees. From the computer's point of view, the computer still has no idea which item the user selected from the list box.
To tell the computer what action a user took, you have to write BASIC code. This BASIC code grabs information off the user interface so that your program can then do something with the information.
** Getting Data from the User Interface
A user interface is a simple way for users to give information to your program. For example, a user interface can offer a list box that displays several filenames. That way the user can just click on the filename they want to use.
When a user gives information to a program (by pressing a particular key or clicking on a command button, files list box, and so on), Visual Basic stores this information in the object's properties. From this point on, the program can use the information.
The nine basic types of objects that can get data from the user interface are as follows:
Check boxes
Radio buttons
Combo boxes
List boxes
Text boxes
Horizontal and vertical scroll bars
Drive list boxes
Directory list boxes
File list boxes
** Finding Information In an Object
To find the information stored in an object, you need to know:
The name of the object (such as txtWriteHere)
The object's property that contains the information (such as the Text property)
The name of the form on which the object is located (such as frmMain)
When the user puts information in an object (such as typing text in a text box), Visual Basic stores the data in one of the object's properties. Different objects store information from the user in different properties. For example, a text box stores information in the Text property, but a check box stores information in the Value property.
Knowing which property you want isn't enough. You also need the name of the object that holds the property you want. And because objects on different forms can have the same names, you may also need to know the name of the form where the object appears.
The combination of form name, object name, and property defines the specific location of the information from the user. Think of objects as mail boxes. To retrieve a letter, you have to know the state (form name), the city (object name), and the address (property).
So if you want to retrieve information from an object, you have to use the following combination:
FormName!ObjectName.Property
For example, you may want to retrieve information from a form named frmAttack, containing a text box named txtWarning, which stores data in the Text property, such as
FrmAttack!txtWarning.Text
.
Whenever Visual Basic sees this code, Visual Basic automatically says to itself, "Okay, let me find the form named frmAttack, look for the object named txtWarning, and find the value stored in the Text setting. Ah, here is the value I want."
If you omit the form's name, Visual Basic looks only for objects that appear on the current form. If that's where you want to look, you can shorten this combination to the following:
ObjectName.Property
* Getting Data from Text Boxes
When a user types something in a text box, Visual Basic stores the information in the Text property. Typing in a text box is equivalent to assigning a value in the text box's Text property.
If the user types "Greetings from Mars!" in a text box named txtSecret, this is the equivalent to the following BASIC code:
TxtSecret.Text - "Greetings from Mars!"
* Determining which radio button the user chose
A radio button can have one of two possible values stored in its Value property:
True (selected)
False (unselected)
An unselected radio button has the Value property set to False. Clicking on an unselected radio button changes the Value property to True.
A selected radio button has the Value property set to True. Clicking on a selected radio button changes the Value property to False.
To determine which radio button the user chose, you have to check the Value properties of all your radio buttons. For example, if the user chooses an unselected radio button named optStation, choosing the button is equivalent to the following BASIC code:
optStation.Value = True
If the user selects a different radio button (other than the radio button named optStation), this is equivalent to the following BASIC code:
optStation.Value = False
* Determining which check box the user chose
A check box can have one of three possible values stored in the Value property:
0 - unchecked
1 - checked
2 - grayed
An unselected check box has the Value property set to 0. Clicking on an unselected check box changes the Value property to 1.
A selected check box has the Value property set to 1. Clicking on a selected check box changes the Value property to 0.
A grayed check box has the Value property set to 2. Clicking on a grayed check box changes the Value property to 0.
Note that a user can still click on a grayed check box, unlike a disabled check box which also looks gray.
A selected check box obviously means that the user chose the command represented by the check box. Likewise, an unselected check box means the user did not choose the command represented by the check box. A grayed check box can draw the user's attention that the grayed check box's command represents a special exception or feature.
You have only two ways to set a check box's Value property to 2 and have the check box appear grayed. The first approach is to change the Value property in the Property window when you design your program. The second way is to use BASIC code. The following BASIC code sets the Value property to 2 for a check box named chkBold:
chkBold.Value = 2
To determine which check box the user chooses, you have to review the Value properties of all your check boxes. For example, if a check box named chkBold is blank, this is equivalent to the following BASIC code:
chkBold.Value = 0
If the check box is selected, this is equivalent to:
chkBold.Value = 1
If the check box is grayed, this is equivalent to:
chkBold.Value = 2
* Retrieving data from Drive, Directory, and File list boxes.
When a user makes a selection from the Drive list box, Visual Basic stores the selection as a string in the Drive list box's Drive property. If a user clicks on c: in a Drive list box named drvWhichDrive, for example, this is equivalent to the following BASIC code:
drvWhichDrive.Drive = "c:"
When a user makes a selection from the Directory list box, Visual Basic stores the selection as a string in the Directory list box's Path property.
If a user clicks on c:\dos in a Directory list box named dirFolder, for example, this is equivalent to the following BASIC code:
dirFolder.Path = "c:\dos"
When a user makes a selection from a File list box, Visual Basic stores the file name as a string in the File list box's FileName property.
If a user clicks on autoexec.bat in a File list box named filGetFiles, for example, this is equivalent to the following BASIC code:
filGetFiles.FileName = "c:\dos\autoexec.bat"
* Obtaining choices from a combo box
When a user chooses or types an item in a combo box, Visual Basic stores this information in the combo box's Text property.
The text that a combo box displays is stored in the combo box's List property. When the user actually types an item or clicks on an item in the combo box, the user's choice is stored in the combo box's Text property.
If the user types or selects an item named " ASAP " in a combo box named cboPriorities, for example, this is equivalent to the following BASIC code:
cboPriorities.Text = " ASAP "
* Getting values from horizontal and vertical scroll bars
Scroll bars that are not part of a text box, list box, or combo box represent a number. This number is stored in the scroll bar's Value property.
The value that a scroll bar can represent is determined by the scroll bar's Min and Max properties. The lowest possible value is -32,768. The highest possible value is 32,767.
Scroll bars let users visually represent a value, rather than typing a number from the keyboard. Ideally, you want to use a scroll bar along with a label that shows the actual value of the scroll bar. That way, as users move the scroll box in the scroll bar, they can also see the value of the scroll bar changing.
Although scroll bars can represent a range of values, your program eventually needs to read a single value from the scroll bar. If you want to retrieve the value of a horizontal scroll bar named hsbSensitivity, for example, you have to set a variable to equal the Value property of the scroll bar. So if you have a variable named MouseSensitivity and a scroll bar named hsbSensitivity, you could use the following command:
MouseSensitivity = hsbSensitivity.Value
* Test your newfound knowledge
1. What does the following BASIC command do?
WhatIsIt = chkBold.Value
a. The command tried to identify UFOs named chkBold.Value
b. The command questions the need for anything named chkBold.Value
c. The command makes the computer ask, "What do you want me to do? Tell me and then leave me alone. I'm feeling bold today."
d. The command yanks a number that's stored in the Value property of a check box named chkBold and stuffs the number in a variable named WhatIsIt. If the check box named chkBold is selected, the value of WhatIsIt is 1.
2. Examine the following command and explain what this code does.
FrmDataSheet!txtMessage.Text = "Warning!"
a. The command tells Visual Basic, "Okay, look for a form named frmDataSheet, and on this form find a text box named txtMessage. When you find that, stuff the Test property with the string "Warning!""
b. The command tells everyone that the person who wrote this command probably had a good reason for not being present to help you interpret this command.
c. The command warns you that your computer is about to explode and you had better take cover immediately.
* Retrieving data from a list box
A user can select one or more items from a list box depending on the list box's MultiSelect property. If the MultiSelect property is set to 0 (the default value) and a user selects an item, the list box stores the item in the Text property.
If the user selects an item named " Cat food " in a list box named 1st Groceries, this is equivalent to the following BASIC code:
1stGroceries.Text = "Cat food"
If a list box's MultiSelect property is 1 or 2, users can hold down the Shift key and click on an item to select two or more items in the list box. Because a list box's Text property can hold only one string at a time, you can't store all the selections made by a user in the list box's Text property.
To retrieve data from a list box that allows multiple selections, you have to do the following:
1. Create a second list box to temporarily store any selected items from the first list box.
2. Set this second list box's Visible property to False so that the list box isn't visible on the user interface.
3. Every time a user selects an Item from the first list box, copy the item into the second (invisible) list box.
The invisible list box contains only those items selected from another list box, and stores all items in a List property. Items in a list are assigned an index number. The first item in the list is given an index number of 0, the second item in a list is given an index number of 1, and so on. To better understand this concept, create on a blank form two list boxes and a command button with the following properties:
If you don't feel like typing in the following program, just load the LISTBOXES.VBP program off the CD-ROM.
Object - First List Box
Property - Name
Value - 1stChoose
Property - MultiSelect
Value - 1 - Simple
Object - Second List box
Property - Name
Value - 1stTemp
Property - Visible
Value - True
Object - Command button
Property - Name
Value - cmdStore
Property - Caption
Value - Store
If the Visible property of the second list box is False, the box is (obviously) invisible and you can't see what is happening. So, when you test the procedure, set the Visible property for the second list box to True so that you can see what happens. Then, after you've become educated, change the Visible property back to the original False setting.
The following procedure stores items in the 1stChoose list box:
Private Sub Form_Load( )
1stChoose.AddItem "Butter"
1stChoose.AddItem "Lard"
1stChoose.AddItem "Cat food"
1stChoose.AddItem "Milk"
1stChoose.AddItem "Ammunition"
1stChoose.AddItem "Ice Cream"
1stChoose.AddItem "Cookies"
1stChoose.AddItem "Sugar"
1stChoose.AddItem "Camouflage"
1stChoose.AddItem "Survival rations"
End Sub
This loads the 1stChoose list box with items for the user to select.
An event procedure to retireve multiple selected items from the 1stChoose list box may look like the following:
Private Sub cmdStore_Click( )
Dim I
1stTemp.Clear
For I = 0 to 1stChoose.ListCount - 1
If 1stChoose.Selected(I) Then
1stTemp.AddItem 1stChoose.List(I)
End If
Next I
End Sub
This is how Visual Basic responds to the code:
1. The first line tells Visual Basic, "Follow these instructions whenever the user clicks on the command button named cmdStore."
2. The second line says, "Declare a variable called I and assume the data type is Variant."
3. The third line says, "Find a list box named 1stTemp and clear out anything the box may be storing."
4. The fourth line says, "Set the value of I to 0. Continue to count by one until the value of I equals the number of items displayed in the 1stChoose list box."
5. The fifth line says, "If the user selected an item displayed in the 1stChoose list box, follow the instructions in the sixth line."
6. The sixth line says, "Copy the item selected in the list box named 1stChoose and put the copied item in the list box named 1stTemp."
7. The seventh line says, "This is the end of all the instructions to follow for now."
8. The eighth line says, "Add one to the value of I and start back at line five."
9. The ninth line says, "This is the end of the instructions to follow when the user clicks on the command button named cmdStore."
These instructions do nothing more than copy all selected items from one list box into a second list box, which is invisible.
Each item in a list box (and a combo box, for that matter) is identified by an index number. The top item of the list is assigned an index number of 0, the second item from the top is assigned an index number of 1, and so on.
To get at the items stored in the list box named 1stTemp, you have to use index numbers. The following command retrieves the top item stored in the 1stTemp list box:
1stTemp.List(0)
The following command retrieves the second item from the top in the 1stTemp list box:
1stTemp.List(1)
Each increase in the index number retrieves the next item further from the top.
++ CHAPTER 17
MATH 101: ARITHMETIC, LOGICAL AND COMPARISON OPERATORS
In This Chapter:
Adding, subtracting, multiplying, and dividing numbers
Using the not, And, Or, and Xor operators
Comparing numbers and strings
Comparing strings and operators
After a program gets data from the user (either as a number or as a string), the next step is to do something with that data. If your program mimics an overworked clerical worker, the program may just lose the data and blame the loss on something else. But most likely, you want your program to calculate some sort of result with the data the program receives from the user.
To calculate a result, your program needs to get data from the user interface and then somehow change, modify, mutilate, or spindle that data. Changing anything involves an operation, so the special commands to work with data are called operators.
Visual Basic provides the following three types of operators:
Arithmetic
Logical
Comparison
** Arithmetic Operators
Arithmetic operators essentially turn your $2,000 computer into a $4.95 pocket calculator. These operators let you add, subtract, multiply, and divide numbers or variables that represent numbers. Table 17-1 shows the most common arithmetic operators.
||
TABLE 17-1
Arithmetic Operators
Operator - +
What the Operator Does - Adds two numbers
Operator - -
What the Operator Does - Subtracts two numbers
Operator - *
What the Operator Does - Multiplies two numbers
Operator - /
What the Operator Does - Divides two numbers and returns a floating-point (decimal) number, such as 3.14, 16.2, or 392.2398
Operator - \
What the Operator Does - Divides two numbers and returns an integer, such as 8, 16, 302, or 25
Operator - Mod (or modulo)
What the Operator Does - Divides two numbers and returns only the remainder
Operator - ^
What the Operator Does - Raises a number to an exponential power
Operator - &
What the Operator Does - Adds (concatenates) two strings
* Adding two numbers with the + operator
To add two numbers, use the addition operator (+), as shown in the following example:
X = 10
Y = 15.4
Sum = X + Y
In case these three BASIC commands mystify you, here's how they work:
1. The first command says, "Create a variable called X and set the value of X to 10."
2. The second command says, "Create a variable called Y and set the value of Y to 15.4."
3. The third command says, "Create a variable called Sum and set the value of Sum equal to the value of X plus the value of Y." In this case, the value of Sum equals 10 + 15.4, or 25.4.
* Subtracting two numbers with the - operator
To subtract two numbers, use the subtraction operator (-), as shown in the following example:
Income = 2000
Taxes = 2500
Real_Income = Income - Taxes
Here's how Visual Basic interprets these three BASIC commands:
1. The first command says, "Create a variable called Income and set the value of Income to 2000".
2. The second command says, "Create a variable called Taxes and set the value of Taxes to 2500."
3. The third command says, "Create a variable called Real_Income and set the value of Real_Income equal to the value of Income minus the value of Taxes." In this case, the value of Rea1_Income equals 2000 - 2500, or -500.
* Negating numbers with the - operator
The subtraction operator ( - ), used by itself, can turn a positive number into a negative number and vice versa. To negate a number, place the -operator in front of any number or variable, as shown in the following example:
Amount = 250
Balance = - Amount
This is how the tiny little brain of Visual Basic interprets these BASIC commands:
1. The first command says, "Create a variable called Amount and set the value of Amount to 250."
2. The second command says, "Create a variable called Balance and set the value of Balance to the negative value of Amount." In this case, the value of Balance is -250.
* Multiplying two numbers with the * operator
To multiply two numbers, use the multiplication operator (*), as shown in the following example:
Hours = 25
Wages = 5.75
Salary = Hours * Wages
To see how Visual Basic understands these three BASIC commands, this is how they work:
1. The first command says, "Create a variable called Hours and set the value of Hours to 25."
2. The second command says, "Create a variable called Wages and set the value of Wages to 5.75."
3. The third command says, "Create a variable called Salary and set the value of Salary equal to the value of Hours multiplied by the value of Wages." In this case, the value of Salary equals 25 * 5.75, or 143.75.
* Dividing two numbers with the /operator
To divide two numbers and calculate a floating-point (decimal) number, use the forward-slash division operator ( / ), as shown in the following example:
GamesWon = 104
TotalGames = 162
WinningPercentage = GamesWon / Total Games
Visual Basic interprets these three BASIC commands as follows:
1. The first command says, "Create a variable called GamesWon and set the value of GamesWon to 104."
2. The second command says, "Create a variable called TotalGames and set the value of Total Games to 162."
3. The third command says, "Create a variable called WinningPercentage and set the value of WinningPercentage equal to the value of GamesWon divided by the value of TotalGames." In this case, the value of WinningPercentage equals 104 / 162, or 0.6419753.
* Dividing two numbers with the \ operator
To divide two numbers and calculate an integer, use the backslash division operator ( \ ), as shown in the following example:
CrateCapacity = 72
Bottles_in_Crate = 1900
FullCrates = Bottles_in_Crate \ CrateCapacity
So how does Visual Basic interpret these three BASIC commands? Glad you asked. Here's how:
1. The first command says "Create a variable called CrateCapacity and set the value of CrateCapacity to 72."
2. The second command says, "Create a variable called Bottles_in_Crate and set the value of Bottles_in_Crate to 1900."
3. The third command says, "Create a variable called FullCrates and set the value of FullCrates equal to the value of Bottles_in_Crate divided by the value of CrateCapacity." In this case, the value of FullCrates equals 1900 \ 72, or 26.
Dividing two numbers often calculates a floating-point (decimal) number, so how does Visual Basic handle rounding? Consider the following example:
Operand1 = 2.5
Operand2 = 1.5
Result = Operand1 \ Operand2
Before Visual Basic performs a calculation using the \ operator, the operands are rounded to the nearest whole number. (If an operand is halfway between two whole numbers, such as 2.5 or 1.5, then the operand is rounded up.) In this example, 0perand1 is rounded up to 3 and 0perand2 is rounded up to 2; therefore, Result = 3 \ 2, or 1.5. Because the \ operator must return an integer, the value of Result is rounded down to 1.
* Dividing with the modulo (Mod) operator
To divide two numbers and calculate the remainder, use the modulo operator ( Mod ), as shown in the following example:
CrateCapacity = 72
Bottles_in_Crate = 1900
LooseBottles = Bottles-in-Crate Mod CrateCapacity
For those curiosity seekers, this is how Visual Basic interprets these commands:
1. The first command says, "Create a variable called CrateCapacity and set the value of CrateCapacity to 72."
2. The second command says, "Create a variable called Bottles_in_Crate and set the value of Bottles_in_Crate to 1900."
3. The third command says, "Create a variable called LooseBottles and set the value of LooseBottles equal to the remainder of the value of Bottles_in_Crate divided by the value of CrateCapacity." In this case, the value of LooseBottles equals 1900 Mod 72, or 28.
* Calculating an exponential with the ^ operator
An exponential is a fancy mathematical term that means to multiply the same number by itself a certain number of times. For example, multiplying the number 2 four times is represented by 2 and small 4, or 2 * 2 * 2 * 2.
Because you can't type 2 and small 4, and typing 2 * 2 * 2 * 2 is a bit cumbersome, Visual Basic provides the caret operator (^), as shown in the following example:
2 ^ 4
* Adding (concatenating) two strings with the & operator
Adding or concatenating two strings means smashing them together. For this operation, use the ampersand operator ( & ), as shown in the following example:
FirstName = "John "
LastName = "Doe"
FullName = FirstName & LastName
When concatenating strings, always make room for a space between the two strings. Otherwise Visual Basic just slams the two strings together as one word like "JohnDoe."
This is how Visual Basic follows these three BASIC commands:
1. The first command says, "Create a variable called FirstName and set the value of FirstName to " John " (note the space at the end)."
2. The second command says, "Create a variable called LastName and set the value of LastName to " Do "."
3. The third command says, "Create a variable called FullName and set the value of FullName equal to the value of FirstName and the value of LastName smashed together. " In this case, the value of FullName equals "John" & "Doe", or "John Doe".
Besides concatenating strings using the ampersand character (&), you can also use the plus sign (+). However, you should use the ampersand character instead because the plus sign is also used with numerical addition. Using the ampersand simply makes your code easier to read.
** Logical Operators
Logical operators manipulate True and False values. Visual Basic represents a value of True as -1 and a value of False as 0. Table 17-2 shows the most common logical operators.
||
Table 17-2
Logical Operators
Operator - And
How to Use - Variable1 and Variable2
Operator - Or
How to Use - Variable1 Or Variable2
Operator - Xor
How to Use - Variable1 Xor Variable2
Operator - Not
How to Use - Not Variable
* Using the Not operator
It's a sad commentary on the negative impact that television has on children when their vocabulary degenerates to the monosyllabic utterance, "Not!"
Of course, the computer world laid claim to Not long before MTV materialized. The Not operator simply changes a True value to False and a False value to True, as in the following example:
Variable Name - Another_Computer_Book
Value - True
Variable Name - Not Another_Computer_Book
Value - False
For clarity, cool programmers like to use parentheses. If you use parentheses in the preceeding example, it would look like this:
Not(Another_Computer_Book)
* Using the And operator
The And operator compares the True or False values of two variables and calculates a new True or False value. This allows your program to mae decisions, as the following example illustrates:
KicktheCat = CatPresent And CatMisbehaving
So when is the variable KicktheCat True or False? This depends on the True or False value of CatPresent and CatMisbehaving.
KicktheCat CatPresent CatMisbehaving
True True True
False False False
False True False
False False True
The And operator returns a True value only if both CatPresent and CatMisbehaving are True.
* Using the Or operator
Like the And operator, the Or operator compares the True or False values of two variables and calculates a new True or False value. This allows your program to make decisions, as the following example illustrates:
LoafInside = GameOnTV Or WeatherBad
So when is the variable LoafInside True or False? This depends on the True or False values of GameOnTV and WeatherBad.
LoafInside GameOnTV WeatherBad
True True True
True False True
False True True
False False False
The Or operator returns a False value only if both GameOnTV and WeatherBad are False.
* Using the Xor operator
As the the And or Or operators, the Xor operator compares the True of False values to two variables and calculates a new True or False value. This allows your program to make decisions, as the following example illustrates:
TellOffBoss = BossPresent Xor AtWork
So when is the variable TellOffBoss True or False? This depends on the True or False value of BossPresent and AtWork.
TellOffBoss BossPresent AtWork
True True False
True False True
False True True
False False False
The Xor operator returns a False value if both BossPresent and AtWork are True or if both are False.
** Comparison Operators
Comparison operators compare two numbers or strings to see whether the numbers or strings are equal to, not equal to, greater than, or less than one another. Table 17-3 shows the most common arithmetic operators.
||
Table 17-3
Comparison Operators
Operator - <
Meaning - Less than
Operator - <=
Meaning - Less than or equal to
Operator - >
Meaning - Greater than
Operator - >=
Meaning - Greater than or equal to
Operator - =
Meaning - Equal to
Operator - < >
Meaning - Not equal to
* Comparing numbers and strings
As the following example illustrates, comparison operators compare the values of numbers and strings in order to return a value of True or False:
Age = 18
MinimumAge 21
Pass = (Age >= MinimumAge)
This is how Visual Basic interprets these three BASIC commands:
1. The first command says, "Create a variable called Age and set the value of Age to 18."
2. The second command says, "Create a variable called MinimumAge and set the value of MinimumAge to 2l."
3. The third command says, "Compare the value of Age and see whether the value is greater than or equal to the value of MinimumAge. If the value of Age is greater than or equal to the value of MinimumAge, create a variable called Pass and set the value of Pass to True. If the value of Age is not greater than or equal to the value of MinimumAge, create a variable called Pass and set the value of Pass to False."
Comparing numbers is fairly easy, but comparing strings is a bit trickier. When comparing strings, Visual Basic calculates the ANSI character code value of each letter.
* ANSI character codes
At the simplest level, computers understand only two numbers: zero and one. You can represent all numbers with zeros and ones, such numbers are called binary numbers.
Since computers only understand numbers, not letters, humans created a simple system where certain numbers represent certain letters, punctuation marks, and characters. So the number 97 represents the letter a, the number 65 represents the letter A, and the number 33 represents an exclamation mark (!).
To make sure that all computers use the same numbers to represent the same letters and punctuation marks, the American National Standards Institute (ANSI) defined an ANSI Character Set that specifies which number represents which letter or punctuation mark.
* Comparing strings with the = and <> operators
Two strings are equal only if they are absolutely identical. As you can see in the following example, the equal to operator ( = ) always calculates a False value unless the operator compares two identical strings, such as " a " = " a ":
Operation - " a" = " a "
Value of Operation - True
Operation - " a " = " A "
Value of Operation - False
Operation - " a " = " aa "
Value of Operation - False
In the next example, however, you see that the not equal to operator ( <>) always calculates a True value unless this operator compares two identical strings, such as " Abott " <> " Abott ":
Operation - " A " <> " a "
Value of Operation - True
Operation - "Abott" <> "Abott"
Value of Operation - False
Visual Basic always treats uppercase and lowercase letters as completely different entitles when comparing strings.
* Comparing strings with the >, >=, <, and <= operators
When comparing strings, Visual Basic calculates the ANSI character code for each letter in each string, beginning with the first letter. The string whose character has the higher ANSI character code is considered greater.
For example, the letter A has an ANSI character code of 65 and the letter a has an ANSI character code of 97. So consider the following line:
Flag = ("Air" < "aardvark")
Because the first letter in " Air " has a lower character code number than the first letter in "aardvark", Visual Basic considers the value of "Air" to be less than "aardvark", so the value of Flag is going to be True.
Now consider the following example:
Flag = ("air" < "aardvark")
Here the value of Flag is False. How does Visual Basic decide whether "air" is less than "aardvark"? First, Visual Basic calculates the ANSI character code for the first letter of each string. Because both begin with a, Visual Basic looks at the second letter. Because i has a higher ANSI character code than a, "air " is considered greater than "aardvark" and Flag is therefore False.
Consider one final example:
Flag = ("air" < "airplane")
In this example, the value of Flag is True. The first three letters of each string are identical, but the fourth letter is not. Because "air" doesn't have a fourth letter and "airplane" does, "airplane" is considered greater and Flag is therefore True.
* Test your newfound knowledge
1. What is the difference between the / operator and the \ operator?
a. One is called a forward slash and one is called a backslash. Other than that, they both look like typos.
b. The / operator divides two numbers and the \ operator puts them back together again.
c. The / operator calculates a floating-point (decimal) number, such as 3.54, and the \ operator calculates an integer, such as 5 or 34.
d. The / operator doesn't work at all, so you have to use the \ operator instead.
2. Is the following statement True or False?
"aeroplane" < "airplane"
a. False, because I don't know what to think; besides, the answer hasn't been the letter (a) for a long time.
b. True, because the second letter in aeroplane is less than the second letter in airplane.
c.. True and False, because I'm hedging my bets.
d. False, because an aeroplane is an oldfashioned way of saying airplane, so both strings are exactly the same.
**Precedence
With all these operators crowding your BASIC commands, what happens if you lump them all together on one line, like this:
Mess = 4 / 7 + 9 * 2
If you guessed that the value of Mess is 18.57143, congratulations! But how does Visual Basic handle this? First, Visual Basic calculates those operators that have higher priority, or precedence.
Not all operators are equal. Some have a higher precedence than others, which means that they demand attention first, just like crying babies. Table 17-4 lists the order in which Visual Basic pays attention to the various operators. The higher an operator appears in Table 17-4, the higher that operator's precedence, so the exponential operator (^) has higher precedence than the less than operator ( < ).
||
Table 17-4
Precedence of Operators
Operator - Exponential ( ^ )
Type of Operator - Arithmetic
Operator - Negation ( - )
Type of Operator - Arithmetic
Operator - Multiplication and Division ( * and / )
Type of Operator - Arithmetic
Operator - Integer division ( \ )
Type of Operator - Arithmetic
Operator - Modulo ( mod )
Type of Operator - Arithmetic
Operator - Addition and Subtraction ( + and - )
Type of Operator - Arithmetic
Operator - String concatenation ( & )
Type of Operator - Arithmetic
Operator - Equality ( = )
Type of Operator - Comparison
Operator - Inequality ( <> )
Type of Operator - Comparison
Operator - Less than ( < )
Type of Operator - Comparison
Operator - Greater than ( > )
Type of Operator - Comparison
Operator - Less than or equal to ( <= )
Type of Operator - Comparison
Operator - Greater than or equal to ( >= )
Type of Operator - Comparison
Operator - Like
Type of Operator - Comparison
Operator - Is
Type of Operator - Comparison
Operator - Not
Type of Operator - Logical
Operator - And
Type of Operator - Logical
Operator - Or
Type of Operator - Logical
Operator - Xor
Type of Operator - Logical
Operator - Eqv
Type of Operator - Logical
Operator - Imp
Type of Operator - Logical
How does Visual Basic calculate the value of Mess in the following equation?
Mess = 4 / 7 + 9 * 2
To help you understand how Visual Basic calculates a result, these are the steps Visual Basic follows:
1. Multiplication and division have a higher precedence than addition, so Visual Basic looks at the multiplication and division operators first.
2. Because multiplication and division have the same precedence, Visual Basic starts with the one furthest to the left. So Visual Basic calculates the value of 4 / 7 and comes up with 0.57143. Now the equation has been simplified to:
Mess = 0.57143 + 9 * 2
3. Visual Basic sees that the multiplication operator has a higher precedence than the addition operator, so Visual Basic calculates the value of 9 * 2 and comes up with 18. The equation is now:
Mess = 0.57143 + 18
The final value of Mess is 18.57143.
What if you really wanted Visual Basic to add the two numbers first before doing any division or multiplication? For clarity, and to make sure that calculations come out the way you intend, enclose particular operations in your equations in parentheses,
as shown in the following example:
Mess = 4 / (7 + 9) * 2
This is how Visual Basic calculates the result:
1. The parentheses tell Visual Basic to add 7 + 9 first, which creates the following equation:
Mess = 4 / 16 * 2
2. Because the division and multiplication operators have the same precedence, Visual Basic begins with the leftmost operator. Visual Basic calculates 4 / 16, and comes up with 0.25. The equation is now:
Mess = 0.25 * 2
3. Finally, Visual Basic multiplies these numbers and assigns the value of 0.5 to the variable Mess.
Whenever you use two or more operators, use parentheses to provide clarity and to ensure that Visual Basic calculates everything in the exact order you want.
To help you better understand precedence, the enclosed CD-ROM provides a program that shows you how precedence works when using parentheses and when omitting parentheses while calculating a numeric result.
++ CHAPTER 18
STRINGS AND THINGS
In This Chapter:
Converting the case of strings
Using parts of strings to search for and replace other strings
Converting strings into numbers and numbers into strings
Converting strings into ASCII values
In addition to manipulating numbers, your program can manipulate strings as well. Strings are any combination of letters, numbers, or symbols that you want the program to treat literally.
For example, computers blindly interpret phone numbers and social security numbers as mathematical expressions. A typical computer interprets the phone number 123-4567 as the expression, "Subtract 4567 from the number 123."
To tell your program to treat strings literally, always surround your strings with quotation marks, "like this". So if you want to assign the string 123-4567 to a variable, you use quotation marks, as in the following example:
Private Sub Count( )
Phone = "123-4567"
End Sub
If you forget to add the quotation marks, Visual Basic stupidly tries to interpret the string of numbers as an actual command. In this case, Visual Basic would try to subtract 4567 from 123.
After you designate particular data as a string, Visual Basic provides all sorts of weird ways to examine, manipulate, and mutilate the string.
** Manipulating Strings
You are not limited to using strings exactly as they originally appear. You can modify them in many ways. You can convert the case of a string, use parts of a string to look for and replace other strings, and shorten a string by removing extra spaces.
* Counting the length of a string
A string is only as long as the number of characters (including spaces) that the string contains. To count the length of a string, use the following BASIC command:
VariableName = Len("String")
For example:
Private Sub Command1_Click( )
Dim Name As String
Dim NameLength As Integer
Name = "Bo the cat"
NameLength = Len(Name)
End Sub
In this case, the length of the string "Bo the cat" is 10 (eight letters and two spaces), so this value is assigned to the variable called Name Length.
* Converting from uppercase to lowercase
To convert a string to all lowercase letters, use the following BASIC command:
Lcase ("String")
For example:
Private Sub Form_Click( )
Dim Name As String, LowerCase As String
Name = "DOESN'T THIS LOOK OBNOXIOUS?"
LowerCase = LCase(Name)
End Sub
In this case, the value of Lower Case is the following string:
doesn't this look obnoxious?
Notice that the LCase command affects only letters. (How do you present a lowercase question mark, anyway?)
To convert a string to all uppercase letters, use the following BASIC command:
Ucase("String")
For example:
Private Sub Form_Click( )
Dim Name As String, UpperCase As String
Name = "whisper when you speak"
UpperCase = UCase(Name)
End Sub
In this case, the value of UpperCase is the following string:
WHISPER WHEN YOU SPEAK
* Reversing strings
The newest string manipulation command in Visual Basic reverses strings, which can be useful for playing with palindromes (phrases that appear the same written forward as they do backward). To reverse a string, use the following BASIC command:
StrReverse("String")
For example:
Private Sub Form_Click( )
Dim Phrase As String
Phrase = "Madam, I'm Adam"
Phrase = StrReverse(Phrase)
End Sub
In this case, the value of Phrase is the following string:
madA m'I ,madaM
* Extracting characters from a string
Sometimes a string contains more information than you want. For example, you may have stored a person's full name in a variable called FullName, as in the following:
FullName = "John Doe"
To extract characters starting from the left of the string, use the following BASIC command:
Left(FullName, N)
The preceding command says, "See that string over there called FullName? Yank out N number of characters, starting from the left." For example:
Private Sub Form_CIick( )
Dim FulIName As String, First As String
FulIName = "John Doe"
First = Left(FullName, 4)
End Sub
In the preceding example, the value of First is John.
To extract characters starting from the right of the string, use the following BASIC command:
Right(FullName, N)
This command says, "See that string over there called FullName? Yank out N number of characters, starting from the right."
For example:
Private Sub Form_Click( )
Dim FulIName As String, Last As String
FulIName = "John Doe"
Last = Right(FulIName, 3)
End Sub
In this example, the value of Last is Doe.
Another command for extracting characters from a string is the Mid command, such as:
Mid(FulIName, X, Y)
This command says, "See that string over there called FullName? Count X number of characters from the left, and rip out the next Y number of characters. For example,
Private Sub Form_Click( )
Dim FulIName As String, Middle As String
FulIName = "John Q. Doe"
Middle = Mid(FulIName, 6, 2)
End Sub
In this example, the value of Middle is Q. (including the period).
* Finding part of a string with another string
If one string is buried in the middle of another string, you can find the location of the burial string by using the following BASIC command:
InStr("TargetString", "WantedString")
This command returns a number defining the exact location from the left where the "WantedString" begins inside the "Target String". For example:
Private Sub Form_Click( )
Dim FulIName As String
Dim Location As Integer
FulIName = "John Plain Doe"
Location = InStr(FulIName, "Plain")
End Sub
In this case, the value of Location is 6.
If the string you want isn't located inside the string you're searching for, the InStr command returns 0.
When you search for a string within another string, you have to search for the exact uppercase or lowercase string. For example, the following command returns a value of 0:
InStr("John Plain Doe", "PLAIN")
In this case, "Plain" is not the same string as "PLAIN", so InStr returns 0. Essentially, a zero is the Visual Basic way of saying, "Sorry, I can't find your exact string anywhere."
* Replacing part of a string with another string
In case you get the creative urge to write your own word processor in Visual Basic (complete with search and replace features), you can do so with the following BASIC command:
Mid("TargetString", Position) = "NewString"
This command says, "See that string called Target String? Find the value defined by Position, count that number of characters from the left, and insert the string called NewString."
Of course, you have to be careful when inserting a new string into an existing one. For example, consider the following code:
FullName = "John Plain Doe"
Mid(FulIName, 6) = "Vanilla"
Here's how Visual Basic interprets this code:
First, Visual Basic assigns the string "John Plain Doe" to the variable called FullName.
Next, Visual Basic looks at the string "John Plain Doe", finds the sixth character from the left, and inserts the string "Vanilla" replacing the original string beginning with the sixth character. So the following is what happens:
John Plain Doe (Original string)
^ (Sixth character from the left)
John Vanillaoe (New string)
After you tell Visual Basic to replace part of a string with another one, Visual Basic gets overzealous and wipes out anything that gets in the way of the new string.
* Trimming spaces from strings
Strings aren't always nice and neat. Sometimes spaces lie in front of or behind the string, as the following examples illustrate:
" This is an example of leading spaces"
"This is an example of trailing spaces "
To strip away leading spaces, use the following BASIC command:
LTrim("TargetString")
For example:
Private Sub Form_Click( )
Dim FullName As String
FullName = " John Doe"
FullName = LTrim(FullName)
End Sub
The value of FullName is "John Doe" with the leading spaces removed.
To strip away trailing spaces, use the following BASIC command:
RTrim("TargetString")
For example:
Private Sub Form_Click( )
Dim FullName As String
FullName = "John "
FullName = RTrim(FullName)
FullName = FullName & " " & "Doe"
End Sub
In the preceding example, the RTrim command removes the trailing spaces so the value of FullName is just plain "John". Then the last command adds the value of FullName("John") to a blank space and the string "Doe" to create the string "John Doe".
In case you have both leading and trailing spaces, you can combine the two commands like this:
LTrim(RTrim("TargetString"))
This command says, "First, remove all trailing spaces and then remove all leading spaces." For an even simpler method, use the following BASIC command instead:
Trim("TargetString")
For example:
Private Sub Form_Click( )
Dim FulIName As String
FulIName = " John Dull "
FulIName = Trim(Ful]Name)
FulIName = FulIName & " " & "Doe"
End Sub
The Trim command removes both the leading and trailing spaces in one quick stroke, then the last command adds the string "John Dull" to a blank string (" ") and the string "Doe" to create "John Dull Doe".
** Converting Strings and Values
Visual Basic handles numbers and strings differently. There may come a time, however, when you need to convert a string into a number so that you can use the string for calculations. Or you may need to convert a number into a string so that you can manipulate the string. You also may need to convert a string into the equivalent ASCII or ANSI value.
* Converting a string into a number
What if you have a text box in which users can type their hourly wages? Unfortunately, the Text property of any text box stores data as a string, not as a number. To convert this string into a number, you have to use one of the following BASIC commands:
CDbl("TargetString")
CSng("TargetString")
The first command says, "Take the string called Target String and convert TargetString to a Double data type."
The second command says, "Take the string called TargetString and convert TargetString to a Single data type.
For example:
Private Sub Form_Click( )
Dim GetNumber As Double
GetNumber = CDbl(txtHourlyWage.Text)
End Sub
The following is how Visual Basic interprets this code:
1. The first statement says, "Declare a variable called GetNumber as a Double data type."
2. The second statement says, "Get the string stored in the Text property of a text box called txtHourlyWage and convert the string to a number that's a Double data type."
3. Finally, the value stored in the txtHourlyWage. Text property is assigned to the GetNumber variable.
If the user types 6.25 in the txtHourlyWage text box, the value of GetNumber is 6.25.
If the user types 6.25 Hourly wage or My hourly wage is 6.25 in the txtHourlyWage text box, Visual Basic chokes and screams about a type mismatch error because neither CDbl nor CSng knows how to handle characters.
* Converting a number into a string
What if you have a number and need to convert it into a string so that you can do fancy string manipulations to the number? Then you have to use the following BASIC command:
CStr(Number)
This command says, "Take the number represented by Number and turn this number into a string."
For example, Visual Basic considers these to be two completely different creatures:
10 ' This is a number
"10" ' This is a string
The following converts a number into a string:
CStr(10) ' The string " 10"
CStr(10.5) ' The string " 10.5"
CStr( - 10) ' The string "-10"
When Visual Basic converts a number into a string, the string has an extra leading space if the string is a positive number or a minus sign (-) if the string is a negative number.
* Converting a string into an equivalent ASCII value
As a programmer, you have to practically memorize the ASCII table at some point, so you may as well find a copy of one and hang it near your computer somewhere so that you can find this table easily.
An ASCII table shows the codes that computers use to represent most of the characters you need. For example, the letter A has an ASCII value of 65, and the letter a has an ASCII value of 97.
Whenever you need the ASCII value of a one-character string, you can use this BASIC command:
Asc("Character")
The following shows how to convert a character into its ASCII value:
X = Asc ("A") ' X = 65
X = Asc ("a") ' X = 97
* Converting an ANSI value into a string
Microsoft Windows doesn't use the ASCII table. Instead, Windows uses the ANSI table, which is practically the same as the ASCII table anyway. (You can read about the ANSI table in Chapter 17)
To use an ANSI value, use the following BASIC command:
Chr("Character")
The only time you need to use the ANSI value of anything is for special control codes, such as for line feeds, carriage returns, and new lines.
The following commands shows common ANSI values:
LineFeed = Chr(I0)
FormFeed = Chr(12)
Carriage = Chr (13)
By using all these fancy string-manipulation commands, you can make sure that your strings look exactly the way you want them to before displaying them in a text box or label. Either that, or you can just have fun playing with words and numbers and pretend you're doing serious research on your job.
To show you how Visual Basic can manipulate strings, examine the ELIZA.VBP program on the enclosed CD-ROM. This program is similar to the famous ELIZA artificial intelligence program that mimics a psychiatrist who simply echoes back portions of what the user types in.
++ CHAPTER 19
DEFINING CONSTANTS AND USING COMMENTS
In This Chapter:
Naming and calculating constants
Declaring the scope of constants
Creating and using the three types of comments
A constant is a fixed value that never changes, no matter what happens to your program. Numbers, strings, and dates can be constant values.
But why bother using constants? Several good reasons exist, none of which make any sense until you start writing your own programs.
For example, suppose that you want to write a program that pays employees according to the current minimum wage. If the minimum wage is $5.95, you have to type the number 5.95 everywhere in your program.
Unfortunately, the number 5.95 means nothing in itself. Even worse, if the minimum wage changes from $5.95 to $6.25, you have to change 5.95 to 6.25 everywhere in your program.
To overcome these problems, you can use constants. A constant is simply a word that represents a specific value. A constant not only uses plain English to describe what the value means, but a constant also lets you change values quickly and easily throughout an entire program.
** Naming Constants
Constant names must meet the following criteria. They must:
Begin with a letter
Be 40 characters or fewer
Contain only letters, numbers, and the underscore character (_); punctuation marks and spaces are not allowed
Be any word except a Visual Basic reserved keyword
To make constant names stand out, use all uppercase letters. For example, the following are acceptable constant names:
AGE
MY_BIRTHDAY
MINIMUM_WAGE
LIFEBOAT CAPACITY
To provide additional information about the type of data a constant represents, use a three-letter prefix as part of the constant name such as:
intAGE (int represents an Integer data value)
curMINIMUM_WAGE (cur represents a Currency data value)
sngGPA (sng represents a Single data value)
Table 19-1 provides Microsoft's list of suggested three-letter prefixes for naming constants (or variables, for that matter).
||
Table 19-1
Three-Letter Prefixes for Naming Constants and Variables
Data Type - Boolean
Prefix - bln
Example - blnIsItDeadYet
Data Type - Byte
Prefix - byt
Example - bytThisNumber
Data Type - Currency
Prefix - cur
Example - curCEOBonus
Data Type - Data (Time)
Prefix - dtm
Example - dtmAnniversary
Data Type - Double
Prefix - dbl
Example - dblHeight
Data Type - Integer
Prefix - int
Example - intTotalNumber
Data Type - Long
Prefix - lng
Example - lngWidth
Data Type - Single
Prefix - sng
Example - sngAverage
Data Type - String
Prefix - str
Example - strMyName
Data Type - Variant
Prefix - vnt
Example - vntWhatEver
* Declaring Constants
Before you can use a constant, you have to declare the constant. To declare a constant, you just give the constant a name and assign it a specific value, such as any of the following:
Numbers
Strings
Dates
The following code declares number, string, and date constants:
Private Sub Command1_Click( )
Const intAGE = 21
Const strCOMPANY = "Acme Manufacturing"
Const dtmCHRISTMAS = #25 December 1995#
End Sub
Place all constant declarations at the top of your event procedures. Instead of typing one constant declaration on each line, you can smash them all together and separate them with commas, as illustrated in the following code:
Private Sub Command1_Click( )
Const intAGE = 21, strCOMPANY = "Acme Manufacturing"
End Sub
Note that number constants are only numbers, string constants are anything enclosed in quotation marks and date constants are dates surrounded by the pound sign (#).
Here are some of the ways in which dates can display:
#12-25-95#
#December 25, 1995#
#Dec-25-95#
#25 December 1995#
* Calculating constants
Constants normally represent a fixed value. However, constants can also be mathematic values based on other constants. For example:
Const intRETIREMENT_AGE = 65
Const sngHALFWAY-THERE = intRETIREMENT_AGE / 2
In this case, the value of the constant intRETIREMENT_AGE is 65 and the value of the constant sngHALFWAY_THERE is 65/2, or 32.5.
* Using constants
After you declare a constant, you can use the constant just like any other value. Consider the following:
Const curMINIMUM_WAGE = 5.75
Salary = curMINIMUM_WAGE * 20
Here's how Visual Basic interprets this code:
1. The first command says, "Create a constant named curMINIMUM_WAGE and set the value of curMINIMUM_WAGE to 5.75."
2. The second command says, "Multiply the value of curMINIMUM_WAGE by 20 and store this value in the variable called Salary." In this case, the value of curMINIMUM_WAGE is 5.75, so you multiply 5.75 by 20, which equals 115. Then Visual Basic stores this value in Salary.
** Scope of Constants
Visual Basic lets you declare the scope of constants in the following three ways:
Local
Module
Public
* Local constants
You can use a local constant only within the procedure in which you declare the constant. The purpose of local constants is to isolate specific constants in a single procedure where they are used.
Declare a local constant within an event procedure, as in the following:
Private Sub Command1_Click( )
Const intSPEED_LIMIT = 55
End Sub
You can use a local constant only in the one event procedure in which you declare the constant. However, what if you want to create a constant that two or more event procedures can share? In that case, you have to create a module constant.
* Module constants
A module constant can be used only in an event procedure stored in the same file.
To declare a module constant, follow these steps:
1. Open the Code window by pressing F7, choosing View->Code, or double-clicking anywhere on the form.
2. Click the Object list box in the Code window and choose (General).
3. Click the Procedure list box and choose (Declarations).
4. Now type your constant declaration using the Const statement, as in the following:
Const intDRINKING_AGE = 21
Module constants are useful for sharing a constant value among one or more event procedures, but isolating the constant to only those event procedures stored in the same file. If you want a constant that any procedure in your program can use, you need to create a public constant.
To help you identify module constants buried in your code, Microsoft recommends that you put the letter m in front of your constant name such as:
Const mintDRINKING_AGE = 21
* Public constants
A public constant (also called a global constant) can be the most convenient to use because every procedure in your Visual Basic program can access such a constant. However, cool programmers use public constants only when absolutely necessary; cluttering up your program with public constants that only a few procedures ever use is bad programming practice.
Using public constants is poor programming etiquette because changing a public constant can affect your entire program. Experienced programmers may blush in embarrassment for you if they catch you using public constants needlessly, and you may never get invited to any of the really great programmer parties as a result.
You need to declare public constants in a BAS (module) file. To declare a public constant, follow these steps:
1. Open the Project window.
To do this, press Ctrl+R and click on the BAS (module) file where you want to put the public constant. (If you need to create a BAS file, click the Module icon or choose Module from the Insert menu.)
2. Open the Code window by pressing F7, choosing View->Code, or double-clicking anywhere on the form.
3. Click on the Object list box in the Code window and choose (General).
4. Click on the Procedure list box and choose (Declarations).
5. Type your public constant using the Public command, as in the following:
Public Const AGE_LIMIT = 18
To help you identify public (or global) constants buried in your code, Microsoft recommends that you put the letter g in front of your constant name such as:
Const gsngHEIGHT_LIMIT = 21.67
If you want to see a list of all the global constants that Visual Basic has already defined for you, follow these steps:
1. Choose Help->lndex. A dialog box appears.
2. Click the Index tab.
3. Type Constants in the Type in the keyword to find text box.
4. Under the Constants category, click on Visual Basic.
5. Click Display. Visual Basic displays the Visual Basic Constants window.
6. Click on the constant category that you want to view, such as Button Constants.
** Using Comments
When you're coding your program (note the proper use of the word coding), the way your program works may be clear to you. Unfortunately, if you put your program aside and try to modify it five years later, you may have forgotten why you wrote certain commands and even how some of those commands work.
For just this reason, add plenty of comments to your programs. Comments are short descriptions that programmers add to their program to explain what certain commands mean or to explain what is supposed to happen in a particular part of the program.
As far as the computer is concerned, comments do absolutely nothing to help or hinder the way your program works. However, from a programmer's point of view, comments help explain how and why a program functions.
* Creating comments
Visual Basic lets you create comments by using the apostrophe ( ' ) symbol, followed by anything you care to type. The following, for example, is a valid comment:
Private Sub Command1_Click( )
' This event procedure does absolutely nothing
End Sub
As far as Visual Basic is concerned, the computer ignores anything that appears to the right of the apostrophe symbol.
Comments can appear on separate lines or they can appear as part of another line, as in the following example:
Private Sub Command1_Click( )
X = Y * 2 ' Y represents the number of employees
End Sub
You can also cram several comments together on multiple lines:
Private Sub Command1_Click( )
Y = 200 ' Y represents the number of employees
X = Y * 2
' X represents the number of employees who would like
' to slash the tires on the boss's car.
End Sub
Just remember that the computer ignores anything that appears to the right of the apostrophe symbol and considers it a comment.
* Commenting for readability
The main reason for using comments is to make your programs easy to understand. For this reason, most cool programmers put comments at the beginning of every procedure.
These comments explain what data the procedure gets (if any) and what calculations the procedure performs. By just looking at the comments, anyone can quickly see what the procedure does without needing to decipher several lines of cryptic BASIC code. For example, can you figure out what the following event procedure does?
Private Sub Command1_Click( )
A = SQR(B ^ 2 + C ^ 2)
End Sub
To make the procedure's function easier to understand, add a bunch of comments at the top of this procedure:
Private Sub Command1_Click( )
' The following equation uses the Pythagorean theorem
' for calculating the length of a side of a right
' triangle if the lengths of two sides are known. In
' this case, the length of one side of the triangle is
' represented by B and the length of the second side of
' the triangle is represented by C.
A = SQR(B ^ 2 + C " 2)
End Sub
If several people share the responsibilities for writing procedures, you can use comments to note the name of the programmer and the date each procedure was last modified. (That way, you know who to blame when the procedure doesn't work.) For example:
Private Sub Command1_Click( )
' Programmer: JOHN DOE
' Last modified: 1/1/80 (our computer clock doesn't work)
A = SQR((B ^ 2 + C ^ 2))
End Sub
Of course, if you get too wordy, your comments can be more intrusive than helpful - like billboards along the highway. Just remember: Provide enough information to be helpful, but not so much that people nod off to sleep while reading your comments. You're not writing a classic novel here, just a brief description that other people can understand.
* Test your newfound knowledge
1. Why do you want to add comments to your program?
a. To summarize and explain how your BASIC code works.
b. To exercise your literary skills and prove that programmers can write, too.
c. To prove that you have something to say besides BASIC commands.
d. So that you can leave cryptic messages for other programmers to decipher
2. Comment on the simplicity and brevity of this lesson.
a. All right! Now I can quit and go home early.
b. Why can't all the lessons in this book be this simple and short?
c. I still can't write a program, but I know how to use comments. Maybe I need to get a job as a commentator.
d. Comments are cool. If we can write comments in our programs, does that mean we can write programs with our word
processors?
* Comments for legibility
If your program contains lots of BASIC code, you can use comments and blank lines to make your code easy to read. For example, to make each chunk more easily seen, separate chunks of code:
Private Sub Command1_Click( )
Const dblINTEREST_RATE = .055 ' 5.5% interest rate
Dim Msg As String ' Declares Msg as a string ' variable
BankBalance = 500
BankBalance = BankBalance * dblINTEREST_RATE
' Subtract bank fees
BankFees = BankBalance * 2
BankBalance = BankBalance - BankFees
' Display a message box saying that the user owes the
' bank a certain amount of money.
Msg = "Please pay this amount: " & - BankBalance
MsgBox Msg, vbCritical, "Amount You Owe"
End Sub
As the preceeding example shows, you can insert hard returns to add blank lines between chunks of code, thereby enabling yourself to see more easily what each chunk actually does.
Stripping out all comments and blank lines gives you the following equivalent, but uglier, version:
Private Sub Command1_Click( )
Const INTEREST_RATE = .055
Dim Msg As String
BankBalance = 500
BankBalance = BankBalance * INTEREST_RATE
BankFees = BankBalance * 2
BankBalance = BankBalance - BankFees
Msg = "Please pay this amount: " & -BankBalance
MsgBox Msg, vbCritical, "Amount You Owe"
End Sub
Notice how this new version seems cramped and cluttered, much like your bathroom counter or your garage.
* Comments for disability
With comments, you can not only add explanations about your program and visually break up your code but also temporarily disable one or more BASIC commands.
For example, as you're writing a program, you may find that a command isn't working as you want it to. To test how your program works without this command, you have two choices:
Erase the command.
Comment the command out.
If you erase a command and then decide you need it, you have to type the command all over again. If you comment the command out, however, you only have to erase the apostrophe symbol in order to put the command back in.
The following example contains a fairly long line of numbers:
Private Sub Command1_Click( )
X = 3.14 * 650 - (909 / 34.56) + 89.323
End Sub
If you erase the second line, typing the line again can be a real pain in the neck. However, you can just comment the line out, as follows:
Private Sub Command1_Click( )
' X = 3.14 * 650 - (909 / 34.56) + 89.323
End Sub
Remember, the computer ignores anything that appears to the right of the apostrophe symbol. So, to the computer, this procedure now looks like the following:
Private Sub Command1_Click( )
End Sub
Placing the apostrophe in front of this statement turns the statement into a comment and disables this new comment as a BASIC command. By removing the apostrophe symbol, you can quickly turn the comment back into a real-life BASIC command.
By using comments wisely, you can ensure that you or another programmer can easily understand any programs you write. Then again, if you really want to sabotage a programming project, add comments that don't make any sense or leave them out altogether and see what happens.
If you want to comment multiple lines of code quickly, follow these steps:
1. Highlight the lines of code you want to turn into a comment.
2. Choose View->Toolbars->Edit.
3. Click on the Comment Block icon. Visual Basic adds an apostrophe to the front of each line that you highlighted in Step 1.
To uncomment blocks of code, just repeat the above three steps except click on the Uncomment Block icon on the Editor toolbar in Step 3.
++ CHAPTER 20
KILLING BUGS
In This Chapter:
Examining the various types of bugs
Embarking on the big bug hunt
Killing the bugs you find
Even if you've written millions of different programs before, you are still going to make a mistake at one time or another. You may misspell a word or forget to type a command. So no matter how carefully you write your program, the program may not work exactly as you design it to. The problems hindering your program from working are bugs.
Every program in the world has bugs, including Netscape Navigator, WordPerfect, Lotus 1-2-3, Quicken, and Microsoft Windows 98. The only difference between the bugs in your program and the bugs in a commercial program is that nobody is paying you to eliminate bugs in your program. However, with a fair amount of planning, application design, and just plain common sense, you can avoid quite a few bugs.
Don't worry. Many bugs are relatively harmless. These minor bugs normally aren't going to prevent you from using a program correctly, but they may slow down your computer or display odd colors or objects on the screen at random times.
Major bugs are more devastating. For example, a major bug allegedly blew up one of NASA's multimillion-dollar weather satellites because someone mistyped a single command in the program.
Nobody is perfect, so no program can be guaranteed to be completely bug-free. Even an experienced professional with a doctorate in computer science regularly writes bug-ridden programs.
Bugs are a fact of life, like cockroaches in a kitchen. You may never get rid of them all, but you can kill as many as possible along the way.
* Why computer problems are called bugs
The first computer in the world used mechanical relays instead of modern electronics. One day the computer stopped working for no apparent reason. The scientists checked their programming (the program was supposed to have worked), the electric cord (it was plugged in), and the wires inside the computer (they were still connected).
Eventually, someone noticed that a moth had gotten smashed in one of the relays, preventing the relay from closing all the way. Because the moth's dead body prevented the computer from working, problems with computers were henceforth known as bugs (which is a lot easier to say than Chihuahuas, so we should be glad that a dog didn't get smashed in the first computer).
** Types of Bugs
The art of killing bugs is known as debugging. Before you can kill a bug, you first have to find the bug. With small programs, such as ones that display Hello, world! on the screen, you have only so many places a bug can hide. With large programs, a bug can be hiding anywhere, and this can be as frustrating as trying to find a single tsetse fly in a high-rise apartment building.
To make hunting for bugs easier, computer scientists classify bugs in three categories:
Syntax errors
Run-time errors
Logic errors
* Syntax errors
A syntax error is a bug that occurs when you misspell a command. If you type INTTEGER instead of INTEGER, for example, Visual Basic is going to have no idea what INTTEGER means and isn't going to even bother trying to run the rest of your program.
When Visual Basic runs across a syntax error, Visual Basic politely high-lights the misspelled word on the screen to show you exactly what the problem is. Just type the correct spelling and run your program again.
Even one syntax error is going to keep your program from running. When you finally get your program to run for the first time, you know that your program is completely free of syntax errors. Then you have to worry only about run-time and logic errors.
* Run-time errors
A run-time error occurs when your program gets data that the program doesn't quite know how to handle. A run-time error is more subtle than a syntax error. Your program may be riddled with run-time errors, but you may never know this until you actually run your program.
To simulate a run-time error in your own life, pull into the drive-through window at your nearest Burger King. When the cashier asks, "May I help you?" order a Big Mac. Because the cashier expects you to order something from Burger King's menu, this person has no idea how to respond to your question and is likely to suffer a run-time error.
For an example of a run-time error in a program, consider this formula for calculating a result:
TaxRate = TaxesOwed / YearlyIncome
This equation normally works - unless the YearIy Income equals 0. Because dividing any number by 0 is impossible, the program stops running if the value in YearlyIncome is 0.
To discover a run-time error, you must test your program for every possible occurrence: from someone pressing the wrong key to some idiot typing a negative number for his or her age.
Because the number of things that can ever go wrong is nearly infinite (Murphy's Law), you can understand why every large program in the world has bugs. (Now isn't this a comforting thought to remember the next time you fly in a computer-controlled airplane?)
* Logic errors
The trickiest type of bug is a logic error. A logic error occurs when the program doesn't work correctly because you gave the program the wrong commands or the commands you issued are out of sequence with other commands. Huh? How can you give a program the wrong commands when you're the one writing the program? Believe it or not, entering the wrong commands is easy.
Anyone raising teenagers knows that when you tell them to mow the lawn or clean up their rooms, they may do the task - but not quite the way you wanted the task accomplished. Instead of mowing the lawn in neat rows, a teenager may move around in circles and give up. Or instead of cleaning a room by picking up dirty clothes and tossing out papers, a teenager may shove the whole mess under the bed or out into the hallway.
In both cases, the teenager followed your instructions, but your instructions weren't specific enough. If a teenager can find a loophole in your instructions, he or she will, and a computer is no different.
Because you thought you gave the computer the right commands to follow, you have no idea why your program isn't working. Now you have to find the one spot where your instructions aren't clear enough. If you have a large program, this may mean searching the entire program, line-by-line. (Isn't programming fun?)
** Bug Hunting
Basically, you need to go through four steps to hunt down and kill bugs in your program:
1. Realize that your program has a bug.
2. Find the bug.
3. Find what's causing the bug.
4. Squash the bug.
* Realizing that your program has a bug
The best way to discover bugs in your program is to let unsuspecting individuals use your program. (In the world of commercial software, these unsuspecting individuals are often called paying customers.)
The more people you find to test your program, the more likely that these guinea pigs are going to uncover bugs you never knew existed. Bugs can be as glaring as ones that cause the computer to crash, or they can be as sneaky as ones that round off numbers to the wrong decimal place.
After you conclude that your program has a bug, you have to track the bug down. (For the optimists in the group, you can call your program's bugs undocumented features.)
* Finding the bug
Finding where a bug is hiding is the toughest part. The simplest (and most tedious) way to find a bug's hiding place is to run your program and examine it line-by-line. The moment the bug appears, you know exactly which line caused the bug.
For small programs, this approach is acceptable. For large programs, this is crazy.
As a faster alternative, just examine the parts of your program in which you think the bug may be hiding. If your program doesn't print correctly, for example, the bug may be in your BASIC code that tells the computer how to print.
* Finding what's causing the bug
After you isolate where you think the bug is hiding, you have to figure out what is causing the bug in the first place.
Suppose that your program is to print your name on the screen but is printing your social security number instead. The program may seem to be printing everything correctly but is simply getting the wrong type of information to print.
By using your incredible powers of deductive reasoning, you realize that the bug is (probably) wherever your program first tries to print your name.
* Squashing the bug
After you find the cause of your bug, you've reached the time to correct your program. But be careful! Sometimes correcting one bug introduces two or three more by mistake. Huh? How can that be?
Compare bug squashing to repairing a problem with the plumbing in your house. The easiest solution may be to tear out a wall and put in new pipes. This may solve the plumbing problem, but tearing out a wall can also tear out electrical wires inside the wall. So now you've fixed your plumbing problems but also created a new electrical problem. If you put up a new wall with electrical wiring, you may inadvertently block a vent for the central air-conditioning. Move the wall back 3 feet, and now the roof may be too weak in the middle to hold up the wall. See - your small plumbing "bug" has just multiplied.
So when fixing a bug, be careful. Sometimes rewriting a huge chunk of code is easier than trying to fix a bug within the code.
The best way to avoid bugs is to not have any in the first place. Of course, that's like saying to avoid money problems, just make sure you always have enough money.
Because bugs appear in even the best of programs, the most you can hope for is to reduce the number of bugs that can pop up in your programs. Here are some tips that may help:
To avoid bugs, write (and test) lots of tiny programs and paste them together to make one huge program. The smaller your programs, the easier isolating any bugs is going to be. In military terms, this is known as the divide and conquer method.
Test your program each time you modify it. If your program worked fine until you changed three lines, the problem probably can be isolated to those three lines.
Have someone you can pin the blame on. If your program refuses to work, blame your spouse, your dog, or your favorite deity. This isn't going to help fix your program, but blaming someone or something else can make you feel better for a moment or two.
** How Visual Basic Tracks and Kills Bugs
Visual Basic provides two primary ways to help you track and kill bugs:
stepping and watching.
Stepping means that you go through your program line-by-line and examine each instruction. After each line runs, look to see what the program did. If your program works the way you wanted it to, the line is okay. If not, you just located a bug.
Watching lets you see what data your program is using at any given time. If you watch for specific data, such as a name or phone number, you can see whether your program is storing, printing, or modifying the specific data correctly.
By stepping through a program line-by-line and watching to see what data your program is using, you can find any bugs in your program.
* Stepping through a program line-by-line
If you have absolutely no idea where your bug may be, you need to examine your entire program line-by-line. To step through a program, Visual Basic provides three commands:
Debug->Step Into (or press F8)
Debug->Step Over (or press Shift+F8)
Debug->Step Out (or press Ctrl+Shift+F8)
The Step Into command runs through your entire program one line at a time, including every line stored in every procedure in your program.
The Step Over command runs through your entire program but whenever Visual Basic runs into a procedure, Visual Basic skips over the instructions that make up that procedure.
The Step Out command is used with the Step Into command. The Step Into command shows you, line-by-line, how a procedure is working. However, if you suddenly decide that you don't want to examine how a procedure is working, line-by-line, use the Step Out command.
You can combine the three commands at any time. First, use the Step Into command to examine your program line-by-line. Next, use the Step Out command to get out of any procedures that the Step Into command starts displaying. Finally, use the Step Over command to skip over any procedures that you're positive already work.
To use the Step Into or Step Over commands, follow these steps:
1. Press F8 or Shift+F8, or choose Debug->Step Into or Step Over.
Visual Basic displays and outlines a line in your program.
2. Choose Run->End when you want to stop.
* Watching your variables
Stepping through your program line-by-line can be even more useful if you watch how your program handles data at the same time. To help you see what values your variables contain at any given time, Visual Basic provides a Watch window.
The Watch window tells Visual Basic, "These are the variables I want to examine. Show me the contents of these variables as I step through my program line-by-line."
To use the Watch window to watch your variables, follow these steps:
1. Open the Code window by pressing F7, choosing View->Code, or clicking on the View Code icon in the Project Explorer window.
2. Highlight the variable that you want to watch and choose Debug->Add Watch. An Add Watch dialog box appears.
3. Click on OK.
4. Press F8 or Shift+F8 to choose the Step Into (if you want to examine the guts of a procedure) or Step Over (if you want to skip over the code trapped inside a procedure) commands. The Watch window displays the value of your watched variable each time you choose the Step Into or Step Over command.
5. Choose Run->End or click on the End icon In the toolbar when you want to stop.
* Setting breakpoints
Both the Step Into and Step Over commands start from the beginning of your program and continue until they reach the end. This is acceptable for small programs, but the process can get tedious for large programs.
To skip over large sections of your program that you know (or hope) already work, you can set a breakpoint. A breakpoint tells Visual Basic, "Run the program up until you reach me. Then wait until I give you the Step Into, Step Over, or Run command."
To set a breakpoint, follow these steps:
1. Open the Code window by pressing F7, choosing View->Code, or clicking on the View Code icon in the Project Explorer window.
2. Click on the line where you want to set your breakpoint
3. Press F9 (or choose Debug->Toggle Breakpoint).
After you set a breakpoint, press F5 to run your program until it reaches your breakpoint. At this point, you can use the Step Into or Step Over commands along with the Add Watch command.
To remove a breakpoint, just follow the above three steps again, clicking on the line that contains the breakpoint you want to remove.
To quickly remove all breakpoints in your program, press Ctrl+Shift+F9 or choose Debug->Clear All Breakpoints.
++ PART V
MAKING DECISIONS (SOMETHING YOU STOP DOING WHEN YOU GET MARRIED)
In this part....
Programs contain nothing more complicated than instructions that tell the computer what to do next. The simplest programs just contain one massive list of instructions that the computer follows, one instruction after another, like a shopping list.
But blindly following instructions doesn't make for a very useful program. Most programs need to receive data and then decide how to use the data.
This decision-making capability can make your computer seem alive, responsive, and intelligent. (Well, alive, anyway.) When a program can tell your computer how to make its own decisions, your computer can begin doing something useful.
++ CHAPTER 21
THE IF-THEN AND IF-THEN-ELSE STATEMENTS
In This Chapter.....
Specifying a condition
Using If-Then and If-Then-End If statements
Using If-Then-Else and If-Then-ElseIf statements
Everyone has made decisions based on certain conditions, such as, "Don't worry, honey. If the ball game is over early, then I am going to mow the lawn." Or "If you stop feeding the cat, then maybe it won't keep getting fatter." Visual Basic is no different when making decisions is at hand. When you write a program, Visual Basic can check for certain conditions and then respond.
** Conditions
A condition must represent a value that is either True or False. Conditions can be:
A single variable
An expression
If a condition is a single variable, that variable must have a value that is either True or False. You can check the value of a variable in two ways. The first way is to specifically check if a variable is equal to True, as in the following example:
If TooHot = True Then
The second way (which is shorter to write) lets you eliminate = True and just write the following:
If TooHot Then
You don't have to specify whether this variable is equal to True because Visual Basic checks whether the value is True or False anyway.
If you want to specifically test whether a variable is False, you can do the following:
If TooCold = False Then
As ashortcut, eliminate = False and just write
If Not TooCold Then
If a condition is an expression, that entire expression must represent a value that is either True or False, such as:
If Age >+ 21 Then
In the above example, the condition is Age >= 21.
For this condition, if the value of Age is greater than or equal to 21, the expression is True. Otherwise, the expression is False.
In the following example, if the string stored in the Text property of a text box called txtName contains the string "Captain Mike", the expression is True. Otherwise, the expression is False.
If (txtName.Text = "Captain Mike") Then
The parentheses in the preceding line of code don't affect your precious code one bit; they just help make the condition easier to identify.
In the following example, the expression Cat Present And CatMisbehaving evaluates to False:
CatPresent = False
CatMisbehaving = True
If CatPresent And CatMisbehaving Then
To make the condition in an If-Then statement easier to see, you can also surround the condition with optional parentheses, as in the following line of code:
If (CatPresent And CatMisbehaving) Then
Now that you understand what and how conditions work, you can find out how to plug them into an If-Then statement in the following section, "The If-Then Statement."
** The If-Then Statement
To make decisions, Visual Basic uses something called an If-Then statement. An If-Then statement is the way Visual Basic checks whether a condition is True or False.
If the condition is True, Visual Basic follows a certain instruction. If the condition is False, Visual Basic ignores this instruction.
All of this may look rather wordy, so here's the condensed version of the statement:
If Condition Then Instruction
Essentially, this code tells Visual Basic that if a certain condition is True, Visual Basic needs to obey the instruction that follows.
Whatever the condition may be, the condition must always return a True or False value.
Here are a few real-life examples:
If Number > 25 Then txtNote.Text = "Full"
Here's how Visual Basic interprets this code:
1. This command says, "Check a variable called Number and see whether the value is greater than 25. If the value is greater than 25, then stuff the string "Full" into the Text property of a text box called txtNote."
2. "If the value of the Number variable is equal to or less than 25, then skip to the next instruction in your Visual Basic program."
If Hungry Or Bored Then Message = "Let's eat."
Here's how Visual Basic interprets this code:
1. This command says, "Check the value of a variable called Hungry and check the value of a variable called Bored. If either one has a value of True, then create a variable called Message and set the value equal to the string " Let ' s eat."
2. "If both the variable Hungry and the variable Bored are False, skip to the next instruction in your Visual Basic program."
The typical If-Then statement tests whether a certain condition is True or False and then follows a single instruction. But what happens if you want to test for a condition and then make the computer follow two or more instructions? In that case, you have to use a different form of the If-Then statement that is called the If-Then-End If statement.
** The If-Then-End If Statement
The If-Then-End If statement lets Visual Basic check a single condition. If the condition is True, the program follows a bunch of instructions. Here's the proper If-Then-End If syntax:
If Condition Then
Instruction1
Instruction2
End If
Essentially, this code tells Visual Basic, "Check a condition; if the condition is True, then obey all the following instructions until you reach End If."
Here is an honest-to-goodness example:
If Electricity_is_Out = True Then
Light-candles = True
TxtWarning.Text = "You just lost all your work."
End If
And here's how Visual Basic interprets this code:
1. The first line says, "Check the value for a variable called Electricity_is_Out. If the variable's value is True, then follow the
next two instructions. If the value is False, then do nothing."
2. "Assign a value of True to the variable Light_candles."
3. "Assign the string "You just lost all your work." to the Text property of the txtWarning text box."
** If-Then-Else Statement
The If-Then statement gives your program the capability to make decisions based on certain conditions. If this isn't the pinnacle of your computer programming career, hold on to your hats for the If-Then-Else statement.
The problem with using an If-Then statement or an If-Then-End If statement is that you may need too many of them to check for both True and False conditions. Is there an easier way? The answer, of course, is yes! Visual Basic offers something called an If-Then-Else statement.
The simplest If-Then-Else statement looks like this:
If Condition Then
Instructions1
Else
Instructions2
End If
This statement tells Visual Basic, "If the condition is True, then follow the first batch of instructions. If the condition is False, then follow the second batch of instructions."
So how can you modify the following?
If Day > 15 Then txtReadMe.Text = "Bills are past due!"
If Day <= 15 Then txtReadMe.Text = "Pay your bills!"
Depending on the condition you use, you can rewrite these statements in two ways. If you use the condition Day > 15, you get the following:
If Day > 15 Then
txtReadMe.Text = "Bills are past due!"
Else
txtReadMe.Text = "Pay your bills!"
End If
But if you use the condition Day <= 15, you get the following:
If Day <= 15 Then
txtReadMe.Text = "Pay your bills!"
Else
txtReadMe.Text = "Bills are past due!"
End If
Both types of If-Then-Else statements are perfectly acceptable. Which statement you choose to use is just a matter of personal preference.
You can shove as many instructions as you want between the If-Then and Else lines and the Else and End If lines.
One possible drawback with an If-Then-Else statement is that if the first condition is False, Visual Basic blindly follows the second group of instructions. If you don't want this to happen, you have to specify a condition for the second set of instructions. To do that, you have to use an If-Then-ElseIf statement.
** The If-Then-ElseIf Statement
An If-Then-ElseIf statement looks like the following:
If Condition1 Then
Instructions1
ElseIf Condition2 Then
Instructions2
End If
This code tells Visual Basic, "If Condition1 is True, then follow the first set of instructions. But if Condition1 is False, then check to see if Condition2 is True. If Condition2 is True, then follow the second set of instructions. If Condition2 is False, then don't do anything at all."
With an If-Then-Else statement, the computer always follows at least one set of instructions. But with an If-Then-ElseIf statement, the computer could possible ignore all instructions - much like a rebellious teenager.
For example:
If Day > 15 Then
txtReadMe.Text - "Bills are past due!"
ElseIf Day > 10 Then
txtReadMe.Text = "Pay your bills!"
End If
So what happens if the value of Day is 12?
1. Visual Basic checks the first condition and concludes that the expression 12 > 15 is False (because the value of Day is 12).
2. Then Visual Basic checks the second condition and concludes that the expression 12 > 10 is True, so Visual Basic assigns the string, "Pay your bills!" to the Text property of a text box called txtReadMe.
Here's the tricky part. What happens if the value of Day is 6?
1. Visual Basic checks the first condition and concludes that the statement 6 > 15 is False, so Visual Basic ignores the first set of instructions.
2. Next, Visual Basic checks the second condition and concludes that the statement 6 > 10 is False, so Visual Basic ignores the second set of instructions.
3. Finally, Visual Basic reaches the end of the If-Then-ElseIf statement. Because none of the statements were True, none of the instructions were followed.
To handle multiple possibilities, you need to add more ElseIf conditions.
** Making multiple choices with If-Then-ElseIf
For checking multiple conditions, use multiple ElseIfs, as follows:
If Condition1 Then
Instructions1
ElseIf Condition2 Then
Instructions2
ElseIf Condition3 Then
Instructions3
End If
This code tells Visual Basic, "If Condition1 is True, then follow Instructions1. But if Condition1 is False, check whether Condition2 is True. If Condition2 is True, then follow Instructions2. If Condition1 is False and Condition2 is False, then check to see whether Condition3 is True. If Condition3 is True, then follow Instructions3."
Once again, the possibility exists that all condi tions are going to be False, so the computer may never follow any of the instructions.
You can use as many ElseIf lines as you need. Of course, the more you use, the more confusing your entire If-Then-ElseIf statement gets. ("Now if Condition3 is False but Condition4 is True, wait a minute, what's supposed to happen?")
** Making sure that the computer follows at least one set of instructions
You can have a huge If-Then-ElseIf statement and still not have a single instruction that the computer follows. To make sure that the computer follows at least one set of instructions, add an Else statement at the end, as shown in the following:
If Condition1 Then
Instructions1
ElseIf Condition2 Then
Instructions2
Else
InstructionsDefault
End If
This code tells Visual Basic, "If Condition1 is True, then follow the first set of instructions. But if Condition1 is False, check the value of Condition2. If Condition2 is True, then follow the second set of instructions. If all conditions are False, then go ahead and follow the last set of instructions."
* * Nesting If-Then statements
If you want, you can nest your statements (cram multiple If-Then statements inside one another), such as:
If Age > 21 Then
If Rating = 10 Then
txtAction.Text "Ask for a date,"
End If
Else
txtAction.Text = "Sorry, you're too young."
End If
If the value of Age were 23 and the value of Rating were 10, Visual Basic interprets this code as follows:
1. Visual Basic checks the first condition and concludes that the expression Age > 21 is True (because the value of Age is 23).
2. Then Visual Basic checks the second condition and concludes that the expression Rating = 10 is True, so Visual Basic assigns the string, "Ask for a date." to the Text property of a text box called txtAction.
If the value of Age were 23 but the value of Rating were only 9, Visual Basic interprets this code as follows:
1. Visual Basic checks the first condition and concludes that the expression Age > 2 1 is True ( because the value of Age is 23).
2. Then Visual Basic checks the second condition and concludes that the expression Rating = 10 is False, so nothing happens.
Finally, if the value of Age were 13 and the value of Rating were 10, Visual Basic interprets this code as follows:
1. Visual Basic checks the first condition and concludes that the expression Age > 21 is False (because the value of Age is 13).
2. Visual Basic skips to the Else part of the If-Then-Else statement and stuffs the string " Sorry, you' re too young. " in the Text property of the text box called txtAction. Notice that in this case, the value of Rating is irrelevant.
Be careful when nesting If-Then statements inside one another because nested If-Then statements may act in ways that you didn't expect. For example, in the preceding code, if the value of Age were 23 but the value of Rating were only 9, you may be surprised to find that this code isn't going to put any string in the Text property of the text box called txtAction.
++ CHAPTER 22
THE SELECT CASE STATEMENT
In This Chapter....
Examining the Select Case statement
Making sure that the computer follows at least one set of instructions.
Using nested control structures
The main problem with using massive If-Then-ElseIf statements, which I discuss in Chapter 21, is that these statements are ugly, hard to read and understand, and cumbersome to write. Consider the following:
If Caller = "Frank" Then
txtReply.Text = "Yes!"
ElseIf Caller = "Matt" Then
txtReply.Text = "Okay, but only if you buy."
ElseIf Caller = "Jeff" Then
txtReply.Text = "I'm washing my hair tonight."
ElseIf Caller = "Steve" Then
txtReply.Text = "This is a recording."
End If
So what's the alternative to an endless proliferation of ElseIfs that can be confusing to look at?
One alternative is to toss your copy of Visual Basic out the window and find someone to write your programs for you. But the more practical alternative is to use something called the Select Case statement.
** The Select Case Statement
The Select Case statement looks like the following:
Select Case VariableName
Case X
Instructions1
Case Y
Instructions2
Case Z
Instructions3
End Select
This statement tells Visual Basic, "Look at the value of the variable called VariableName. If this value is equal to X, then follow Instructions1. If this value is equal to Y, then follow Instructions2. If this value is equal to Z, then follow Instructions3."
Replacing the multiple If-Then-ElseIf statement at the beginning of this chapter with the Select Case statement changes the code to look like the following:
Select Case Caller
Case "Frank*
txtReply.Text "Yes!"
Case "Matt"
txtReply.Text - "Okay, but only if you buy."
Case "Jeff"
txtReply.Text "I'm washing my hair tonight.
Case "Steve"
txtReply.Text "This is a recording."
End Select
Notice the cleaner look and the elimination of repetitive words such as ElseIf and Then.
Depending on how many values you need to check, you can sandwich as many Case lines in a Select Case statement as you want.
** Using the Select Case Statement with Comparison Operators
Normally, the Select Case statement requires an exact value to examine. However, by using comparison operators, such as <, <=, or <>, you can make the Select Case statement examine whether a variable falls within a range of values.
To make a Select Case statement use comparison operators, you have to use the magic reserved word is. Therefore, the following Select Case statement:
Select Case Day
Case is > 15
txtReadMe.Text = "Bills are past duel"
Case is > 10
txtReadMe.Text = "Pay your bills!"
End Select
is equivalent to the following If-Then statement:
If Day > 15 Then
txtReadMe.Text - "Bills are past due!"
ElseIf Day > 10 Then
txtReadMe.Text = "Pay your bills!"
End If
** Making Sure the Computer Follows at Least One Set of Instructions
Like the If-Then-ElseIf statement, the possibility that none of the instructions within the Select Case statement are going to be followed exists. To make sure that the computer follows at least one set of instructions, you have to use the magical Else command again. Take a look at the following example:
Select Case Day
Case 1
Instructions1
Case 2
Instructions2
Case3
Instructions3
Case Else
InstructionsDefault
End Select
The preceding code tells Visual Basic, "If the value of Day equals 1, then follow the first set of instructions. If the value of Day equals 2, then follow the second set of instructions. If the value of Day equals 3, then follow the third set of instructions. If the value of Day doesn't equal 1, 2, or 3, then follow the last set of instructions."
** Nesting Case Statements
Some of the simplest toys that amuse children to no end are those Chinese boxes stacked one inside the other. Each time you open a box, you find a smaller one inside. Eventually, you reach a point where no more boxes remain and you have to stop.
Normally an ordinary Select Case statement contains one or more groups of instructions, such as the following:
Select Case ID
Case 123
chkFrank.Value = True
Case 124
chkBob.Value
Case 125
chkMartha.Value = True
End Select
Rather than shoving boring old instructions inside a Select Case statement, however, you can shove If-Then and Select Case statements within a Select Case statement, as the following example illustrates:
Select Case IQ
Case 120
Select Case Age
Case is <= 9
txtAnalysis.Text = "You must be a smart kid."
End Select
End Select
Here's how Visual Basic interprets this code:
1. The first line says, "Check the value stored in a variable called IQ. Then continue to the second line."
2. The second line says, "If the value of IQ is exactly equal to 120, continue to the third line. If the value of IQ is anything else (such as 119, 121, or 3), skip to the seventh line.
3. The third line says, "Check the value stored in a variable called Age. Then continue to the fourth line."
4. The fourth line says, "If the value stored in the variable called Age is equal to or less than 9, then continue to the fifth line. If the value of Age is anything greater than 9 (such as 13, 86, or 10), then skip to the sixth line."
5. The fifth line says, "Assign the string, " You must be a smart kid." to the Text property of a text box called txtAnalysis."
6. The sixth line says, "This is the end of one Select Case statement."
7. The seventh line says, "This is the end of another Select Case statement."
For kicks and grins, you can put an If-Then statement inside a Select Case statement or a Select Case statement inside an If-Then statement. (Some fun, huh?)
Although no theoretical limit exists as to how many If-Then or Select Case statements you can place inside one another, the fewer you use, the easier your code is going to be to figure out. As a general rule, if you have nested more than three If-Then or Case Select statements inside one another, you probably don't know what you're doing.
When nesting multiple statements, you should indent statements so that seeing where they begin and end is easier. For example, notice how confusing the following program appears without indentation:
Select Case Salary
Case 1200
If Name = "Bob" Then
txtReview.Text = "No raise this year, ha, ha, ha!"
ElseIf Name = "Karen" Then
txt Review.Text = "Okay, how about a 5% raise?"
End If
End Select
Here's what the same program looks like with indentation:
Select Case Salary
Case 1200
If Name = "Bob" Then
txtReview.Text = "No raise this year, ha, ha, ha!"
ElseIf Name = "Karen" Then
txt Review.Text = "Okay, how about a 5% raise?"
End If
End Select
From the computer's point of view, both programs are the same. But from a programmer's point of view, the program using indentation is much easier to read and understand.
* Test your newfound knowledge
1. What is the limit to the number of control structures (If-Then or Select Case statements) you can nest?
a. The limit is determined by the restrictions your government may place upon you.
b. The limit is determined by the theoretical applications pursuant to the implications of Einstein's theory of relativity, as reworded by a lawyer.
c. The limit is 65. If you go over that, you risk getting pulled over by a state trooper.
d. No limit exists. But if you have too many nested control structures, your program is going to be harder to read and understand.
2. To make nested control structures easier to read and understand, what do you need to do?
a. Avoid using nested control structures
b. Avoid programming altogether.
c. Limit the number of nested control structures you use, and use indentation to make each If-Then or Select Case statement easy to find.
d. Print in big, bold, block letters and use short statements like "See Dick run. Dick runs fast."
++ PART VI
GETTING LOOPY
In this part....
Every program contains instructions that tell the computer what to do. Sometimes these instructions are used only once, such as instructions that tell the computer what to do when the program loads.
But other times, the computer may need certain instructions over and over again. Rather than retyping these instructions each time, programmers invented something magical called loops.
Essentially a loop tells the computer, "See those instructions over there? Keep repeating them again and again a certain number of times, and then stop." By using loops, programmers spare themselves the tedium of repeatedly writing the same instructions - and saving time is what programmers know how to do best.
++ CHAPTER 23
THE DO WHILE AND DO-LOOP WHILE LOOPS
In This Chapter ....
Using the Do While loop
Finding out how often the Do While loop runs
Examining how the Do-Loop While loop works and when to use it
Do While loops don't do anything without checking if a certain condition is True first. If a certain condition is True, then the Do While loop goes ahead and does something. Otherwise, it jumps to the next instruction in the Visual Basic program.
You can find Do While loops in such everyday experiences as when office workers tell themselves, "Keep stuffing office supplies in my briefcase while no one is looking. The moment someone looks in my direction, stop and do something else."
A Do While loop looks like this:
Do While Condition
Instructions
Loop
The condition must be a variable or an expression that represents a True or False value. A Do While loop can hold one or more instructions.
** How the Do While Loop Works
The first time Visual Basic sees a Do While loop, it says, "Okay, is the value of the condition True or False? If it's False, ignore all the instructions inside the Do While loop. If it's True, follow all the instructions inside the Do While loop."
For example, the following code has a Do While loop:
Counter = 0
Do While Counter <> 5
Counter = Counter + 1
txtCounter.Text = CStr(Counter)
Loop
Visual Basic interprets the code like this:
1. The first line says, "Stuff the value of 0 (zero) inside a variable called Counter."
2. The second line says, "As long as Counter is not equal to 5, keep repeating all the instructions sandwiched between the Do While line and the Loop line."
3. The third line says, "Add 1 (one) to the value of the Counter variable."
4. The fourth line says, "Take the value of Counter, convert it into a String, and stuff it in the Text property of a text box called txtCounter."
5. The fifth line says, "This is the end of the Do While loop. Go back to the second line where the Do While loop begins as long as the condition that Counter <> is True."
Each time this loop runs, it increases the value of Counter by one. As soon as the value of Counter equals 5, the condition Counter < > 5 suddenly becomes False and the Do While loop stops.
* How many times does the Do While loop run?
If the condition of a Do While loop is False, none of the instructions inside the Do While loop runs. In that case, the Do While loop runs zero times.
If the condition of a Do While loop is True, the Do While loop runs at least once.
If the condition of a Do While loop is always True, the Do While loop repeats itself an endless number of times until you turn off the computer or the universe explodes in another Big Bang.
When a loop repeats itself endlessly, it is called an endless loop.
Endless loops send your program into infinity and keep it from working properly (because the loop never ends). To avoid an endless loop, make sure that at least one instruction inside your Do While loop changes the True or False value of the condition that the loop checks.
* When to use a Do While loop
Use a Do While loop whenever you need to do the following:
Loop zero or more times
Loop as long as a certain condition remains True
** How the Do-Loop While Loop Works
Do-Loop While loops essentially tell Visual Basic, "Go ahead and do something until a certain condition tells you to stop."
You can find Do-Loop While loops in such everyday experiences as when parents tell their kids, "Go ahead and do what you want, just as long as you don't bother me."
A Do-Loop While loop looks like the following:
Do
Instructions
Loop While Condition
The condition must be a variable or an expression that represents a True or False value. A Do-Loop While loop can hold one or more instructions.
The first time Visual Basic sees a Do-Loop While loop, it says, "Let me follow all the instructions inside the loop first. After this, check whether the value of the condition is True or False. If it's False, stop. If it's True, repeat all the instructions inside the Do-Loop While loop again."
For example, the following code has a Do-Loop While loop.
Counter = 0
Do
Counter = Counter + 1
Loop While Counter < 5
Visual Basic interprets the code like this:
1. The first line says, "Create a variable called Counter and set its value to 0."
2. The second line says, "This is the beginning of the Do-Loop While loop."
3. The third line says, "Take the value of Counter and add 1 (one) to it. Now store this new value in the Counter variable."
4. The fourth line says, "This is the end of the Do-Loop While loop. As long as the value of Counter is less than 5, keep repeating all the instructions sandwiched between the Do line and the Loop While line. Otherwise, exit the loop."
Each time this loop runs, it increases the value of Counter by one. As soon as the value of Counter equals 5, the condition Counter < 5 suddenly becomes False and the Do While loop stops.
* How many times does the Do-Loop While Loop repeat itself?
No matter what the condition of a Do-Loop While loop may be, it always runs at least once. As long as the condition remains True, the Do-Loop While loop can keep running until infinity (or until you neglect to pay your electricity bill).
To avoid an endless Do-Loop While loop, you have to make sure that at least one instruction inside your Do-Loop While loop changes the True or False value of the condition that the loop checks.
* When to use a Do-Loop While loop
Use a Do-Loop While loop whenever you need to do the following:
Loop at least once
Loop as long as a certain condition remains True
Choose your loops carefully because loops may look similar but act differently. To avoid confusion, try to stick with one type of loop throughout your program so that it is easier for you to figure out how all your program's loops may work.
* Test your newfound knowledge
1. What are two main differences between a Do While loop and a Do-Loop While loop?
a. The words are in a different order, and they use a different number of consonants.
b. The Do While loop runs zero or more times and repeats only if its condition is True. Regardless of its condition, the Do-Loop While Loop runs at least once, and if its condition is True, it can run an infinite number of times.
c. Both loops run endlessly, faster and faster, until your computer is flung up against a wall because they're running so fast.
d. Four out of five dentists recommend the Do While loop, sugar-free chewing gum, and Crest toothpaste.
2. Why is it possible that a Do While loop runs zero or more times?
a. Because it's the number one loop used by two out of three programmers employed at Microsoft, IBM, and Symantec.
b. Nobody knows, but I remember seeing a segment on Unsolved Mysteries, asking viewers to call in if they had any information that might help resolve this question.
c. Because it checks its condition before it runs even once.
d. Because it used steroids; So not only does it run once, it runs faster than any other loop that isn't doped up.
++ CHAPTER 24
TO DO UNTIL AND DO-LOOP UNTIL LOOPS
In This Chapter...
Using the Do Until loop
Finding out how often the Do Until loop repeats itself
Examining how the Do-Loop Until loop works and when to use it
Do Until loops keep repeating until some condition becomes True, such as, "Keep stealing from the cash register until someone catches you."
A Do Until loop looks like this:
Do Until Condition
Instructions
Loop
The condition must be a variable or an expression that represents a True or False value. A Do Until loop can hold one or more instructions.
** How the Do Until Loop Works
The first time Visual Basic sees a Do Until loop, it says, "Let me check whether the value of the condition is True or False. If it's False, follow all the instructions inside the Do Until loop. If it's True, exit the Do Until loop."
For example, the following is a typical Do Until loop that counts:
Counter = 0
Do Until Counter > 4
Counter = Counter + 1
Loop
Here's an explanation of the code:
1. The first line says, "Stuff the value of 0 inside a variable called Counter."
2. The second line says, "This is the beginning of the Do Until loop. As long as the value of Counter > 4 is False, keep repeating all the instructions sandwiched between the Do line and the Loop line. Otherwise, if the value of Counter > 4 is True (when Counter is 5), exit the loop."
3. The third line says, "Take the value of Counter and add 1 (one) to it. Now store this new value in the Counter variable."
4. The fourth line says, "This is the end of the Do Until loop."
Each time this loop runs, it increases the value of Counter by one. As soon as the value of Counter equals 5, the condition Counter > 4 suddenly becomes True and the Do Until loop stops.
* How many times does the Do Until loop repeat itself?
The Do Until loop can run zero or more times. This loop keeps repeating itself until its condition becomes True.
Notice that this is the opposite of the Do While and Do-Loop While loops. These two loops keep running as long as their condition is True. The Do Until loop keeps running until its condition becomes True. (This is the same thing as saying that the Do Until loop keeps running as long as its condition is False.)
To avoid an endless Do Until loop, make sure that at least one instruction inside your Do Until loop changes the True or False value of the condition that the loop checks.
* When to use a Do Until loop
Use a Do Until loop whenever you need to do the following:
Loop zero or more times
Loop until a certain condition becomes True
The Do Until loop works like the following two Do While loops:
Do While Not Condition
Instructions
Loop
Do While Condition = False
Instructions
Loop
* How the Do-Loop Until Loop Works
Do-Loop Until loops not only sound like you're stuttering, but they keep repeating until a condition becomes True.
A Do-Loop Until loop looks like this:
Do
Instructions
Loop Until Condition
The condition must be a variable or an expression that represents a True or False value. A Do-Loop Until loop can hold one or more instructions.
The first time Visual Basic sees a Do-Loop Until loop, it says, "Follow all the instructions inside the loop once. Then check whether the value of the condition is True or False. If it's True, stop. If it's False, follow all the instructions inside the Do-Loop Until loop again."
For example, the following is a typical Do-Loop Until loop that counts:
Counter = 0
Do
Counter = Counter + 1
Loop Until Counter > 4
Here's what this code means:
1. The first line says, "Stuff the value of 0 inside a variable called Counter."
2. The second line says, "This is the beginning of the Do-Loop Until loop."
3. The third line says, "Take the value of Counter and add 1 (one) to it. Now store this new value in the Counter variable."
4. The fourth line says, "This is the end of the Do-Loop Until loop. As long as the value of Counter is 4 or less, keep repeating all the instructions sandwiched between the Do line and this Loop Until line. Otherwise, exit the loop."
Each time this loop runs, it increases the value of Counter by one. As soon as the value of Counter equals 5, the condition Counter > 4 suddenly becomes True and the Do-Loop Until loop stops.
* How many times does the Do-Loop Until loop repeat itself?
No matter what the condition of a Do-Loop Until loop may be, it always runs at least once. As long as the condition remains False, it keeps running until its condition becomes True.
Notice that this is the opposite of the Do While loop and the Do-Loop While loop. These two loops keep running as long as their conditions are True. The Do-Loop Until loop, on the other hand, keeps running as long as its condition is False.
To avoid an endless Do-Loop Until loop, make sure that at least one instruction inside your Do-Loop Until loop changes the True or False value of the condition that the loop checks.
* Test your newfound knowledge
1. If you need to loop until a certain condition becomes True, which type of loop would you use?
a. Either a Do Until loop or a Do While Not loop.
b. A loop twisted in the shape of a pretzel.
c. A loop-the-loop.
d. Have you noticed that if you stare at the word loop long enough, it starts to look funny?
2. When do you use a Do Until loop and when do you use a Do-Loop Until loop?
a. Whenever I need to confuse myself on how different loops work.
b. When I get mixed up and use the wrong loop by mistake.
c. I use the Do Until loop if I need to loop zero or more times. I use the Do-loop Until loop if I need to loop at least once.
d. When I can't remember how to use a Do While loop. Can you explain it to me again?
* When to use a Do-Loop Until Loop
Use a Do-Loop Until loop whenever you need to do the following:
Loop at least once
Loop until a certain condition becomes True
The Do-Loop Until loop is equal to the following two Do-Loop While loops:
Do
Instructions
Loop While Not Condition
Do
Instructions
Loop While Condition = False
When creating loops, always make sure that they eventually end and that they do exactly what you want them to do. If anything goes wrong with your program, look to see if your loops are causing the problem first.
++ CHAPTER 25
FOR NEXT LOOPS THAT CAN COUNT
In This Chapter....
Using the For Next loop
Counting backward and forward
Using the Step increment
If you want to loop until a certain condition becomes True or False, use one of the following principles:
Loops at least once:
As long as condition = True - Do Loop While condition
As long as Condition = False - Do loop Until condition
Loops zero or more times:
As long as Condition = True - Do While condition Loop
As long as Condition = False - Do Until condition Loop
All four of these types of loops keep running until a certain condition becomes True or False. But if you already know how many times you want a loop to run, use a For Next loop.
A For Next loop looks like the following:
For Counter = Start to End Step X
Instructions
Next Counter
The counter is a variable that represents an integer. Start represents the first number assigned to the value of the counter. End is the last number assigned to the value of the counter. Step is the interval to count by. If Step is omitted, the interval defaults to 1.
** How the For Next Loop Works
If you want to loop exactly three times, you can use the following code:
For X = 1 To 3
Instructions
Next X
Here's what this code means:
1 . The first line says, "Create a variable called X and set its value equal to 1. Keep looping as long as the value of X is either 1, 2, or 3. The moment the value of X is no longer one of these values, stop looping."
2. The second line is where you can shove one or more instructions (including additional For Next loops if you want).
3. The third line says, "Okay, get the next value of X (by adding 1 to the value of X) and go back to the first line. At this point, the value of X is now 2."
The following line tells Visual Basic to loop three times:
For X = 1 To 3
By default, Visual Basic counts by one. For grins and laughs, you can use any combination of numbers that you want, such as:
For X = 1209 To 1211
Instructions
Next X
This For Next loop also loops three times, although it's not as easy to tell that just by looking at it:
The first time, X = 1209.
Thesecond time, X = 1210.
The third and last time, X = 1211.
You can count by such bizarre numbers if these numbers somehow make sense to your program. For example, you can count by employee numbers:
For EmployeeNumber = 11250 To 11290
' Use the value of EmplyeeNumber to search a database
' of employees and print their background information
Next EmployeeNumber
In this case, the instructions inside your For Next loop use the value of EmployeeNumber to find a specific employee.
If you just need to loop a particular number of times, such as five, use the simplest and most straightforward method, as shown in the following:
For X = 1 To 5
Instructions
Next X
Only if your numbers must be used inside your For Next loop should you resort to bizarre, hard-to-read counting methods, such as:
For Counter = 3492 To 12909
Instructions
Next Counter
** Counting Backward and Forward
Normally, the For Next loop counts forward by 1s. However, if you want to count by 5s, 10s, 13s, or 29s, you can. To count by any number other than 1, you have to specify a Step increment. For example:
For Counter = start To end Step increment
Instructions
Next counter
Adding the Step increment instruction tells Visual Basic, "Instead of counting forward by ones, count by the value of the increment that follows the word Step." If you wanted to count by 16s, you would use the following code:
For X = 0 To 32 Step 16
Instructions
Next X
This For Next loop actually loops just three times:
The first time, X = 0
The second time, X = 16
The third time and last time, X = 32
If you want, you can even count backward. To count backward three times, you could use the following code:
For X = 3 To 1 Step - 1
Instructions
Next X
Here's what the preceding code means:
1. The first line says, "Create a variable called X, set its value to 3, and count backward by -1"
2. The second line contains one or more instructions to follow.
3. The third line says, "Choose the next value of X. Because we're counting backward by -1, the new value of X will be X -1. The second time, X will be 2."
Although Visual Basic doesn't care about how you count, always choose the simplest method whenever possible. That way, you and any other programmers can quickly see how many times a For Next loop keeps looping.
Count backward or by unusual numbers (increments of 3, 5, 16, and so on) only if instructions in a For Next loop need the numbers. Otherwise, you only make your program harder to read.
So what happens if you write a For Next loop like the following?
For J = 1 To 7 Step 5
Instructions
Next J
Here's what this code means:
1. This For Next loop repeats itself twice. The first time, the value of J is 1.
2. The second time, the value of J is 1 + 5 (remember, the value of Step is 5), or 6.
3. Before it can repeat a third time, the loop changes the value of J to 6 + 5, or 11. Because 11 is greater than the specified range of J = 1 To 7, the For Next loop refuses to loop a third time and quits.
** Use Caution When Using a For Next Loop with the Step Increment
A For Next loop must create its own variable to do its counting. For example, the following code creates a variable called X Y Z that counts by 10s:
For XYZ = 1 To 50 Step 10
Instructions
Next XYZ
And the next code creates a variable called TUV that counts by increments of 1.5:
For TUV = 1 To 7 Step 1.5
Instructions
Next TUV
For Next loops usually count by whole numbers, such as 1, 2, 5, or 58, so it's not difficult to determine the number of loops there will be. The first code example just shown counts by 10, so the number of times it will loop is pretty easy to figure out. (It loops five times.)
The second example, however, counts in increments of 1.5; because of this decimal increment, it's harder to tell how many times it will loop. (It loops five times.) When using the Step increment, use whole numbers so that you can see the number of loops more easily.
When using a For Next loop, never (and I repeat, never) change the value of the counting variable within the loop. The loop will get messed up, as the following example illustrates:
For X = 1 To 5
X = 3
Next X
Here's what happens:
1. The first line says, "Create a variable called X and set its value to 1."
2. The second line says, "Assign the value of 3 to a variable called X."
3. The third line says, "Add 1 to the value of X. Because X is equal to 3, now make X equal to 4."
Because X always equals 4 at the end of each loop, this For Next loop becomes an endless loop, which never stops. So when using For Next loops, make sure that none of the instructions inside the loop changes the counting variable. Otherwise, you'll be sorry...
** When to Use a For Next Loop
Use a For Next loop whenever you want to loop a specific number of times.
Just to show you that it is possible, you can also use other types of loops to count. The following two loops will loop exactly six times:
X = 0
Do While X < 6
X = X + 1
Instructions
Loop
For X = 1 To 6
Instructions
Next X
Notice how simple and clean the For Next loop is compared to the Do While loop. There are an infinite number of ways to write a program that works (and an even greater number of ways to write programs that don't work), but the simplest way is usually the best.
* Test your newfound knowledge
1. How many times will the following For Next loop repeat itself?
For ID = 15 To 1 Step -1
Instructions
Next ID
a. Fifteen times.
b. One time, but fifteen times as fast.
c. Zero or more times, or something like that. Wait a minute. I think I'm in the wrong lesson.
d. None, because only history can repeat itself.
2. What is the main advantage of a For Next Loop over a Do While loop?
a. A Do While loop is more complicated to use, and a For Next loop doesn't work at all.
b. It all depends on your of view, man. Like, all things are good if we only love one another and live in peace and harmony.
c. You can specify how many times you want a For Next loop to repeat itself.
d. There is no advantage to learning Visual Basic. You should be learning C++ or Java instead.
** Try a For Next Loop for Yourself
If you don't want to type any code right now, just run and study the source code for this chapter, stored on the enclosed CD-ROM.
This sample program runs through a For Next loop and prints the value of X on the screen each time it loops. To create this program for yourself, use the settings defined in the following table:
Object - Form
Property - Caption
Setting - A For Next Loop Example
Object - Command1
Property - Caption
Setting - Next X
Property - Name
Setting - cmdNext
Type the following in the Code window:
Private Sub cmdNext_Click( )
For X = 1 To 10 Step 2
Print X
Next X
End Sub
This program does nothing more exciting than loop five times and print the value of X on the form each time.
++ CHAPTER 26
NESTED LOOPS AND QUICK EXITS
In This Chapter:
Using nested loops
Making nested loops work
Exiting quickly from loops
For the ultimate in flexibility and complexity, you can jam loops inside other loops to create an endless series of loops. Whenever you have one loop stuffed inside another loop, it's called a nested loop. So which loop runs and completes first? The answer is simple.
** Using Nested Loops
When you have nested loops, the loop inside finishes first as shown in the following example:
Do While Employee = "Supervisor"
For J = 1 To 5
Instructions
Next J
Loop
Now, here's what this code means:
1. The first line says, "Create a variable called Employee and check to make sure its value is equal to "Supervisor". If it is, move to the second line. If it isn't, don't even bother looking at the For Next loop inside; simply skip to the fifth line."
2. The second line says, "Create a variable called J and set its value to 1."
3. The third line says, "Follow these instructions, whatever they may be."
4. The fourth line says, "Increase the value of J by 1 and jump back to the first line again. Keep doing this until the value of J is greater than 5."
5. The fifth line says, "This is the end of the Do While loop. Keep repeating as long as the variable Employee is equal to "Supervisor "."
In the preceding example, the For Next loop finishes before the Do While loop. The For Next loop also runs one complete time through for every thime you go through the Do While loop.
** Making Nested Loops Work
Naturally, Visual Basic gives you the complete freedom to cram as many loops inside one another as you want. When creating nested loops, indent each loop to make it easier to see where each loop begins and ends. For example, notice how confusing the following nested loops look without indentation:
Do While Name = "Sam"
Do
For K = 20 To 50 Step 10
Do
Do Until Sex = "Male"
' Change some variables here
Loop
Loop While Age > 21
Next K
Loop Until LastName = "Doe"
Loop
This is what the same code looks like with indentations:
Do While Name = "Sam"
Do
For K = 20 To 50 Step 10
Do
Do Until Sex = "Male"
' Change some variables here
Loop
Loop While Age > 21
Next K "Doe"
Loop Until LastName = "Doe"
Loop
From the computer's point of view, both nested loops work the same. But from a programmer's point of view, the nested loops using indentation are much easier to read and understand.
With so many nested loops, make sure that the inside loops don't accidentally mess up the conditions or counting variables of the outer loops. Otherwise, you may create an endless loop and have to examine all your loops to find the problem.
Another problem that can prevent nested loops from running is if they are tangled, as in the following example:
For K = 1 To 4
For J = 2 To 20 Step 2
Next K
Next J
In this example, the two For Next loops intertwine because the first For Next loop ends before the second, inner For Next loop can end. Fortunately, if you run the above BASIC code, Visual Basic catches this mistake so this type of problem is easy to correct.
** Quick Exits from Loops
A Do loop continues running until a certain condition becomes True or False. A For Next loop continues running until it finishes counting. But what if you need to exit a loop prematurely? In that case, you can bail out of a loop by using the magic Exit command.
To bail out of a Do loop, use the Exit Do command, as shown in the following example:
X = 0
Do While X < 6
X = X + 1
If X = 4 Then Exit Do
Loop
This Do While loop continues looping as long as the value of a variable called X is less than 6. The moment the value of X equals 4, Visual Basic runs the Exit Do command.
The Exit Do command bails Visual Basic out of the loop, even though the value of X is still less than 6.
To bail out of a For Next loop, use the following:
Exit For
For example:
For Y = 1 To 100
If Y = 50 Then Exit For
Next Y
Normally this For Next loop repeats 100 times but the second line tells Visual Basic to ball out of the For Next loop as soon as the value of Y equals 50 - even if the value of Y is still less than 100.
Usually (notice the emphasis on usually), a good idea is to provide a way to bail out of a loop prematurely, just in case the user needs to do something else. However, make sure that using Exit Do or Exit For doesn't kick you out of the loop before you want it to. Otherwise, you'll have another bug to hunt and track down.
If you use the Exit Do/For commands within a loop nested inside another loop, the Exit Do/For commands exit out of only the current loop and then return control to the outer loop.
* Test your newfound knowledge
1. What is the limit to the number of loops you can nest?
a. Theoretically, the number is infinity. Practically, the number is as many as you feel like typing, although the more nested your loops are, the harder it is to see what each one does.
b. The number of loops is limited to your yearly allotment, as defined by Microsoft when you send in your registration card.
c. Five.
d. Discovered by Einstein, the limit to the number of nested loops is equal to the same value that represents the speed of light.
2. To make nested loops easier to read and understand, what should you do?
a. Avoid using loops, control structures, variables, or anything else that requires,thinking.
b. Absolutely nothing. If people can't understand my nested loops, that's their problem.
C. Avoid indentation, because only amateurs need to rely on such editing tricks to write programs.
d. Use plenty of indentations to make the beginning and ending of each loop easy to find.
++ PART VII
WRITING SUBPROGRAMS (SO YOU DON'T GO CRAZY ALL AT ONCE)
In this part....
Now is the time to find out how to divide your Visual Basic program into smaller programs so you can create programs that are easier to write, modify, and understand. Rather than write one huge, monolithic program (which is like carving a mansion out of a single piece of granite), subprograms let you create miniature programs and paste them together to make one larger program (like using bricks to build a house).
By dividing one large program into several smaller ones, you can test each part of your program before moving on to writing another part of the program. By conquering each task of your program one by one, you can also maintain your sanity so that you don't go nuts trying to create one monster program in one sitting.
++ CHAPTER 27
GENERAL PROCEDURES (SUBPROGRAMS THAT EVERYONE CAN SHARE)
In This Chapter....
Creating general procedures
Naming general procedures
Using general procedures
Pocedures are small programs that make up a single larger program, much like bricks make up an entire wall. Visual Basic has two types of procedures: event procedures and general procedures.
An event procedure is part of a user interface object, such as a command button, check box, or scroll bar. Event procedures run only when a certain event occurs to a certain object, such as clicking the mouse on a command button or a check box.
A general procedure isn't attached or connected to any specific objects on the user interface. A general procedure doesn't do anything until an event procedure (or another general procedure) specifically tells it to get to work.
So do you need event procedures? Yes. Event procedures make your user interface responsive. Do you need general procedures? No. General procedures exist solely for the programmer's convenience.
If two or more event procedures contain nearly identical instructions, typing the same instructions over and over would be repetitive. Even worse, if you needed to modify the instructions, you would have to change these instructions in every event procedure that used those same instructions.
As an alternative, you can use general procedures. The whole purpose of general procedures is to hold commonly used instructions in one place. That way, if you need to modify the instructions, you change them in just one place.
** How to Create a General Procedure
After you've drawn all the objects that make up your user interface, Visual Basic automatically creates empty event procedures for all your user interface objects.
Unfortunately, Visual Basic doesn't create a single general procedure for you; you have to create it yourself. You can create and save general procedures in two types of files:
FRM (form) files
BAS (module) files
When you save a general procedure in an FRM (form) file, that general procedure can be used only by event or general procedures stored in that same FRM file. (The one exception is that any part of your program can use a general procedure that is stored in an FRM form file, but only if that form file is loaded into memory at the time.) When you save a general procedure in a BAS (module) file, however, the general procedure can be used by any event or general procedures that make up your Visual Basic program.
If you save your general procedures in a BAS file, you can create a library of useful general procedures that you can plug into any other Visual Basic programs you write. If your general procedures are useful for only one specific program, store them in an FRM file.
To create and save a general procedure in an FRM (form) file, follow these steps:
1. Click on a form file in the Project Explorer window and then press F7; choose View->Code; or click on the View Code icon to open the Code window.
2. Select (General) In the Object list box.
3. Choose Tools->Add Procedure. Visual Basic displays the Add Procedure dialog box.
4. Select the Sub radio button, type your procedure name in the Name box, and press Enter or click on OK.
Visual Basic displays an empty general procedure.
To create and save a general procedure to a new BAS (module) file, follow these steps:
1. Choose Project->Add Module. An Add Module dialog box appears.
2. Click on the Module icon and then click on Open.
3. Choose Tools->Add Procedure. Visual Basic displays the Add Procedure dialog box.
4. With the Sub radio button selected, type your procedure name in the Name box, and press Enter or click on OK.
Visual Basic displays an empty general procedure.
To create a general procedure and save it to an existing BAS file, follow these steps:
1. Choose View->Project Explorer, press Ctrl+R, or click on the Project Explorer icon on the toolbar.
2. Click on the module file where you want to save your general procedure; then click on the View Code icon.
Visual Basic displays the Code window for that particular BAS module file.
3. Choose Tools->Add Procedure. Visual Basic displays an Add Procedure dialog box.
4. With the Sub radio button selected, type your procedure name in the Name box, and press Enter or click on OK.
Visual Basic displays an empty general procedure.
** How to Name General Procedures
Unlike with event procedure names (which identify the object name and the event), you can name general procedures anything you want, with the following restrictions:
The name must be 40 characters or fewer.
The name must begin with a letter and can consist of only letters, numbers, and the underscore character (_).
The name can't be a reserved word that Visual Basic already uses such as End, Sub, or Private.
Ideally, you want to use names for your general procedures that describe what they do. For example:
CubeRoot
Ask4Password
DisplayWindow
These complete procedure names would appear in the Code window as follows:
Public Sub CubeRoot( )
End Sub
And
Public Sub Ask4Password( )
End Sub
And
Public Sub DisplayWindow( )
End Sub
Notice that a complete general procedure name consists of four parts:
Public (or Private)
Sub
Your general procedure name
A pair of parentheses ( )
The word Public tells Visual Basic that the general procedure is public. So, if the general procedure is stored in a BAS file, any event or general procedures stored in other FRM or BAS files can use it. (If you don't want procedures stored in other files to use your general procedure, just use the word Private instead.)
The word Sub identifies your subroutine as a procedure. (If you replace the word Sub with the word Function, you can create a general function. I explain functions in Chapter 29.) Your procedure name is the name that event procedures and other general procedures use to call your general procedure. Calling a procedure means telling a particular procedure, "Okay, do something now!"
The pair of parentheses is called the argument list (which you can read about in Chapter 28). The simplest general procedures have an empty argument list, represented by an empty pair of parentheses.
** How to Use a General Procedure
A general procedure contains one or more instructions. When another procedure wants to use those instructions that are stored in a general procedure, it calls this general procedure by name.
You can call a procedure in two ways. You can state the procedure's name:
ProcedureName
Or, you can state the procedure's name along with the word Call:
If you want to use a general procedure that is stored in another FRM form file, you can call it by specifying the form name and then the procedure name, such as:
FormName.ProcedureName
Or you can state the procedure's name along with the word Call:
Your computer doesn't care which method you use. But whatever method you choose, use it consistently - your program's easier to read that way.
Stating only the procedure name is simpler than using the word Call, but the latter helps identify all the procedure calls in your program. For example, consider the following general procedure stored in a form called frmMainForm:
Public Sub Warning( )
MsgBox "Your computer will blow up in 3 seconds!", 16,
"Warning!"
End Sub
This general procedure simply displays a dialog box with the message "Your computer will blow up in 3 seconds!".
If an event procedure, stored in a different form, wanted to use or call this general procedure, it would look like the following:
Public Sub cmdAlert_Click( )
frmMainForm.Warning
EndSub
If the event procedure used the Call method of calling a procedure, it would look like this:
Public Sub cmdAlert_Click( )
Call frmMainForm.Warning
End Sub
Both of these event procedures are equivalent to the following:
Public Sub cmdAlert_Click( )
MsgBox "Your computer will blow up in 3 seconds!", 16,
"Warning!"
End Sub
Although this example is simple, you can see that, if you cram two or more instructions inside a general procedure, you won't have to type these same instructions over and over again in multiple places. Instead, you can just call a single general procedure.
* Test your newfound knowledge
1. Why would you need to use a general procedure?
a. Because a more specific procedure won't do the job.
b. To store commonly used instructions in one place so that the procedure is easy to modify.
c. To keep your event procedures from getting lonely.
d. There's no reason to use general procedures - cool programmers have no need of such crutches.
2. What are the two ways to use, or call, a general procedure?
a. Telephone or mail a letter.
b. State the general procedure's name or insert the word Call before the general procedure's name.
c. See your favorite psychic and participate in a seance.
d. Get your local hog-calling contest winner to shout real loud for you.
++ CHAPTER 28
PASSING ARGUMENTS
In This Chapter....
Sending arguments to a procedure
Passing arguments by value
Quitting a procedure early
When a procedure calls a general procedure, it does so by name. Calling a general procedure by name essentially tells that general procedure, "Hey, get busy and start doing something."
Many times, the called general procedure needs no further instructions when its name is called. Sometimes, though, the called general procedure needs additional information before it can do anything.
Any procedure can call a general procedure and give it data to work with. This data is called an argument. Essentially, the first procedure is saying, "Don't argue with me. Here's all the information you need to get busy. Now get to work."
** Why Use Arguments?
An argument is data - numbers, strings, or variables (which represent a number or string) - that a general procedure needs to work with. By using arguments, you can write a single, nonspecific procedure that can replace two or more specialized general procedures.
For example, you can create two general procedures, as follows:
Public Sub DisplayWarning( )
TxtReadMe.Text = "Warning! Nuclear meltdown has occurred!"
End Sub
and
Public Sub DisplayCaution( )
txtReadMe.Text = "Caution! Turn off the reactors now!"
End Sub
To use either procedure, you call them in one of two ways:
DisplayWarning or Call DisplayWarning
DisplayCaution or Call DisplayCaution
However, creating two procedures that do almost the same thing is tedious and wasteful, which makes it a perfect technique to use in a job you don't like.
Instead of writing near-duplicate copies of the same procedure, you can replace both of those procedures with a single one, such as the following:
Public Sub Display(Message As String)
txtReadMe.Text = Message
End Sub
This new procedure says, "Create a variable called Message that holds any string that another procedure will give me. Whatever this value may be, stuff it in the Text property of the text box called txtReadMe.
Using a general procedure, you can choose the type of text by calling the Display procedure like this:
Display ("Warning! Nuclear meltdown has occurred!")
Or
Display ("Caution! Turn off the reactors now!")
** Sending Arguments to a Procedure
To call a procedure and send an argument to it, you can use one of three methods:
ProcedureName Argument
ProcedureName (Argument)
Call ProcedureName (Argument)
Suppose you had the following general procedure:
Public Sub Display(Message As String)
TxtReadMe.Text = Message
End Sub
To call the preceding procedure and display the message "Warning! Nuclear meltdown has occurred!", you can use one of three methods:
Display "Warning! Nuclear meltdown has occurred!"
Display ("Warning! Nuclear meltdown has occurred!")
Call Display ("Warning! Nuclear meltdown has occurred!")
All three methods are equivalent.
Now here's what calling the Display general procedure does:
1. First it tells Visual Basic, "Find a general procedure named Display and send it one argument." In this case, the one argument is the string, "Warning! Nuclear meltdown has occurred!"
2. Visual Basic finds a general procedure called Display. The Display procedure says to assign whatever argument it gets to the variable Message.
3. Then the Display procedure says, "Stuff the value of Message into the Text property of the text box called txtReadMe. Because the value of Message is the string "Warning! Nuclear meltdown has occurred!", stuff this value into the Text property."
If you call the procedure in the following way:
Display ("Caution! Turn off the reactors now!")
the Text property of the text box called txtReadMe gets stuffed with the string "Caution! Turn off the reactors now!"
By using the same procedure but feeding it different arguments, you replace two specialized general procedures with a single general procedure.
** Accepting Arguments
Before a general procedure can accept arguments, you have to define the procedure's argument list. Essentially, this list defines how many arguments the general procedure can take.
For example, to define a general procedure that won't take any arguments, you use a pair of empty parentheses, as follows:
Public Sub NoBackTalk( )
End Sub
To call this procedure, you can choose one of two ways:
NoBackTalk
Call NoBackTalk
To define an argument list that takes one argument, you do the following:
Public Sub BackTalk(Something)
End Sub
In this case, the variable called Something is by default a Variant data type (see Chapter 15 for more information about data types), which can represent a number or string. To call this procedure and give it the argument 4, you can do one of the following:
BackTalk 4
BackTalk (4)
Call BackTalk (4)
To define an argument list that takes two or more arguments, you have to specify a variable for each argument and separate each by a comma. For example, the following defines three arguments:
Public Sub Chatty(Message, Reply, Gossip)
End Sub
This argument list defines three arguments that can represent a number or a string. To call this procedure and give it the arguments 30, "Hello", and 12.9, you must do one of the following:
Chatty 30, "Hello", 12.9
Call Chatty (30, "Hello", 12.9)
There is no practical limit to the number of arguments that a procedure can accept. However, the longer your argument list is, the more complicated your procedure must be and the more likely you may get confused trying to understand exactly what your procedures do.
** Defining argument types
Besides defining the number of arguments in an argument list, you have the option of defining the type of data each argument must represent.
For example, you can define an argument to represent only one of the following data types (see Chapter 15 for more information about data types):
Integer
Long
Single
Double
Currency
String
You have to define argument types in the argument list. For example:
Public Sub Convert(Fahrenheit As Integer, Celsius As Integer)
The preceding line of code defines two arguments, both of which must represent an integer. The following code shows the only procedure call that works:
Public cmdTest_Click( )
Dim X, Y As Integer
Dim A, B As String
Dim M, N As Single
Call Convert(X, Y) ' This would work
Call Convert(A, B) ' Neither A nor B are Integers
Call Convert(M, N) ' Neither M nor N are Integers
Call Convert("Hello", X) ' "Hello" is not an Integer
End Sub
The main reason to specify an argument's type is to prevent your procedure from trying to work with incorrect data. For example, if your procedure expects a string but gets an integer, your program can crash. Even worse, it may run correctly but contain the wrong information. In this case, you have created a bug called a logic error. (For more about logic errors, check out Chapter 20)
Although you don't have to declare an argument's type, doing so is a good idea, just to help prevent any bugs from breeding inside your precious code.
** Problems with sending arguments
Two problems may occur when calling procedures. One problem is when the number of arguments sent doesn't match the number of arguments defined by the procedure. The other problem occurs when the types of arguments sent don't match the types of arguments defined by the procedure.
** Giving the wrong number of arguments:
When you define a procedure with an argument list, the argument list defines the number of arguments it needs to run. If you call this procedure and don't give it the correct number of arguments, the procedure doesn't work. For example:
Public Sub ArgueWithMe(Flame)
End Sub
This procedure expects one argument, which can be a number or a string. None of the following calls to this procedure works, because the number of arguments is not one:
ArgueWithMe
ArgueWithMe 9, "Shut up!"
ArgueWithMe("Why?", "Go away!", 4500, "Okay.")
** Giving the wrong type of arguments:
Likewise, when calling a procedure, always make sure that the arguments have the same data types as the types defined in the argument list. For example:
Public Sub ArgueWithMe(Flame As String)
End Sub
This procedure expects one argument, which must be a String data type. None of the following calls to this procedure work because the arguments are not String data types:
ArgueWithMe(78.909)
ArgueWithMe(9)
Call ArgueWithMe(34)
** Passing arguments by value
To further protect your procedures from messing around with each other's variables, you can pass arguments by value. Normally, when you pass an argument to a procedure, the new procedure can change the argument's value. This is like giving somebody a drink and having them spit in it and hand it back to you.
If another procedure needs certain data, but you don't want that other procedure to risk changing the data used by other parts of your program, you can pass an argument by value. So, you give a procedure an argument, and the procedure can change the argument all it wants. However, any changes it makes to this variable are limited within that particular procedure. This situation is like pouring part of your drink into another cup and giving it to another person. If that person decides to spit in his or her cup, your drink remains uncontaminated.
To define an argument as passed by value, you use the ByVal keyword in the argument list, like this:
Public Sub ShowMe(ByVal Name As String)
You never need to use the ByVal keyword to call a procedure.
Suppose you have an event procedure such as the following:
Private Sub Command1_Click( )
Dim MyString As String
MyString = "John Doe"
ShowMe MyString
txtMessage.Text = MyString
End Sub
and a general procedure such as the following:
Public Sub ShowMe (ByVal Name As String)
Name = UCase(Name)
txtNewMessage.Text = Name
End Sub
Visual Basic follows the instructions in the Command1_Click( )event procedure in the following way:
1. When the user clicks on a command button named Command 1, Visual Basic starts to follow the instructions in the Private Command1_Click( ) event procedure.
2. The second line in the Command1_Click( ) event procedure tells Visual Basic to declare a variable called MyString that can only hold string values.
3. The third line stuffs the string "JohnDoe" into the MyString variable.
4. The fourth line calls the general procedure called ShowMe and sends it the value of the MyString variable as a single argument.
5. Visual Basic immediately jumps to the general procedure called ShowMe. The ByVal keyword tells Visual Basic that any changes it makes to the argument received are isolated within the ShowMe general procedure.
6. The first line of the ShowMe general procedure defines a new variable called Name and defines it to hold a string.
7. The second line of the ShowMe general procedure says to use the UCase keyword to turn all the letters of the Name variable into uppercase and store the result back in the Name variable. In this case, the value of the Name variable is now "JOHN DOE ".
8. The third line of the ShowMe general procedure says to display the Name value in the Text property of a text box named txt NewMessage. In this case, the txtNewMessage text box displays "JOHN DOE " .
9. The fourth line of the ShowMe general procedure says this is the end of the general procedure; now go back to the procedure that was originally called the ShowMe general procedure. This information tells Visual Basic to jump back to the fifth line of the Command1_Click( ) event procedure.
10. The fifth line of the Command1_Click( ) event procedure says to take the value of MyString and stuff it in the Text property of a text box named txtMessage. In this case, the txtMessage text box displays "John Doe".
11. The sixth line of the Command1_Click event procedure tells Visual Basic to stop thinking anymore.
If you remove the ByVal keyword from the ShowMe general procedure, the txtMes sage text box in Step 10 displays "JOHN DOE" . The ByVal keyword simply keeps the general procedure from messing up variables used by other procedures.
To specify that an argument be passed by value, you must put the ByVaI keyword in front of each argument:
Public Sub BlackBox(ByVal X As Integer, Y As Integer)
In the preceding example, only the argument X is passed by value. The Y variable is not. To specify that the Y argument also be passed by value, you have to do the following:
Public Sub BlackBox(ByVal X As Integer, ByVal Y As Integer)
** Quitting a Procedure Prematurely
Normally, a procedure runs until all its instructions are followed. However, you may want to exit a procedure before it finishes.
To exit a procedure prematurely, you have to use the following code:
Exit Sub
For example, you may have a procedure like the one that follows:
Public Sub Endlessloop( )
X = 0
Do
X = X + 1
If (X - 13) Then
Exit Sub
End If
Loop Until X = 25
End Sub
Normally, this loop would keep repeating until the value of X equals 25. However the If-Then statement inside the Do - Loop causes this procedure to end when X equals 13.
** Try Passinig Arguments Yourself
If you want to try your hand at a passing argument, head over to the CD. There you find a sample program that lets you type text in a text box and see how passing a variable by value (using the ByVal keyword) causes the variable's value to change only within that particular procedure. This program is stored under the name CALL.VBP.
* Test your newfound knowledge
1. Explain why argument passing is useful in writing programs.
a. Argument passing is like passing the buck. Programmers do this all the time to avoid taking responsibility when their project is behind schedule.
b. Passing arguments lets you write one general-purpose procedure to replace two or more specialized procedures.
c. Arguments let you give wrong information to your procedures, so they have twice as many chances of wrecking your entire project.
d. Argument passing is like scream therapy. Each side argues for its own point until both sides are exhausted, which prevents people from shooting each other at work.
2. Explain what the following argument list means:
Public Sub Confusion(ByVal Catch As String, X As Integer, Z)
End Sub
a. Crud, I have to review this section all over again.
b. Now I finally understand why programmers look and act the way they do, if they have to spend eight hours a day deciphering cryptic commands like this.
c. Someone didn't comment the code correctly to make this argument easy to understand.
d. The procedure expects three arguments. The first argument is called Catch, is passed by value, and must be a String data type. The second argument is called X and must be an Integer data type. The third argument is called Z and can be any data type.
++ CHAPTER 29
FUNCTIONS, A UNIQUE TYPE OF SUBPROGRAM
In This Chapter....
Calling functions
Defining argument types
Quitting a function early
Functions return a single value. Procedures on the other hand, return zero or more values. When you need to calculate only a single value, use a function. When you need to calculate zero or more than two values, use a procedure.
A typical function looks like the following:
Public Function FunctionName(ArgumentList) As DataType
FunctionName = SomeValue
End Function
The word Public tells Visual Basic that if the function is stored in a BAS file, that this function can be used by all event and general procedures in your Visual Basic program. (If the function is stored in an FRM file, this function can be used only by event and general procedures stored in that same FRM file.)
The word Function defines the subprogram as a function. The FunctionName can be any valid Visual Basic name, preferably one that describes what the function does. The Argument List can containzero or more arguments. The DataType defines what type of data the function returns, such as an integer or string.
Visual Basic includes several built-in functions that you can use in your programs. Table 29-1 lists a few of these built-in functions.
||
TABLE 29-1
SOME COMMONLY USED VISUAL BASIC FUNCTIONS
Built-In Visual Basic Function - Abs(number)
What It Does - Returns the absolute value of a number
Built-In Visual Basic Function - Date
What It Does - Returns the current system date
Built-In Visual Basic Function - Lcase(string)
What It Does - Converts a string to lowercase
Built-In Visual Basic Function - Sqr(number)
What It Does - Returns the square root of a number
** How to Create a Function
You can create and save functions in two types of files:
FRM (form) files
BAS (module) files
When you save a function in an FRM (form) file, that function can be used only by procedures or functions stored in that same FRM file. (The one exception is that any part of your program can use a function stored in an FRM form file, but only if that form file is loaded into memory at the time.) When you save a function in a BAS (module) file, the function can be used by any procedures or functions that make up your Visual Basic program.
If you save your functions in a BAS file, you can create a library of useful functions that you can plug into any other Visual Basic programs you write. If your functions are useful for only one specific program, store them in an FIRM file.
To create and save a function in an FRM (form) file, follow these steps:
1. Click on a form file in the Project Explorer window and then press F7, choose View->Code, or click on the View Code Icon to open the Code window.
2. Select (General) in the Object list box.
3. Choose Tools->Add Procedure. Visual Basic displays the Add Procedure dialog box.
4. Click the Function radio button. Then type your function name In the -Name box, and press Enter or click on OK.
Visual Basic displays an empty function.
To create and save a function to a new BAS (module) file, follow these steps:
1. Choose Project->Add Module. An Add Module dialog box appears.
2. Click on the Module Icon and then click on Open.
3. Choose Tools->Add Procedure. Visual Basic displays the Add Procedure dialog box.
4. Click the function radio button. Then type your function name In the Name box, and press Enter or click on OK. Visual Basic displays an empty function.
To create a function and save it to an existing BAS file, follow these steps:
1. Choose View->Project Explorer, press Ctrl+R, or click on the Project Explorer Icon on the toolbar.
2. Click on the module file where you want to save your general procedure. Then click on the View Code Icon.
Visual Basic displays the Code window for that particular BAS module file.
3. Choose Tools->Add Procedure. Visual Basic displays an Add Procedure dialog box.
4. Click the function radio button. Then type your function name in the Name box, and press Enter or click on OK. Visual Basic displays an empty function.
** Assigning a Value to a Function
Somewhere inside the function, you must assign the function's name to a value or an expression, such as the following:
Public Function YardsToMeters(Yards As Single) As Single
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
If you don't assign a value to a function's name, the function can't return any value - and the whole point of using functions is to return a value.
You can also define the specific data type of the value that a function returns, such as Integer, String, or Currency. For everything you've ever wanted to know about data types, check out Chapter 15.
The three main differences between a function and a procedure are as follows:
A function can return only one value. A procedure can return zero or more values.
Somewhere inside the function, the function's name must be assigned a value. You never have to do this with a procedure.
You can define the data type that a function represents. You cannot define a procedure to represent a data type (but you can define the data types of a procedure's argument list).
** Calling Functions
Calling a function is different than calling a procedure. Because functions represent a single value, you call a function by assigning the function name to a variable:
Public Function YardsToMeters (Yards As Single)
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
Private Sub cmdConvert_Click( )
Dim Meters As Single
Meters = YardsToMeters(CSng(txtYards.Text))
TxtMetric.Text = Cstr(Meters)
End Sub
This event procedure says, "When the user clicks on a command button called cmdConvert, do the following:"
1. Create a variable called Meters and define this variable to hold only Single datatype.
2. Take whatever value is stored in the Text property of a text box called txtYards and use the value as an argument for the YardsToMeters function.
3. The YardsToMeters function takes txtYards.Text as its argument, multiplies the argument by 0.9, and stores this new result in the YardsToMeters function name. The result stored in the YardsTo Meters function name gets stuffed into the Meters variable.
4. The value stored in the Meters variable gets converted into a string and stuffed into the Text property of a text box named txtMetric.
Note the differences in calling procedures and calling functions. When calling a procedure, you can use one of three methods:
ProcedureName ArgumentList
ProcedureName(ArgumentList)
Call ProcedureName(ArgumentList)
You have only one way to call a function:
Variable = FunctionName(ArgumentList)
Because a function name represents a single value, you can use a function name in any mathematical expression, such as:
Variable = FunctionName(ArgumentList) + Variable
So a procedure that calls a function called YardsToMeters may look like this:
Private Sub cmdConvert_Click( )
Dim Meters As Single, NewValue As Single
NewValue = (YardsToMeters (Meters) + 32) * 4
End Sub
** Defining a Function as a Certain Data Type
Because a function returns a single value, you can specify what data type that value represents.
Take a look at the following, for example:
Public Function YardsToMeters (Yards) As Single
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
This defines the value of YardstoMeters as a Single datatype. You can define a function to represent any one of the following:
Integer
Long
Single
Double
Currency
String
No matter what data type a function represents, any variables assigned to the function must be of the same data type. For example:
Public Function YardsToMeters(Yards) As Single
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
Private Sub cmdConvert_Click( )
Dim Meters As Single
Meters = YardsToMeters(CSng(txtYards.Text))
txtMetric.Text = CStr(Meters)
End Sub
In this example, the variable Meters is defined as a Single data type, and the YardsToMeters function is also defined as a Single data type.
If Meters is defined as the following:
Dim Meters As String
the line
Meters = YardsToMeters(CSng(txtYards.Text))
can't work because Meters is a String data type, and YardsToMeters returns a Single data type value. Because Meters expects a string but YardsToMeters gives Meters a number, the program won't work.)
* Defining argument types
Arguments are data (numbers, strings, or variables, which represent a number or a string) that a function needs to be able to work.
In addition to defining the number of arguments in an argument list, you have the option of defining the type of data each argument must represent.
For example, you can define an argument to represent only one of the following:
Integer
Long
Single
Double
Currency
String
To define an argument type, you have to define the type in the argument list:
Public Function Convert(Fahrenheit As Integer, Celsius As Integer)
This example defines two arguments and both must represent an Integer data type. The following shows the only procedure call that can work:
Private Sub cmdTest_Click( )
Dim X, Y, Z As Integer
Dim A, B, C As String
Dim L, M, N As Single
Z = Convert(X, Y) ' This would work
C = Convert(A, B) ' Neither A nor B are Integers
L = Convert(M, N) ' Neither M nor N are Integers
Z = Convert("Hello", X) ' "Hello" is not an Integer
End Sub
The main reason to specify an argument's type is to prevent a function from working with the wrong type of data. For example, if your function expects an integer but gets a string, your program can crash. Even worse, your program may create a logic error. Although you don't have to declare an argument's type, you should - just to prevent any bugs from wrecking your
program.
* Test your newfound knowledge
1. When do you want to use a function and when do you want to use a procedure?
a. In Chapter 27, 1 read about procedures, so that's where you use a procedure. in this chapter, you use only functions, because that's what this chapter covers.
b. Use a function when you need to calculate a single value. Use a procedure to calculate zero or more values.
c. Functions and procedures are identical except that they use different names, have different purposes, and don't look the same.
d. You use a function only if you're too wimpy to use a procedure, like a real programmer does.
2. Which line is a function call and which line is a procedure call?
Private Sub cmdDisplay( )
Dim Alex, Pete, George As Double
Pete = 3
George = 0
Alex =
Clockworkorange(Pete, George) ' Line 5
ConditionedBehavior(Alex) ' Line 6
End Sub
a. line five is a function call, because a variable is assigned to the value represented by the function name. Line six is a procedure call, because the function name is not assigned to a variable.
b. Both lines five and six are procedure calls, because I think this is a trick question and because guessing wrong 99 percent of the time since I started answering these question sections of this book still hasn't convinced me that the right answer is always the most obvious one.
c. Line six is a function call because this line looks different from line five, which is also a function call.
d. I don't know the answer, but this question looks like something organic that's really mechanical, much like a clockwork orange. Have you read the book, with the original ending, by Anthony Burgess?
* Problems sending arguments
Two types of problems may occur when calling functions. One problem is when the number of arguments sent doesn't match the number of arguments defined by the function.
Another problem is when the types of arguments sent don't match the types of arguments defined by the function.
Giving the wrong number of arguments:
When you define a function with an argument list, the list specifies the number of arguments that the function needs to run. If you call this function and don't give the function the right number of arguments, the function isn't going to work. For example:
Public Function Flame(Mail)
End Function
This function expects one argument, which can be a number or a string. None of the following calls to this function work, because the number of arguments is not one:
X = Flame
X = Flame(9, "Shut up!")
X = Flame("Why?", "Go away!", 4500, "Okay.")
Giving the wrong type of arguments:
Likewise, when calling a function, always make sure that the arguments have the same data types as the types defined in the argument list:
Public Function Flame(Mail As String)
End Function
This function expects one argument, which must be a String data type. None of the following calls to this function can work because the arguments are not String data types:
X = Flame (78.909)
X = Flame (9)
X = Flame (34)
* Quitting a Function Prematurely
Normally, your function runs until all the function's instructions have been followed. However, you may want to exit a function before the function is finished.
To exit a function prematurely, you have to use the following code:
Exit Function
Before you exit a function, make sure that you also assign a value to the function name; otherwise, your program may not work correctly.
** Try Using Functions Yourself
The following sample program lets you see how a function works. In this example, the function converts yards to meters.
In case you don't feel like typing everything out, this program is stored on the CD-ROM under the name FUNCTION.VBP.
Object - Form
Property - Caption
Setting - Converts Yards into Meters
Object - Label 1
Property - Caption
Setting - Yards:
Property - Height
Setting - 495
Property - Left
Setting - 360
Property - Top
Setting - 360
Property - Width
Setting - 1215
Object - Textbox 1
Property - Height
Setting - 495
Property - Left
Setting - 1680
Property - Name
Setting - txtYards
Property - Text
Setting - (Empty)
Property - Width
Setting - 1215
Object - Label2
Property - Caption
Setting - Meters:
Property - Height
Setting - 495
Property - Left
Setting - 360
Property - Top
Setting - 1320
Property - Width
Setting - 1215
Object - Textbox2
Property - Height
Setting - 495
Property - Left
Setting - 1680
Property - Name
Setting - txtMetric
Property - Text
Setting - (Empty)
Property - Width
Setting - 1215
Object - Command1
Property - Caption
Setting - &Convert
Property - Height
Setting - 495
Property - Left
Setting - 840
Property - Name
Setting - cmdConvert
Property - Top
Setting - 2400
Property - Width
Setting - 1215
Object - Command2
Property - Caption
Setting - E&xit
Property - Height
Setting - 495
Property - Left
Setting - 2520
Property - Name
Setting - cmdExit
Property - Top
Setting - 2400
Property - Width
Setting - 1215
Type the following in the Code window:
Public Function YardsToMeters(Yards) As Single
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
Private Sub cmdExit_Click( )
Unload Me
End Sub
Private Sub cmdConvert_Click( )
Dim Meters As Single
Meters = YardsToMeters(CSng(txtYards.Text))
txtMetric.Txt = CStr(Meters)
End Sub
++ CHAPTER 30
GETTING SOME CLASS WITH OBJECT-ORIENTED PROGRAMMING
In This Chapter....
Defining object-oriented programming
Creating objects in Visual Basic
Using Visual Basic objects
In an attempt to keep up with the latest programming fads, Visual Basic gives you the ability to use object-oriented programming to develop your next killer application. Of course, unless you know what object-oriented programming is and what this form of programming can do for you, this latest feature is about as useful as giving a chain saw to a monkey and expecting the monkey to build a tree house.
So before you get all excited about using object-oriented programming with Visual Basic, take some time to find out what the heck object-oriented programming is supposed to do for you in the first place.
**What the Heck Is Object-Oriented Programming?
Despite the rigors of college computer science curriculums, programming Is still more an art than a science, which means that even having a Ph.D. in computer science is no guarantee that you can write better programs than a high school dropout.
To help turn programming into a science, the computer industry has been trying to develop guidelines to help people write bug-free programs as quickly and easily as possible. The first attempt at this "Holy Grail" of programming was something called structured programming, which encouraged you to divide a program into subprograms where each subprogram performed exactly one function.
While structured programming helped people create programs that were easier to write and modify, a problem still remained. Different parts of a program often accessed the same data, such as a file stored on the disk. This meant that if you wanted to change the way the program accessed specific data, you had to exhaustively search through the entire program and find all the commands that accessed that data. Miss one command and you've just introduced a bug into your program. In case you understand pictures better than text.
So the latest focus in computer science revolves around object-oriented programming, often abbreviated as OOP as in "Oops, you just wasted four years of college studying structured programming."
The central idea behind object-oriented programming is to divide your program into isolated parts called (what else?) objects. Each object contains two parts:
Data (called properties in Visual Basic)
Commands for manipulating that data (called methods in Visual Basic)
So when your program wants to access data, your program just gives a command to the object that contains that data. Your main program never directly accesses the data but just gives commands to the object to access the data.
If you ever need to change the way your program accesses specific data, you just have to modify the commands inside that object instead of modifying commands throughout your entire program. In this way, object-Oriented programming helps isolate the commands that affect specific data, thereby reducing the chance of introducing new bugs into your program when you need to modify your program.
Object-oriented programming is being credited with making programs easier to write, modify, and reuse. However, object-oriented programming alone won't make your program useful or bug-free in any way. A poor programmer using object-oriented programming will still be less efficient than a great programmer who doesn't use object-oriented programming. Don't let the hype about object-oriented programming fool you into thinking your programs will run faster, work better, or sell more copies just because you used object-oriented programming.
** Theoretical Stuff about Class Modules
Now that you have a general idea how objects work, you may be curious how objects work in Visual Basic. To create an object in Visual Basic, you have to create something called a class module, which has a file extension of .CLS, such as NOCLASS.CLS.
The class module defines:
The type of data the object can hold (but not any actual data itself)
BASIC commands (the methods) for manipulating the data in that object
The types of data defined by an object is called the properties of an object. The BASIC commands that manipulate the data in an object are called the methods of an object. If the terms properties and methods don't seem to clarify anything, don't worry; you're not the only one who feels that way.
A class module acts like a cookie cutter because this type of module defines the object but doesn't contain any data itself, much like a cookie cutter defines the shape of a cookie but doesn't contain any cookie dough itself.
A typical class module, as shown below, consists of three parts:
Variable declarations
Property declarations
Methods, which are BASIC commands, stored in procedures, that manipulate variables and properties
In the example class module code below, the variable declaration is the line Private mvarMessage As String.
The first property declaration begins with the line Public Property Let Message (ByVal vData As String) and the second property declaration begins with the line Public Property Get Message( ) As String.
The method begins with the line Public Sub CorporateSpeak( ).
Private mvarMessage As String 'local copy
Public Property Let Message(ByVal vData As String)
MvarMessage = vData
End Property
Public Property Get Message( ) As String
Message = vbarMessage
End Property
Public Sub CorporateSpeak( )
Dim NewString As String, FrontString As String, TailString As String
Dim Location As Integer, PickOne As Integer
NewString = ""
TailString = ""
PickOne = Cint((4 * Rnd) + 1
Select Case PickOne
Case 1
NewString = "manufacture high-quality, customer-empowered"
Case 2
NewString = "service and deliver world-class"
Case 3
NewString = "create product-driven, solution-oriented "
Case Else
NewString = "customize functional, creatively-packaged "
End Select
Location = InStr(mvarMessage, "make")
If Location = 0 Then
MvarMessage = "You need to type a mission statement that uses the word 'make' in it. """
Else
FrontString = Left(mvarMessage, Location - 1)
TailString Right(mvarMessage, Len(mvarMessage) - (Location + 4))
mvarMessage = FrontString & NewString & TailString
End If
End Sub
* Declaring your variables
Declaring all variables used by your class at the beginning of your class is a good idea, just so you know what type of information your class is using. If you want to declare a variable that any part of your program can use to store or retrieve information, you can declare a public variable, such as:
Public MoneyStolen As Currency
If you declare a public variable, any part of your program (including other objects) can put data into the variable, which means debugging your program can be extremely difficult. In general, don't declare a public variable unless you have a really good reason to do so.
If you want to declare a variable that only your class needs to use, you can declare a private variable, such as:
Private Counter As Integer
* Defining an object's properties
As far as the rest of your Visual Basic program is concerned, two types of properties exist:
Properties that the program can assign a value to
Properties that the program can retrieve a value from
For example, if you have a property called Direction and you want to assign a value to this property, you have to declare the Direction property in its class by using a property definition like this:
Private mWay As Integer
Property Let Direction(ByVal WhichWay As Integer)
mWay - WhichWay
End Property
Visual Basic interprets the above code as follows:
1. The first line defines a private variable, called mWay, that can hold an integer value.
2. The second line tells Visual Basic, "Let another part of the program assign an integer value to the mWay private variable." To assign a value to an object's property, you use code such as the following:
Set m_Object = New cObject
M_Object.Direction = 5
These two lines of code tell Visual Basic to create an object using the Set and New keywords (you can find out more about these keywords, later in this chapter, in the section "Creating an object"), and then assign a value to the object's properties just like you assign a property to a text box or a label on a form.
3. The third line tells Visual Basic, "Whatever integer value gets assigned to the Direction property, store that value in the mWay private variable."
4. The fourth line tells Visual Basic, "This is the end of the property definition that lets another part of the program assign a value to a property."
To yank out information trapped in an object's property, you have to declare the Direction property in its class by using a property definition like this:
Property Get Direction( ) As Integer
Direction = mWay
End Property
Visual Basic interprets the above code as follows:
1. The first line tells Visual Basic, "The Direction property of this object can only hold integer values."
2. The second line tells Visual Basic, "Assign the integer value stored in the mWay private variable to the Direction property."
3. The third line tells Visual Basic, "This is the end of the property definition so another part of your program can get a value from the property"
To actually yank out information stored in an object's property, you can use BASIC code, such as the following:
TxtDirection.Text = CStr (mObject.Direction)
Note the subtle differences between the two property definitions. To allow another part of your program to define a value for an object's property, you need to:
Use the magical Let keyword.
Define a variable and its data type in parentheses, such as (WhichWay As Integer).
Assign a private variable (declared earlier in the class module) to equal the value of the variable identified in parentheses, such as mWay = WhichWay.
To allow another part of your program to retrieve (or Get) a value from an object's property, you need to:
Use the magical Get keyword and assign a data type to the property such as an integer.
Leave the parentheses empty, such as ( ).
Assign the actual property name (such as Direction) to a private variable, such as Direction = mWay.
You need to use the Property Let and Property Get statements for each of your object's properties.
* Writing an object's methods
After you declare any variables your object may need and define your object's properties, the next step is to write methods (procedures or functions) that do something with the data stored inside of your object.
Writing a procedure or function is fairly straightforward (for more information check out Chapters 27, 28, and 29). The main difference is that instead of using the Private keyword in front of your procedure or function declarations, you use the Public keyword such as:
Public Sub Move( )
' Some useful BASIC code goes'here
End Sub
Public Function XLocation ( ) As Integer
' Some useful BASIC code goes here
End Function
When you want to call an object's methods, you just use the object's name plus the object's method such as:
M_Object.Move
The above code tells Visual Basic, "Find an object called m-Object and call the procedure named Move." As far as your main program is concerned, the program has no idea how the Move procedure works.
* Designing a class on paper
While you could rush right into writing the BASIC code that makes up a class module, you should take some time to design your class module first. What's the best way to design a class module? None. (Now aren't you glad you bought a book that told you that?)
Actually, the optimum design for a class module depends on how you plan to use the class module. The optimum design of a class module for one program may be horrible for another type of a program. To give you some pointers in designing classes, consider the following tips:
* To determine a class module's properties, decide the basic building block of data your program needs to manipulate. If you're writing a program to store information about employees, your class module needs to include properties that contain names, addresses, phone numbers, IQs, or felony records. If you're writing a video game where cartoon aliens pop up on the screen so you can shoot them, your class module may contain the X- and Y-coordinates of your cartoon alien's position on the screen.
* To determine a class module's methods, decide what your main Visual Basic program needs to do with the information stored in your class module. For example, a class module containing employee names and addresses may need methods that allow the main program to search, sort, and print employee data. Likewise, a class module containing cartoon alien X- and Y-coordinates may need methods that allow the main program to move, display, and blow up the alien.
* After you sketch out the type of properties and methods your class module needs to include, then you're ready to create an actual class module.
** Creating a Class Module with the VB Class Builder
Because creating classes can be messy if you don't know what you're doing, Visual Basic provides a handy program called the VB Class Builder. Essentially, the VB Class Builder lets you define your class methods and properties, and then the VB Class Builder writes the BASIC code for you.
Of course, if you like doing things the hard way, you can still create class modules from scratch, but at least with the VB Class Builder, you don't have to if you don't want to waste your time.
Rather than rush right into creating a class module, you should design your object's properties and methods on paper first. Only after you're satisfied that you've designed your objects correctly should you rush into creating a class module.
* Creating a new class
You need to create a separate class module for every different object you want to use in your program. To create a class module (or to edit an existing class module), follow these steps:
1. Choose Project->Add Class Module. An Add Class Module dialog box appears.
2. Click on the VB Class Builder icon and click on Open.
3. Choose File->New->Class or click on the Add New Class icon on the toolbar.
A Class Module Builder dialog box appears. (If you just want to edit an existing class module, skip Steps 3 and 4, and just click on the class name that appears in the class window.)
4. Type a name for your class in the Name text box and click on OK.
If you don't choose a name, Visual Basic names your class something dull and generic like Class1.
The above steps only let you create and name a class module, but you still need to define properties and methods inside your class.
* Creating properties for your classes
To define the properties for a class module, follow these steps (assuming that the VB Class Builder window is already displayed):
1. Click on the class module where you want to define your properties.
2. Choose File->New->Property or click on the Add New Property to Current Class. The Property Builder dialog box
appears.
3. Type the name for your property in the Name text box.
4. Click on the downward-pointing arrow in the Data Type list box and choose a data type (such as Integer or Single) for your property.
5. Click on OK.
* Creating methods for your classes
To define the methods for a class module, follow these steps (assuming that the VB Class Builder window is already displayed):
1. Click on the class module where you want to define your methods.
2. Choose File->New->Method or click on the Add New Method to Current Class. The Method Builder dialog box appears.
3. Type the name for your method in the Name text box.
4. Click on the Add a new argument icon (the plus sign), if your methods need arguments. An argument dialog box appears.
5. Type a name for your argument in the Name text box.
6. Click on a data type (such as Integer or Currency) in the Data Type list box.
7. Click on OK.
8. Click on the downward-pointing arrow in the Return Data Type list box and choose a data type.
(Skip this step if you don't want the method to return a value.
9. Click on OK.
* Quitting the VB Class Builder
When you're done creating or editing your class modules, you can get rid of the VB Class Builder window by one of these methods:
Click on the close box of the VB Class Builder window
Choose File->Exit
If a dialog box appears, asking if you want to update your Visual Basic project, click on Yes. After the VB Class Builder window goes away, you can click on your newly created class module and view the BASIC code that Visual Basic kindly created for you automatically.
After you use the VB Class Builder to create a class module, you must still write code inside your class module to make it do something useful. Then you must write code in your main program to use the class module.
** Using a Class Module in a Visual Basic Program
After going through all this trouble creating a class module, you still have to write BASIC code in your main program to actually use the program. Before you can use an object, you have to create one. After you create an object, then you can use that object's methods or store or retrieve information in that object's properties.
* Creating an object
Even though you may have gone through the trouble of creating and defining your class module, you still have to create an object based on the design of your class module. In the world of object-oriented programming, creating an object is called creating an instance. (Once again, you can see that computer scientists are no better at choosing self-explanatory terms than other experts are in their fields.)
To create an instance of an object, you have to create an object to represent your class module using the Set and New keywords such as:
Set ObjectName = New ClassName
This is how Visual Basic interprets this single line of code:
1. The Set keyword tells Visual Basic, "Get ready to create an object."
2. The 0bjectName variable is the name of your object.
3. The New keyword tells Visual Basic, "Create a new object based on the class module defined by ClassName."
* Using an object
After you create an object, the final step is to use that object to:
Stuff a value into an object's property
Retrieve a value out from an object's property
Use an object's method to do something with the object's data
To stuff a value into an object's property, you just have to use the following code:
ObjectName.Property = Value
To retrieve a value out from an object's property, you have to use the following code:
Variable = ObjectName.Property
To use an object's method, you can use the following code:
ObjectName.Method
** Try Class Modules Yourself
Naturally the best way to understand anything is to do what you don't understand yourself, so in case this entire chapter doesn't make a whole lot of sense, try the following program. This sample program demonstrates how a main program can create an object from a class module, call an object's method to manipulate the data, and then retrieve the data afterwards.
In this particular program, type your company's mission statement in the top text box, making sure that this mission statement includes the word "make," such as "We make cars." Then this program substitutes the word "make" for a more acceptable corporate term that makes no sense such as "manufacture high-quality, customer-empowered."
This program is stored on the CD-ROM so you don't have to type it all in if you don't want to. Just load the MISSION.VBP file.
If you get nothing else out of this chapter, just remember that object-oriented programming is supposed to help you organize your programs so that you have fewer chances of introducing bugs into any programs that you write or modify.
Object - Form
Property - Caption
Setting - Mission Statement Maker
Property - Height
Setting - 3600
Property - Left
Setting - 0
Property - Top
Setting - 0
Property - Width
Setting - 4800
Object - Text1
Property - Height
Setting - 615
Property - Left
Setting - 480
Property - MultiLine
Setting - True
Property - Name
Setting - txtInput
Property - TabIndex
Setting - 0
Property - Text
Setting - (Empty)
Property - Top
Setting - 360
Property - Width
Setting - 3850
Object - Text2
Property - Height
Setting - 615
Property - Left
Setting - 480
Property - MultiLine
Setting - True
Property - Name
Setting - txtOutput
Property - Text
Setting - (Empty)
Property - Top
Setting - 1440
Property - Width
Setting - 3850
Object - Command1
Property - Caption
Setting - &Create
Property - Height
Setting - 495
Property - Left
Setting - 600
Property - Name
Setting - cmdCreate
Property - Top
Setting - 2400
Property - Width
Setting - 1575
Object - Command2
Property - Caption
Setting - E&xit
Property - Height
Setting - 495
Property - Left
Setting - 2640
Property - Name
Setting - cmdExit
Property - Top
Setting - 2400
Property - Width
Setting - 1575
Double-click on the two command buttons on the form and create the following event procedures:
Private Sub cmdCreate-Click( )
Set DoubleSpeak = New CorporateTalk
DoubleSpeak.Message = txtInput.Text
DoubleSpeak.CorporateSpeak
TxtOutput.Text = DoubleSpeak.Message
End Sub
Private Sub cmdExit_Click( )
Unload Me
End Sub
Create a separate class module, give this module a name of CorporateTalk, create a property named Message, and create a method named CorporateSpeak. You will need to type cod in the CorporateSpeak method:
Private mvarMessage As String ' local copy
Public Property Let Message(ByVal vData As String)
MvarMessage = vData
End Property
Public Property Get Message( ) As String
Message = mvarMessage
End Property
Public Sub CorporateSpeak( )
Dim NewString As String, FrontString As String, TailString As String
Dim Location As Integer, PickOne As Integer
NewString = ""
TailString = ""
PickOne = Cint((4 * Rnd) + 1)
Select Case PickOne
Case 1
NewString = "manufacture high-quality, customer-empowered "
Case 2
NewString = "service and deliver world-class "
Case 3
NewString = "create product-driven, solution-oriented "
Case Else
NewSTring = "customize functional, creatively-packaged "
End Select
Location = InStr(mvarMessage, "make")
If Location = 0 Then
MvarMessage = "You need to type a mission statement that uses the word 'make' in it." " "
Else
FrontString = Left(mvarMessage, Location - 1)
TailString = Right(mvarMessaage, Len(mvarMessage) - (Location + 4))
MvarMessage = FrontString & NewString & TailString
End If
End Sub
++ CHAPTER 31
MANAGING FILES
In This Chapter....
Determining how Visual Basic stores a program
Playing with the Project Explorer window
Adding Form, Module, and Class files
Plugging ActiveX controls into your programs
In the old days, a single program consisted of a single file. You just had to modify that one file and you could modify the entire program. But as programs have gotten more complicated, they now consist of two or more files that make up a single program. This chapter explains how Visual Basic manages these multiple files so you can keep track of them all.
** How Visual Basic Stores a Program
When you use a word processor, you can type one sentence or 3,000 pages and your word processor can store all your text in a single file. However, when you use Visual Basic, Visual Basic always (yes, always) saves your program as two or more separate flies. Table 31-1 lists the most common types of Visual Basic files you may see, along with their identifying threeletter file extension.
Every Visual Basic program must contain exactly one Project (.VBP) file. The Project file lists all the separate files (Form files, Module files, ActiveX controls, and Class files) that make up a single Visual Basic program.
A Form file contains one window that makes up the user interface of your program along with the BASIC code that tells any button (check boxes and so on) on the window how to work. Most Visual Basic programs consist of one or more Form (.FRM) files.
A Module file contains BASIC code that performs some sort of calculation independent of the user interface. Visual Basic programs can contain zero or more Module (.BAS) files.
An ActiveX control is a miniature program that you can plug into your own program to add features. Some popular ActiveX controls let you quickly add word processing, charting, or spreadsheet features to your Visual Basic programs. You can buy ActiveX controls or write your own. A Visual Basic program may contain zero or more ActiveX (.OCX) controls.
A Class file contains BASIC code that defines different classes used by your Visual Basic program. A Class is simply a funny way to organize data that your program uses. (For more information about classes, see Chapter 30.) A Visual Basic program may contain zero or more Class (.CLS) files.
You can store your Project file in one folder and your other files (Form, Module, Class, or ActiveX files) in entirely different folders. However, if you don't store all files for a single program in the same folder, you may have difficulty modifying your program later if you can't find all the files that make up your program.
||
TABLE 31-1
TYPICAL VISUAL BASIC FILES AND THEIR FILE EXTENSIONS
File Type - Project File
Three-Letter File Extension - .VBP
Sample File Name - VIRUS.VBP
File Type - Form File
Three-Letter File Extension - .FRM
Sample File Name - MAINMENU.FRM
File Type - Module File
Three-Letter File Extension - .BAS
Sample File Name - LIBRARY.BAS
File Type - ActiveX Control
Three-Letter File Extension - .OCX
Sample File Name - RICHTX32.OCX
File Type - Class File
Three-Letter File Extension - .CLS
Sample File Name - OBJECTS.CLS
** VBP Project
A VBP project file contains a list of all the FRM, BAS, CLS, and OCX custom control files that make up a single Visual Basic program. The Project Explorer window shows you all the files stored in a VBP project file.
To help organize your files, the Project Explorer window displays them as if they were stored within separate folders, such as:
Forms
Modules
Class Modules
The Project Explorer window does not list all the ActiveX controls that your Visual Basic program uses. To see a list of ActiveX controls that a program uses, press Ctrl+T.
If you don't want to see all the files within a folder, double-click on the folder so that a Plus Sign Icon appears to the left. Then, when you want to see a list of your files within a particular folder, double-click on that folder to display all the files again.
To create a new VBP project file, follow these steps:
1. Choose File->New Project or press Ctrl+N. Visual Basic displays a New Project dialog box.
2. Click on the type of program you want to create (such as a Standard EXE or ActiveX DLL) and click on OK.
To load an existing VBP project file, follow these steps:
1. Choose File->Open Project, press Ctrl+O, or click on the Open Project icon. Visual Basic displays an Open Project dialog box.
2. Type or click on the name of the VBP project file you want to load and then click on Open.
Whenever you load a VBP project file, Visual Basic automatically loads all the files listed in the VBP project file.
To save your Visual Basic project for all eternity (or until you erase the project from your hard disk), follow these steps:
1. Choose File->Save Project or click on the Save Project Icon on the toolbar.
If you haven't saved your files (such as FRM form or BAS module flies), a Save File As dialog box appears.
2. Type a name for each of your FRM Form files and click on Save.
After you save all your Form files, Visual Basic displays a Save Project As dialog box.
3. Type a name for your VBP Project file and click on Save.
Be sure to save your flies before you try running your program. A program often has bugs that can crash your computer, which means that if you don't save your program before running it, you are going to lose all (yes, all) your changes if the computer crashes.
If you want Visual Basic to save any changes automatically before you run your program, follow these steps:
1. Choose Tools->Options. An Options dialog box appears.
2. Click on the Environment tab.
3. Click on the Save Changes or the Prompt to Save Changes radio button and then click on OK.
* Adding files to a VBP project file
The more Visual Basic programs you write, the more useful certain parts of them are going to be for future programs you may write. Fortunately, Visual Basic makes it easy to take FRM Form, BAS Module, and CLS Class files from another Visual Basic program and add them to a new Visual Basic program.
To add an FRM Form, BAS Module, or CLS Class file to a VBP project file, choose Project and then choose the type of file you want to add, such as Add Form or Add Module.
If you add a Form, Module, or Class file from another Visual Basic program, make sure that you save the file under another name. If two or more Visual Basic programs share the same Form, Module, or Class file, any changes you make to that file will affect every Visual Basic program that uses that particular file.
* Removing files from a project file
Sometimes you may want to remove a file permanently from a project file. To remove an FRM file, BAS file, or, CLS file from a VBP project file, follow these steps:
1. Press Ctrl+R; choose View->Project Explorer, or click on the Project Explorer Icon in the toolbar.
2. Click on the file (such as a Form, Module, or Class) that you want to remove.
3. Choose Project->Remove.
You can also right-click on a file in Step 2, and then left-click on the Remove command.
When you remove a file from a project file, the removed file still exists on your floppy or hard disk. Removing a file simply tells Visual Basic, "See that file over there? I don't want that file to be part of this particular program anymore, so get rid of it, but keep it saved on disk in case I want it back again." To erase or delete a file from your floppy or hard disk, use the Windows Explorer and erase the file off your floppy or hard disk for good.
** Adding (Or Removing) ActiveX Controls
An ActiveX control is a miniature program that you can plug into your Visual Basic programs to give them added features without writing much BASIC code yourself. Because ActiveX controls can help you write a program quickly and easily, many people's Visual Basic programs are nothing more than globs of ActiveX controls that they've connected by using a little bit of BASIC code.
Although Visual Basic comes with several ActiveX controls, you can buy tons of them by mail order or through the Internet. Just be careful that you don't rely too much on ActiveX controls. If a company wrote an ActiveX control that doesn't work properly, then your Visual Basic program isn't going to work properly either.
Whenever you add a custom control to a V13P project file, the icon for that custom control appears in the Visual Basic Toolbox.
To add an ActiveX control to a VBP project file, follow these steps:
1. Choose Project->Components or press Ctrl+T.
Visual Basic displays a Components dialog box that lets you add custom controls.
2. Click in the check box of the ActiveX control that you want to use and then click on OK.
Visual Basic displays that custom control's Icon in the Visual Basic Toolbox.
After you add an ActiveX control to your program, you still have to draw the ActiveX control on a form and then modify the properties or write BASIC code to make the control do anything.
Believe it or not, you can actually write your own ActiveX controls by using Visual Basic itself. (I explain ActiveX controls in more detail in the latest edition of MORE Visual Basic For Windows For Dummies, written by yours truly and also published by IDG Books Worldwide, Inc.) While writing these controls, just keep in mind that if you create a particularly useful feature in your program that you think others may find useful, you can save the feature as an ActiveX control and give or sell copies to other people.
++ PART VIII
DATABASE FILES AND PRINTING
In this part....
Most people don't care about knowing how to use a database program, and even fewer people care about all the different file formats in which data may be trapped. So to cater to the majority of people, Visual Basic can cheerfully read, write, and modify information stored inside a variety of database file formats including Microsoft Access, dBASE, Paradox, and FoxPro.
In addition to providing employment for hundreds of computer book authors and publishers, the purpose of most programs is to store information and then print or display the information again. After all, what good would a database be if you had to keep typing names and addresses into your computer every time you turned on your computer? So in this part of the book, you find out all about creating, storing, and printing information for your Visual Basic programs.
++ CHAPTER 32
CREATING DATABASE FILES
In This Chapter....
Answering the question, "What the heck is a database?"
Organizing a database
Creating your very own database
In case you need to write a program that stores and retrieves organized information such as names and addresses or inventory part numbers and customer phone numbers, you have two choices. You can either write a Visual Basic program along with tons of BASIC code to store, retrieve, and organize information on your hard disk, or you can take the easy way out and just use a database file instead.
By using database files with Visual Basic, you can spend your time designing your user interface without worrying about the details in saving, retrieving, and sorting your data.
You can use one of two existing ways to create a database file. One, you can rush out and spend hundreds of dollars buying a separate database program with a weird name like Microsoft Access, Paradox, or FoxPro. Two, you can just use the Visual Basic special add-in program called the Visual Data Manager. Because this book is about doing things the easy way, this chapter focuses primarily on using the Visual Data Manager. After all, the program comes free with Visual Basic.
** Databases For Dummies or (What the Heck Is a Database?)
Basically, a database file is nothing more than a special file designed to hold organized information. Just as a word processor file contains sentences and paragraphs, a database file contains information such as names, addresses, phone numbers, Social Security numbers, shoe sizes, secret passwords to sell to foreign agencies, and so on.
Typically a database file consists of the following parts:
One or more tables
Records
Fields
The database is (usually) a single file that contains all your information.
Tables let you organize the information stored within a database. For example, rather than toss names and addresses into a database like throwing clothes in a hamper, tables let you store one set of names and addresses as Customers and another set of names and addresses as Suppliers, Mailing List, Hot Dates, or any other type of classification you can dream up. Every database must have at least one table.
Each table consists of one or more records, where a single record contains all the information about a single item such as a person's name, address, and phone number. In the real world, a person's business card is a record.
Each record consists of one or more fields, where a field contains a single bit of information such as a name, address, hat size, or marital status.
** Organizing the Structure of a Database
Now that you have a general idea about the purpose of each part of a database file, you still need to decide exactly how you want to organize your information. Before creating a database, ask yourself, "What type of information do I need for the future?"
For example, if you're creating a database of business contacts, you may want to store names, phone numbers, e-mail addresses, and fax numbers. If you're creating an inventory list, you may want to store part numbers, part names, quantities left in stock, and the price per item.
After you decide what type of information you want to save, the next step is to create field names to identify each bit of information. Fields have two purposes in life:
To store a chunk of information
To provide a way to sort and search through a database
For example, if you want to store the name "John Doe," you can store the name by using one of the following two methods:
Method - #1
Field Name - Name
Actual Data - John Doe
Method - #2
Field Name - First Name
Actual Data - John
Field Name - Last Name
Actual Data - Doe
The first method stores a person's name as one complete chunk of information, which means that you can't search your database by first or last name.
The second method stores a person's name in two chunks of information, which can be more annoying to type in but allows you to search the database later by first or last name.
Neither method is right or wrong. The method you choose to use all depends on how you want to use your data. After you list all the fields in which you want to store your information, the next step is to decide if you want to divide your database into tables (categories). For example, if you're storing business cards, you may want to organize them according to Personal Contacts, Business Contacts, Contacts Who Owe Me a Favor, and so on.
You don't have to divide your database into two or more tables. A database can consist of just one table if that's what you need.
** Creating a Database File
After you design the structure of your database on paper, you can finally start designing your database on your computer. Although you can use a separate database program for this task (such as Microsoft Access or dBase), you may just want to use the Visual Basic add-in program called the Visual Data Manager.
* Running the Visual Data Manager
To run the Visual Data Manager program, follow these steps:
1. Load Visual Basic.
2. Choose Add-Ins->Visual Data Manager. The Visual Data Manager window appears.
3. Choose File->New. A list of database file formats such as Microsoft Access, Paradox, and FoxPro appears.
4. Choose Microsoft Access->Version 7.0 MDB.
(Unless you need to create database files in another format because everyone else you know uses Paradox or FoxPro database files, use a Microsoft Access 7.0 database.) A dialog box appears.
5. Type the name of your database file and click on Save. The Visual Data manager displays a Database Window.
6. Point the mouse cursor on the Properties Icon In the Database Window and click on the right mouse button. A pop-up menu appears.
7. Click on New Table. The Table Structure dialog box appears.
8. Type a name for your database table in the Table Name text box.
9. Click on the Add Field button. An Add Field dialog box appears.
10. Type the name of your field in the Name box, such as First Name, Employee ID, or Age.
11. Use the pull-down Type list box to choose the type of data you want to store In your field (such as Text, Currency, or Date/Time) and type the size of the data in the Size field); and then click on OK.
12. Repeat Steps 10 and 11 for each field you want to add, and then click on Close when you finish.
13. Click on the Build the Table button. The Database Window displays your table as an icon.
After you create a table, you can always edit it later. To do so, simply point the mouse cursor over the Table icon in the Database Window, click the right mouse button, and choose Design.
* Stuffing information into Your database
After you create a database file, one or more tables, and one or more fields in each table to hold your information, you eventually need to stuff your database full of information such as people's names, phone numbers, or employee ID numbers.
You don't have to stuff a database with information. You may want to leave your database empty so someone using your Visual Basic program can stuff her own information into the database.
To stuff information into a database by using the Visual Data Manager program, make sure that the Visual Data Manager program is running and follow these steps:
1. Move the mouse cursor over the Table lcon in the Database Window, click with the right mouse button, and choose Open.
A dialog box appears.
2. Click on Add.
3. Type the information you want to appear In each field.
4. Click on Update when you finish.
5. Click on Close.
* Exiting the Visual Data Manager
To exit the Visual Data Manager program, just choose File->Exit.
The Visual Data Manager is a simple program for creating databases. If you really need to create complicated databases, you are better off buying a separate copy of Microsoft Access and using that program to create your database files.
++ CHAPTER 33
USING FILES FROM DATABASE PROGRAMS YOU'D RATHER NOT USE
In this Chapter....
Connecting to a database file
Displaying different records
Searching for specific records
If you're planning to store a great deal of data, you may as well store the data in a database file. Visual Basic can store and retrieve data in any one of the following four database formats:
Microsoft Access MDB files
Borland dBASE DBF files
Microsoft FoxPro DBF files
Borland Paradox DB files
If you have no clue as to what any of these files or databases are, consider yourself lucky
Visual Basic comes with a separate program called the Visual Data Manager (see Chapter 32 for more information), which lets you create Microsoft Access database (MDB) files without shelling out the money to buy a real copy of Microsoft Access. If you want to create really sophisticated databases, you are better off buying a separate copy of that database program, such as dBASE, Microsoft Access, or Paradox.
** What Are Database Files?
Whenever one of those fancy database programs such as Paradox, Access, or dBASE saves information, that program stores the information in a disk file. Because the disk file contains a bunch of data that somebody thinks is important, the file is called a database file. Essentially, a database file is like a Rolodex jammed with junk.
To organize the data in the file, a database file is made up of one or more records. A record is like a 3-x-5 index card. Each record contains fields, which contain the specific information (names, addresses, and stuff like that).
To organize this information further, database files let you organize data into tables. A table is a subset of your entire database. The table contains only specific information, such as the names of all the people who live in Oregon or the phone numbers of everyone you can't stand.
** How to Connect to a Database File
When you want a Visual Basic program to store and retrieve information in a database file, you have to specify the following:
Which database file to use
Which recordset type to use
Which database table to use
Which database fields to display
* Connecting to a database manually
To connect your program to a database file, follow these steps:
1. Make sure that the database file to which you want to connect exists.
If not, you need to create the file by using a database program (such as Paradox or FoxPro) or the Visual Data Manager program.
2. Click on the Data Control icon from the Visual Basic Toolbox and draw the data control on your form. Figure 33-2 shows the Data Control icon on the Toolbox and drawn on a form. For more about how to use a data control, check out the section "Manipulating Different Records," later in this chapter.
3. Open the Properties window. (Press F4; choose View->Properties Window; or click on the Properties Window icon on the toolbar.)
4. Double-click on the DatabaseName property. Visual Basic displays the DatabaseName dialog box.
5. Select the database file that you want to use, such as BIBLIO.MDB (which is the sample database that comes with Visual Basic) and click on Open.
6. Click on the Recordset Type property and click on the gray downarrow button in the settings box. Visual Basic displays a list of recordset types to choose from:
0 - Table
1 - Dynaset
2 - Snapshot
7. Click on a recordset type, such as 0 - Table.
8. Click the RecordSource property and click on the gray down-arrow button in the settings box. Visual Basic displays a list of database tables you can choose from.
9. Click on a database table (such as Titles, if you choose the BIBLIO sample database that comes with Visual Basic).
If you're using Microsoft Access MDB files, choose the 0 - Table recordset type in Step 7. If you're using any other type of database, choose the 1 - Dynaset recordset type. If you need to read but not update data, go ahead and choose the 2 - Snapshot recordset type.
* Connecting to a database using the VB Data Form Wizard
In case you'd rather not do everything the hard way, Visual Basic offers a VB Data Form Wizard, which can create a new form specifically for displaying data in your Visual Basic project.
To accept default settings for your data form, you can click the Finish button any time during the following steps.
To use the VB Data Form Wizard, follow these steps:
1. Choose Project->Add Form. An Add Form dialog box appears.
2. Click on the VB Data Form Wizard Icon and click on Open. The Data Form Wizard - Introduction dialog box appears. If you have created a form that you want to base your new form on, click on the down arrow in the list box to choose a profile to use.
3. Click on Next >. The Data Form Wizard - Database Type dialog box appears.
4. Click on a database format (such as Access) and click on Next >. The Data Form Wizard - Database dialog box appears.
5. Click on the Browse button. An Access Database dialog box appears.
6. Click on the database file you want to use, click on Open, and click on Next >. The Data Form Wizard - Form dialog box appears.
7. Type a name for your form and click on one of the following:
* Single record: Creates a form that displays one record at a time.
* Grid (Datasheet): Creates a form that displays data in a spreadsheet (grid) format.
* Master/Detail: Creates a Master record form that displays data in a single record format and a Detail record source in a spreadsheet (grid) format.
* MS HFlexGrid: Creates a form that displays data in a spreadsheet (grid) format that merges and consolidates data by pivoting rows or columns.
* MS Chart: Creates a form that displays your data as a chart.
8. Click on Next >. The Data Form Wizard - Record Source dialog box appears.
9. Click on the downward-pointing arrow in the Record Source list box and choose the record you want to display.
10. Click on a field displayed in the Available Flelds list box and click on the > button for each field you want to display on your form. Click on Next > when you're done. The Data Form Wizard - Control Selection dialog box appears.
11. Click in the check boxes for each type of button you want displayed on your form (such as an Add or Delete button), and then click Next >. The Data Form Wizard - Finished! dialog box appears.
12. Click on the three black dots to the right of the down arrow if you want to save your form as a profile for future use. Click Finish. Visual Basic makes up your form.
13. Click on OK. Your form appears, ready for your editing or modifications.
If you use the VB Application Wizard to create a skeleton program for you, you can create a form for displaying database information. By creating a program that displays database information with the VB Application Wizard, you don't have to go back later and use the VB Data Form Wizard.
** Displaying Data Fields on the Screen
After you connect your program to a database file, the next step is to display the information stored in the fields of the database. (If you use the VB Data Form Wizard to create a form to display your data, you can edit your form and add or delete additional data fields.) Visual Basic gives you several ways to display database fields on a form:
Check boxes
Image and Picture boxes
Labels and Text boxes
List and Combo boxes
Grids
Check boxes display Yes and No or True and False values. If a field has a Yes or True value, the check box appears selected on the screen. If the field has a No or False value, the check box appears blank.
Image and picture boxes enable you to display graphics, as long as the database file itself contains graphics.
Text boxes and labels display information such as names, addresses, phone numbers, and quantities.
List and combo boxes enable you to display multiple choices. The combo box gives users a chance to type in something that's not included in the combo box's list.
Grids let vou display entire records as rows in a spreadsheet format.
Use a text box if you want to give the user the ability to change the displayed data. Use a label when you want to prevent a user from changing data.
To create a text box, label, check box, image box, combo box, list box, grid, or picture box to display a database field, make sure that you draw at least one data control on the form and then follow these steps:
1. Draw the text box, label, check box, image box, combo box, list box, grid, or picture box on the form that contains the data control.
2. Open the Properties window. (Press F4; choose View->Properties Window; or click on the Properties Window icon on the toolbar.)
3. Click on the DataSource property and click on the gray down-arrow button in the settings box. Visual Basic lists the names of all the data controls on the form.
4. Select a data control name (such as Data1).
5. Click on the DataField property and click on the gray down-arrow button In the settings box.
Visual Basic lists all the field names you can display. (Skip this step if you drew a grid in Step 1.)
6. Click on a field name.
When selecting a field name for a check box, make sure that the database field contains only Yes/No or True/False values. When selecting a field name for an image or a picture box, make sure that the database field contains only graphic images. Otherwise, Visual Basic becomes confused and won't know how to handle this data when your program is running.
** Manipulating Different Records
When Visual Basic loads a database, Visual Basic makes the first record of the database the current record. Any commands you issue affect only that first database record.
Choosing different records is like thumbing through a library card catalog. (For those younger people who have never seen a library card catalog, think of thumbing through a CD music collection in a music store.)
Visual Basic provides two ways to make a different record the current record:
Using the data control object
Using BASIC code
* Using the data control object to display the records in a database
The data control object has four arrows that let the user scroll throbgh different records.
The data control object is designed to look like the controls on a tape cassette or VCR. (Then again, how many people know how to program their VCR?)
The arrows on the far left and far right move to the first and last database record. The other two arrows move forward or backward, one record at a time.
The data control object lets a user choose a record. Just make sure that you have a text box or label on the screen to display the data from the current record. If you don't, the user is going to have no idea which record is current and is not going to be able to see or edit the data either.
* Using BASIC code to display the records in a database
To display the first record by using BASIC code, issue the following command:
Data1.Recordset.MoveFirst
In all these commands, Data1 refers to the name of your data control object. If you change the name of the data control, you have to use your new name instead.
To display the last record in the database, use the following command:
Data1.Recordset.MoveLast
To display the next record in the database, use the command that follows:
Data1.Recordset.MoveNext
To display the previous record in the database, use the following command:
Data1.Recordset.MovePrevious
* Adding a record
To add a record to a database, follow these steps:
1. Create a new record, using the following code:
2. Put data in the fields of this newly created record by using the following code:
Data1.Recordset.AddNew
2. Put data in the fields of this newly created record by using the following code:
Data1.Recordset.Fields("FieldName") = "NewData"
FieldName is the specific field you want to change in your database, and yes, you need the quotation marks surrounding the field name. NewData is the new information you want to store in the field, and can be a string or a number. If the information is a string, you need to place quotation marks around the information. If the information is a number, you don't need the quotation marks.
3. Save the record to the database with the following code:
Data1.Recordset.Update
Visual Basic always adds new records to the end of a database file.
* Editing a record
To edit an existing record in a database, follow these steps:
1. Display the record that you want to edit.
2. Use the following command to tell Visual Basic that you want to edit this record:
Data1.Recordset.Edit
3. Assign the new data to the specific field whose contents you want to change:
Data1.Recordset.Fields("FieldName") = "NewData"
4. Save the record to the database:
Data1Recordset.Update
* Deleting a record
To delete a record from a database, find the record that you want to obliterate from the face of the earth and then use the following two BASIC commands:
Data1.Recordset.Delete
Data1.Recordset.MoveNext
This command says, "See the currently displayed record? Delete this record now and then display the next record so that the deleted record is no longer visible."
* Counting the number of existing records
Sometimes knowing how many records a database file contains is handy. To calculate this value, use the commands that follow:
Dim TotalRecords As Long
Data1.Recordset.MoveLast
TotalRecords = Data1.Recordset.RecordCount
1. The first line says, "Create a variable called TotalRecords, and define this variable as a Long data type."
2. The second line says, "Move to the last record in the database."
3. The third line says, "Count the number of records and store this value in the variable called Total Records."
** Finding a Specific Record
To find a specific record in a database, you have to use one of the following commands:
Data1.Recordset.FindFirst "criteria"
Data1.Recordset.FindNext "criteria"
Data1.Recordset.FindPrevious "criteria"
Data1.Recordset.FindLast "criteria"
The search criteria (or criterion, in case you think the singular form of criteria doesn't look silly) specifies which field to search and the specific data you want to find. For example, to find all the records containing information on people with the last name Jefferson, your search criteria may look like the following:
*LastName = 'Jefferson'"
Notice that the data you're searching for ('Jefferson') must appear surrounded by single quotation marks. If you fall to use single quotation marks, Visual Basic isn't going to know what you want to find and isn't going to do anything, out of spite. Of course, this example also assumes that a field called LastName exists.
If you try searching for data in a nonexistent field, you get an error message.
To find the first record that meets your search criteria, use a command like the following:
Data1.Recordset.FindFirst "LastName = 'Jefferson'"
After you find one record that meets your search criteria, you can search for another record that meets your criteria. To find the next record that meets your search criteria, use a command like the following:
Data1.Recordset.FindNext "LastName = 'Jefferson'"
To find the previous record that meets your search criteria, use a command like the following:
Data1.Recordset.FindPrevious "LastName = 'Jefferson'"
To find the last record that meets your search criteria, use a command like this:
Data1.Recordset.FindLast "LastName = 'Jefferson'"
* Yanking data from a database field
Before you yank out information from a database field, find the record that you want by using the FindFirst, FindNext, FindLast, or FindPrevious command. Then yank information from the database field by using one of two methods:
Create a text box or label and set the DataField property to the field you want.
Use BASIC code to yank information out of a database and assign the information to a variable:
Dim Store As String
Store = Data1.Recordset.Fields("Fieldname").Value
1. The first line says, "Create a variable called Store and define this variable as a String data type."
2. The second line says, "Yank the data stored in the field called Fieldname and store the value in the Store variable."
* Changing data in a database
To change the contents of a database field when the field is displayed in a text box, check box, image box, or picture box, just type or paste new information inside.
To change the contents of a database field by using BASIC code, use the following commands:
Data1.Recordset.Edit
Data1.Recordset.Fields("Fieldname").Value = NewValue
Data1.Recordset.Update
1. The first command tells Visual Basic, "I'm getting ready to change some data in a database."
2. The second command replaces the current value of the field " Fieldname " with the value stored in NewValue.
3. The third command says, "Save my changes to the database."
Any time you add, delete, or edit a record in a database using BASIC code, you have to use the magic Update command to update the database file.
* Test your newfound knowledge
1. What are the steps you must follow to make your Visual Basic program use a database file?
a. First, create a database file using a separate database program such as Access or Paradox. Then draw the Data Control icon on a form, choose the database file name for the DatabaseName property of the Data Control icon, and choose a database table for the RecordSource property of the Data Control icon.
b. Buy a computer, take it back to the store because it doesn't work, hire a database programmer, fire the database programmer, and throw up your hands in despair.
c. Visual Basic can use database files? Hey, where can I find out more about this?
d. None. This is a trick question and I refuse to reveal my ignorance by selecting any of these choices.
2. What do the following lines of BASIC code do?
Data1.Recordset.AddNew
Data1.Recordset.Fields("Question") = "Did you ever accept money from overseas governments?"
Data1.Recordset.Fields("Answer") = "Yes, but I made them pay with American money."
Data1.Recordset.Update
a. The first line erases any existing databases on your hard disk, the second and third lines are superfluous, and the fourth line wipes out the rest of the files stored on your hard disk.
b. The first line adds a new record to a database file; the second line stores thestring "Did you ever accept money from overseas governments?" in the database field called "Question ";the third line stores the string "Yes , but I made them pay with American money " in the database field called "Answer"; and the fourth line updates the database with this new information, adding this new record at the end of the database file.
c. The first line confuses Visual Basic, the second line asks Visual Basic a question, the third line displays the Visual Basic answer as given under the advice of a lawyer, and the fourth line ignores the whole problem and hopes that everyone can forget what really happened.
d. Hey! You're making fun of somebody important, aren't you?
* Placing a bookmark on a specific record
You can put a bookmark on a record so that you can quickly jump back to the record at any time. To create a bookmark, you have to create a variable and define this variable as a Variant or String data type. For example:
Dim MyBookMark1 As Variant
Or
Dim MyBookMark2 As String
Each bookmark can point to only one record at a time.
To place a bookmark, display the record where you want to put it and then use the following command:
MyBookMark1 = Data1.Recordset.Bookmark
To jump back to a bookmark, use the following command:
Data1.Recordset.Bookmark = MyBookmark1
Note: Not all database files let you use bookmarks. Microsoft Access MDB files are always bookmarkable (is that a real word?), but other database types may not be, depending on whether an index exists for that particular database. (If you have no idea what an index is and don't care to know, feel free to ignore this whole discussion.) To determine whether a database file supports bookmarks, examine the Bookmarkable property, as in the following example:
If Data1.Recordset.Bookmarkable = True Then ' Can place bookmarks
ElseIf Data1.Recordset.Bookmarkable = False Then ' Cannot use bookmarks
End If
Be careful about database formats. Each time a company such as Microsoft upgrades their database program (such as Access), the new version of that database program may use a different file format. This means that Visual Basic may not be able to use the new, updated file format.
++CHAPTER 34
MAKING YOUR PROGRAM PRINT STUFF
In This Chapter....
Printing your Visual Basic project
Printing a form
Using the printer object to print lines and circles
Counting pages
A program can suck in data, manipulate data, and display data on the screen. But eventually, your program may need to print stuff out. Visual Basic provides two ways to print stuff:
Display data on a form and then print the form
Send data directly to the printer using the Printer object
** Printing a Form
Printing a form is the simplest method for printing out information. To print a form, use the following syntax:
FormName.PrintForm
This tells Visual Basic, "Find the form named FormName and send this form to the printer."
To print a form named frmAbout, use the following command:
frmAbout.PrintForm
You can print any form in your program, including invisible forms and minimized forms. The only drawback with printing forms is that the print resolution may not be very high.
Most screens display a resolution of 96 dots per inch (dpi). Most laser printers can print at resolutions of 300 or 600 dots per inch. For quick printing, the PrintForm command works well. But for higher resolution printing, print by using the Printer object instead.
** Printing with the Printer Object
The Printer object is a temporary storage space that intercepts data, cleans up data, and then sends the clean version to the printer. To print a form using the Printer object, you essentially have to redraw your form on the Printer object, using BASIC code.
The advantage of the Printer object is that this object uses the resolution of your printer. The disadvantage is that you have to write lots of BASIC code just to print a simple form.
Before you print anything using the Printer object, you have to define the top/bottom page margins and the right/left page margins.
To define the top page margin, use the following syntax, where TopValue measures the size of the top margin:
Printer.ScaleTop = TopValue
To define the bottom page margin, use the syntax that follows, where BottomValue measures the size of the bottom margin:
Printer.ScaleBottom = BottomValue
To define the left page margin, use the following syntax, where LeftValue measures the size of the left margin:
Printer.ScaleLeft = LeftValue
To define the right page margin, use the following syntax, where RightValue measures the size of the right margin:
Printer.ScaleRight = RightValue
You may have to experiment with different values for your margins until they look the way you want.
* Printing text on the Printer object
Before you can print text through the Printer object, you have to define the X and Y location where you want the text to appear.
To define this location, use this syntax:
Printer.CurrentX = Xvalue
Printer.CurrentY = Yvalue
These commands say, "Start printing all text at the location defined by the CurrentX and CurrentY properties."
To start printing at the upper-left corner of a page, use the following commands:
Printer.CurrentX = 0
Printer.CurrentY = 0
After you define where to start printing, the next step is to actually print some text. To print text on the Printer object, follow this syntax:
Printer.Print "Text string"
This command says, "Put a text string on the Printer object and print this text string at the location previously defined by the CurrentX and the CurrentY properties."
* Test your newfound knowledge
1. Why would you want to have your Visual Basic program print anything?
a. To waste paper and help contribute to global deforestation in Third World countries.
b So I can justify buying a $1,000 laser printer.
c. To provide hard copies of any important information that my Visual Basic program may create.
d. I can never justify a reason to print 'anything. Hasn't watching American politics taught you to never put down
on paper anything that may implicate you in the future?
2. Why do you need to specify X- and Y- coordinates when printing text or drawing lines or circles?
a.To make you feel like you actually learned something useful in school geometry class.
b. So you can tell Visual Basic exactly where you want your lines, circles, or text to appear on the printed page.
c, The X- and Y-coordinates define the location of your printer in relation to your computer, If you define the X and Y-coordinates incorrectly, your documents may start printing out from your toaster or air conditioner.
* Printing lines and circles on the Printer object
Printing plain text can be boring, so Visual Basic gives you the option to spice up your printouts with lines and circles. If this sounds like Visual Basic is turning your $2,000 computer and $1,000 laser printer into an Etch-ASketch, you're right.
Defining line thicknesses:
Before you start drawing lines and circles, you have to define the draw width. The smaller the draw width, the thinner your lines are going to look.
To define the draw width, use the syntax that follows:
Printer.DrawWidth = Value
To define the skinniest draw width possible, use this command:
Printer.DrawWidth = 1
To define a fatter draw width, use this command:
Printer.DrawWidth = 5
For those who really care, the values of DrawWidth can vary from 1 to 32,767. A value of 1 specifies a line one pixel wide. A value of 32,767 specifies a line of 32,767 pixels wide, which is probably wider than you are going to ever need.
Printing a 1-pixel wide line on a 600 dots-per-inch laser printer may look different than if you print a 1-pixel wide line on a 300-dpi inkjet printer.
Drawing lines on the Printer object:
To draw a line, use the syntax that follows, where x1 and y1 define the starting point of the line and x2 and y2 define the ending point:
Printer.Line (x1,y1) - (x2, y2)
Drawing circles on the Printer object:
To draw a circle, use this syntax, where x and y are the center of the circle and Radius defines the circle's radius:
Printer.Circle(X,y), Radius
* Printing Multiple Pages
Usually Visual Basic keeps printing your data until the printer runs out of room on the page. Then Visual Basic cuts your text off and starts printing a new page automatically.
However, if you want to control when Visual Basic starts printing a new page, use the following command:
Printer.NewPage
This command tells Visual Basic to start printing on a new page. (Wow! What are they going to think of next?)
* Defining the Print Quality
Depending on your printer, you can specify a range of resolutions for printing. The magic command to control print resolution is
In the preceding command, x represents a negative number between -4 and -1, or a positive number representing the specific resolution you want to use, measured in dots per inch. For example, the following list shows the print resolution of the negative numbers -4 to -1:
Printer.PrintOuality = -1 Draft resolution
Printer.PrintOuality = -2 Low resolution
Printer.PrintOuality = -3 Medium resolution
Printer.PrintOuality = -4 High resolution
In case you want to use the predefined Visual Basic constants, you can use them as follows:
Constant - vbPRPQDraft
Value - -1
Print Resolution - Draft resolution
Constant - vbPRPQLow
Value - -2
Print Resolution - Low Resolution
Constant - vbPRPQMedium
Value - -3
Print Resolution - Medium resolution
Constant - vbPRPQHigh
Value - -4
Print Resolution - High resolution
If you're the type who likes to specify the exact print resolution to use, just specify a positive number. To define 300 dpi, which is what most cheap laser printers can produce these days, you want to use the following command:
Printer.PrintQuality = 300
* Keeping a Page Count
When you're printing multiple pages, Visual Basic automatically keeps track of the page count in a property called Page. To use this page count, use the following BASIC command:
Printer.Page
* When You Finish Printing
Use the following command to tell Visual Basic you've finished printing:
Printer.EndDoc
If you neglect to use this command, guess what? Visual Basic assumes this command exists anyway as the last command to the printer, so you really don't need to use this after all. However, for good programming practice, use this command so you know exactly when your program has finished printing.
In case you want to stop printing, you can tell Visual Basic, "Hey, I changed my mind. Stop printing." To stop the printer right away, use the following BASIC command:
Printer.KillDoc
As you can see, trying to torture Visual Basic into printing data can be cumbersome and tedious. If you have the time, write your own printing routines and reuse them for each program you write. As a quicker alternative, buy an ActiveX control called vsView from VideoSoft (www.videosoft.com). This ActiveX control makes it easy to add printing capabilities to your program without writing a lot of BASIC code.
** Try Creating a Print Program Yourself
The following sample program prints a short message when you click the Click to Print command button. To create this program, use the settings specified in the following table:
Object - Form
Property - Caption
Setting - An Example of Printing
Object - Command1
Property - Caption
Setting - &Click to Print
Property - Name
Setting - cmdPrint
Type the following code in the Code window. If you don't want to give your fingers such a strenuous workout, just load the PRINTME.VBP file off the CD-ROM at the back of this book.
Privat Sub cmdPrint_Click( )
Dim TotalPages As Integer
Dim PageCount As String
' Specifies the text position to print
Printer.CurrentX = 100
Printer.CurrentY = 100
Printer.Print "This appears at the top of the page"
' Specifies a line width and location
Printer.DrawWidth = 3
Printer.Line(100, 100) - (10000, 100)
Printer.Line(100, 350) - (10000, 350)
TotalPages = Printer.Page
' Specifies a line width and location
Printer.Circle (2000, 3500), 450
Printer,CurrentX = 2750
Printer.CurrentY = 3500
Printer.Print "This is a circle. Whee!"
' Specifies where to print the page count
Printer.CurrentX = 1000
Printer.CurrentY = 400
PageCount = "TotalPages = " & Str$(TotalPages)
Printer.Print PageCount
Printer.EndDoc
End Sub
** Printing a Visual Basic Project
While this chapter has explained how to make your Visual Basic program print out data while the program is running, you may have another question, "How the heck do I print out the source code to my Visual Basic project itself?"
Fortunately, after you take all the time and effort to write a program in Visual Basic, you can print the program out for posterity to admire. Visual Basic provides three ways to print out a Visual Basic project:
Form Image: Prints your form exactly as the form appears on the screen.
Code: Prints only your BASIC code.
Form As Text: Prints the property values of all the forms and objects that make up your user interface.
To print a Visual Basic project, follow these steps:
1. Choose File->Print or press Ctrl+P. Visual Basic displays the Print dialog box.
2. Click on the Current Module (to print only the highlighted file in the Project Explorer window) or the Current Project radio button (to print the entire Visual Basic project).
3. Click on one or more of the following check boxes: Form Image, Code, or Form As Text.
4. Make sure vour printer Is turned on and click an OK.
++ PART IX
THE PART OF TENS
In this part....
Now that you've made it this far in the book (or maybe you're one of those readers who jumps to the end of a great book), you're looking for a few ideas to help you write the best Visual Basic programs possible without losing your mind in the process.
With the help of this book and Visual Basic, you can now write your own programs or get a job writing programs for others. But no matter what you may plan to do with your programming skills, browse through this part of the book to pick up tips for making the most of your newly acquired Visual Basic programming skills.
++ CHAPTER 35
THE TEN VISUAL BASIC TOPICS THAT DIDN'T FIT ANYWHERE ELSE
In This Chapter....
Finding magazines and newsletters
Visiting Visual Basic Web sites
Joining a user group
Attending technical conferences
Writing your own DLL files
Writing Visual Basic programs for the Macintosh and Linux
Now that you've reached the end of this book (even if you just skipped to this section while browsing through this book in the bookstore), you may be wondering what to do next to continue your Visual Basic education without going through the process of trial and error and driving yourself crazy in the process.
You can pick up the latest edition of MORE Visual Basic For Windows For Dummies (written by yours truly and also published by IDG Books Worldwide, Inc.; order by calling 800-762-2974 or 415-655-3172). You can also use the following list of sources to find out more about Visual Basic and get help in creating the most powerful Visual Basic programs possible.
** Buy, Read, or Steal Visual Basic Programmer's journal
Every month, look for a fresh copy of Visual Basic Programmer's Journal at your favorite magazine stand or bookstore. This magazine comes loaded with articles exploring the intricate details of Visual Basic, reviews of Visual Basic add-ons, and samples of Visual Basic code that you can copy (steal) for your own use.
Unlike other magazines, one of the more useful features is the advertising. If you need an ActiveX control to make your programming task easier, then browse through the ads and you'll probably find what you need. For more information about this magazine, contact
Visual Basic Programmer's Journal
Fawcette Technical Publications
209 Hamilton Avenue
Palo Alto, CA 94301-2500
Tel: 415-833-7100
Fax: 415-853-0230
www.windx.com
** Spend a Bundle of Money and Get a Visual Basic Newsletter
For more detailed information, buy a Visual Basic newsletter. The Cobb Group publishes monthly newsletters for a variety of programs, including Paradox, Microsoft Office, and Visual Basic. Although a year's subscription is about $60, the Inside Visual Basic newsletter provides plenty of source code examples that you can study, modify, and claim you wrote yourself to impress your boss, who doesn't know any better.
Another company, Pinnacle Publishing, Inc., publishes another Visual Basic monthly newsletter, Visual Basic Developer, that comes with source code examples on an enclosed floppy disk. Of course, this newsletter costs a bit more (about $179 a year), but if you can get your company to buy this for you, who cares about the price?
For more information on the two publications discussed in this section, contact:
Inside Visual Basic
The Cobb Group
9420 Bunsen Parkway
Louisville, KY 40220
Tel: 502-493-3300
Fax: 502-491-8050
www.cobb.com
Visual Basic Developer
Pinnacle Publishing, Inc.
P.O. Box 72255
Marietta, GA 30007-2255
Tel: 770-565-1763
Fax: 770-565-8232
www.pinpub.com
** Visit a Web Site Dedicated to visual Basic
If you have access to the Internet, you can often find entire Web sites devoted to Visual Basic programming. While the number of specialized Visual Basic Web sites continue to grow, here's a short list of the more popular Web sites you may want to visit:
Carl and Gary's Visual Basic Home Page at the following Web address:
www.apexsc.com/vb
Chris & Tim's VB Programming Resources at the following Web address:
www.zetnet.co.uk/rad/index.html
VB Tips & Tricks Home Page at the following Web address:
www.vbtt.com
** Attend a Visual Basic Technical Conference
Every few months, Microsoft and Visual Basic Programmer's Journal sponsor a Visual Basic Technical Summit somewhere around the world in the United States, Europe, and Asia. These conferences are great places to learn techniques from real-life Visual Basic programming experts, listen to the latest propaganda talks from Microsoft representatives, buy Visual Basic add-ons cheaply from vendors, and make lots of contacts in the Visual Basic world. For more information about these technical conferences, contact:
Visual Basic Programmer's Journal
Fawcette Technical Publications
209 Hamilton Avenue
Palo Alto, CA 94301-2500
Tel: 415-833-7100
Fax: 415-853-0230
www.windx.com
** Shop from Mail-Order Dealers
Don't buy Visual Basic or any Visual Basic add-on programs direct from the publisher. Most software publishers cheerfully charge full retail price for their programs, which makes as much sense as paying full sticker price for a used car.
Rather than buy direct from the software publisher, shop by mail order instead. Mail-order dealers give you even deeper discounts (up to 50 percent in some cases), with the added advantage of saving you from having to pay sales tax.
Two popular mall-order dealers are VBxtras and ZAC Catalog. Both companies specialize in selling Visual Basic add-ons at substantial discounts, so give them a call and ask for a free catalog.
For more information on these mail-order companies, contact:
ZA Catalog
1090 Kapp Drive
Clearwater, FL 33765-2111
Tel: 813-298-1181
Fax: 813-461-5808
www.zaccatalog.com
VBxtras
1905 Powers Ferry, Suite 100
Atlanta, GA 30339
Tel: 770-952-6356
Fax: 770-952-6388
www.vbxtras.com
** Extend Visual Basic by Writing your Own DLL Files
If you plan to write lots of Visual Basic programs, you need to organize your commonly used procedures in separate BAS module files. That way you can quickly plug in BAS module files to any Visual Basic programs you write.
Unfortunately, BAS module files can contain only Visual Basic commands. Because languages such as C++ and Pascal can be more flexible in digging into the guts of your computer and manipulating individual bits and bytes, many programmers write commonly used procedures in C++ or Pascal and store them in separate Dynamic Link Library (DLL) files. A DLL file contains commonly used procedures that you can share among different programs.
You can write a DLL file using C++, Pascal, or even Visual Basic. But no matter which language you use, a DLL file makes your programs run faster than if you just stored BASIC code in a BAS module file instead. (For more information about writing DLL files, pick up the latest edition of MORE Visual Basic For Windows For Dummies.)
Although using DLL files can be cumbersome, using these files is still easier than your other alternative, which is to use the Windows Application Programming Interface. (See the following section "Extend Visual Basic By Using the Windows API").
** Extend Visual Basic By Using the Windows API
Visual Basic makes programming so easy because it insulates you from all the messy details needed to write Windows programs. But the price you pay for this insulation is a loss of flexibility that C++ programmers love to flaunt (as they slowly go mad, dealing with the complexities of Windows programming).
Eventually, you are likely to find Visual Basic's ease of use frustrating because you can't perform certain tasks. The quickest and easiest alternative is to buy an add-on program to give Visual Basic the features you want. But if you can't find such an add-on program, you are going to have to dig into the guts of Windows itself.
To help programmers write programs for Windows, Windows has many commands called the Microsoft Windows Application Programming Interface (API). Although these commands are complicated to learn and understand, the complexity of these commands gives you the ability to manipulate (and of course, crash) Windows to your heart's content.
If you like living dangerously, take the time to read about the Windows API. (You can find out more about the Windows API in the latest edition of MORE Visual Basic For Windows For Dummies.) Otherwise, just pretend this section never existed and skip to another part of the book.
** Buy a Program to Create Help Files
Every good Windows program has an online help system so that panicky users can browse through a hypertext reference on the screen instead of wading through hundred-page manuals that don't make any sense anyway. If you're serious about writing Visual Basic programs, you are going to need to provide a help system with your programs, too.
Creating a help system isn't difficult, just incredibly dull and tedious. Fortunately, you can get a special program to make the process a little more enjoyable. Two popular help file creation programs are RoboHelp and ForeHelp, which let you design help screens as easily as writing a document in a word processor. When you're finished, each of these programs lets you test your creations by showing you exactly how your help screens are going to look when added to your own Visual Basic programs.
In today's competitive world of software development, a good help file is crucial to your program's professional appearance. Then again, if you don't care about making your programs easier to use, go work for any of the major software companies instead.
For more information on RoboHelp and ForeHelp, contact
RoboHelp
Blue Sky Software
7777 Fay Avenue, Suite 201
La Jolla, CA 92037
Tel: 619-459-6365
Fax: 619-459-6366
www.blue-sky.com
ForeHelp
ForeFront, Inc.
4710 Table Mesa Drive, Suite B
Boulder, CO 80303
Tel: 303-499-9181
Fax: 303-494-5446
www.ff.com
** Buy a Program to Create Installation Disks
After you finish creating a program in Visual Basic, the final step is to distribute your program to others. While you can just copy your Visual Basic program onto a floppy disk and trust that the other person knows how to use the disk, you are better off using a special installation program instead.
Installation programs can guide someone, step-by-step, through the oftentimes complicated process of copying a program onto another computer. Visual Basic comes with an installation program called the Application Setup Wizard. However, you may want to buy a special installation program that offers you the chance to customize the installation with your own corporate logo, advertisements, or sound effects.
Two popular installation programs are InstallShield and PC-Install. Both programs can help you create foolproof installation programs for all your Visual Basic programs.
For more information on these installation programs, contact
PC-Install
20/20 Software, Inc.
8196 SW Hall Blvd., Suite 200
Beaverton, OR 97008
Tel: 503-520-0504
Fax: 503-520-9118
www.twenty.com
InstallShield
InstallShield Corporation
900 National Parkway, Suite 125
Schaumburg, IL 60173-5108
Tel: 847-240-9111
Fax: 847-240-9120
www.installshield.com
** Write Visual Basic Programs for the Macintosh and Linux
When Microsoft first introduced Visual Basic back in 1991, programmers went nuts over it because they could design applications quickly and easily. So Microsoft promised that they would port Visual Basic to other operating systems so you could write Visual Basic programs for other computers.
After releasing Visual Basic for MS-DOS and then letting it die, Microsoft has done nothing but continue to hint of future releases of Visual Basic for other platforms. So if you want to use Visual Basic to write a Macintosh program, you can't.
However, you can use a Macintosh Visual Basic clone called (surprise!) Visual MacStandard Basic. Visual MacStandard Basic lets you visually draw your user interface and then write BASIC code to make it do something useful.
While Visual MacStandard Basic lacks many of the advanced features of Visual Basic 6.0, it does provide you with the ability to write Macintosh programs quickly and easily, just like Visual Basic 1.0 did for Windows back in 1991. So if you know how to program in Visual Basic, you can use your Visual Basic programming skills to write Visual MacStandard Basic programs for the Macintosh.
For the really adventurous, try XBasic, a BASIC compiler that runs under both Windows and Linux. While not as intuitive to use as Visual Basic, XBasic lets you write programs for Windows or Linux without resorting to learning C or C++. Since Linux may be the operating system of tomorrow, you can leverage your BASIC programming skills by writing Linux programs so you can insure job security for the future.
Visual MacStandard Basic
Zcurve Software
8206 Rockville Road #280
Indianapolis, IN 46214
www.zcurve.com
Zbasic
Basmark Corporation
P.O. Box 40450
Cleveland, OH 44140
Tel: 440-871-8855
Fax: 440-871-1715
www.basmark.com
++ APPENDIX A
ABOUT THE CD
Provided that somebody didn't take a razor blade and slice out the plastic envelope attached to the back cover, you should find a compact disc (CD or, more specifically, CD-ROM) embedded inside a plastic envelope like a mosquito trapped in a block of amber. This CD contains the Visual Basic source code from the various exercises contained within the chapters of this book, along with a few bonus demo and trial programs provided by third-party companies. These demo or trial programs are Visual Basic ActiveX programs that you can add to your own programs, which can make your programming much easier.
The sample Visual Basic programs show actual working examples of the BASIC commands that I describe in different chapters. By studying this honest-to-goodness Visual Basic code, you can spend more time learning and less time typing tedious commands into your computer yourself.
** System Requirements
Make sure that your computer meets the minimum system requirements listed below. If your computer doesn't match up to most of these requirements, you may have problems using the contents of the CD.
A PC with a 486 or faster processor.
Microsoft Windows 95/98/NT.
At least 8MB of RAM installed in your computer. For best performance, I recommend that PCs have at least 16MB of RAM installed. (The more RAM, the better.)
A CD-ROM drive - double-speed (2x) or faster.
Visual Basic version 6.0 (earlier versions won't work with the enclosed CD, so there).
If you need more information on the basics, check out PCs For Dummies, 5th Edition, by Dan Gookin, and Windows 98 For Dummies by Andy Rathbone (both published by IDG Books Worldwide, Inc.).
** How to Get to the Good Stuff
If you are running Windows 95, 98 or NT, follow these steps to get to the items on the CD:
1. Insert the CD into your computer's CD-ROM drive.
Give your computer a moment to take a look at the CD.
2. When the light on your CD-ROM drive goes out, double-click on the My Computer Icon (which is probably In the top left corner of your desktop).
This action opens the My Computer window, which shows you all the drives attached to your computer, the Control Panel, and a couple other handy things.
3. Double-click on the !con for your CD-ROM drive.
Another window opens, showing you all the folders and files on the CD.
To use the CD, follow these steps:
1. Double-click on the file called License.txt.
This file contains the end-user license that you agree to by using the CD. When you finish reading the license, close the program (most likely NotePad) that displayed the file.
2. Double-click on the file called Readme.txt.
This file contains instructions about installing the software from this CD. It may be helpful to leave this text file open while you are using the CD.
3. Double-click on the folder for the software you are Interested in.
Be sure to read the descriptions of the programs in the next section this appendix (much of this information also shows up in the Readme file). These descriptions give you more precise information about the programs' folder names and about finding and running the installer program.
4. Find the file called Setup.exe, or Install.exe, or something similar, and double-click on that file.
The program's installer walks you through the process of setting up your new software.
To run some of the programs on the Visual Basic 6 For Dummies CD-ROM, you may need to keep the CD inside your CD-ROM drive. This is a Good Thing. Otherwise, the installed program requires you to install a very large chunk of the program to your hard drive, which may keep you from installing other software.
**What You Find
Sample Visual Basic code appears in separate folders, named according to the chapter they belong to. (See Table A-1 to find out what's in each folder.) For example, to find all the Visual Basic source code for Chapter 3, look for a folder on the CD named Chapter 3. You can load and run the source code directly off the CD or you can copy it to your hard disk. (Just remember that if you run the source code off the CD, you can't modify it.)
||
Table A-1
Folder Contents
Folder - Chapter 3
What's In It - Contains two programs, Hello1 and Hello2. By loading these files off the disk, you don't have to go through the tedium of typing BASIC code directly out of the book.
Folder - Chapter 4
What's In It - Contains the final Hello program for you to examine and enjoy.
Folder - Chapter 5
What's In It - Contains the Caption program that demonstrates how BASIC code can change the caption of an object displayed on the user interface.
Folder - Chapter 6
What's In It - Contains the Forms and Buttons program that demonstrates how to use command buttons to modify the appearance of a form. (As a bonus, this sample program also demonstrates how to use dialog boxes, which you can find out about in Chapter 13).
Folder - Chapter 7
What's In It - Contains the Boxes and Buttons program that demonstrates how to use check boxes, radio buttons, combo boxes, and list boxes.
Folder - Chapter 8
What's In It - Contains the TextBoxes program that demonstrates how to use a password text box and an ordinary text box.
Folder - Chapter 9
What's In It - Contains the ScrollBars program that demonstrates how to use scroll bars and display the scroll bar value on the screen.
Folder - Chapter 10
What's In It - Contains the Shapes program that shows how to change a circle during run-time from the user interface.
Folder - Chapter 13
What's In It - Contains the DialogBox program that shows how to use the built-in Visual Basic Common Dialog Box object.
Folder - Chapter 15
What's In It - Contains the Variables program that shows how to change the properties of objects stored on different forms.
Folder - Chapter 16
What's In It - Contains the ListBoxes programs that shows you how to use a Visual Basic list box.
Folder - Chapter 17
What's In It - Contains the Precedence program that shows you how precedence works when using parentheses and when omitting parentheses while calculating a numeric result.
Folder - Chapter 18
What's In It - Contains the ELIZA program that demonstrates how to manipulate strings. ELIZA mimics a psychotherapist who simply echoes back anything you type, giving the illusion that the computer is actually responding to you.
Folder - Chapter 23
What's In It - Contains the DoWhile program that demonstrates how a simple Do While loop works.
Folder - Chapter 25
What's In It - Contains the ForNext program that demonstrates how a simple For Next loop works.
Folder - Chapter 28
What's In It - Contains the Arguments program that shows how you can pass arguments among procedures.
Folder - Chapter 29
What's In It - Contains the Functions program that shows you how a functions that converts yards to meters works.
Folder - Chapter 30
What's In It - Contains the Mission program, which demonstrates how class modules work and how to use them within a Visual Basic program.
Folder - Chapter 34
What's In It - Contains the PrintExample program that shows how you can print stuff from within a Visual Basic program.
** What You Find in the ActiveX folder
The ActiveX folder contains all the trial ActiveX programs (see table A-2 for details) kindly provided by third-party companies eager to show you their wares and sell you the full-featured versions of their programs. While you can experiment with these trial versions, you can't use them within your own Visual Basic programs that you may distribute for sale.
Each trial ActiveX program comes with its own setup program (usually called SETUP.EXE). Just run this setup program by double-clicking on the SETUP.EXE file within Windows Explorer, or by clicking the Start button on the Windows taskbar, choosing Run, and choosing Browse until you find the SETUP.EXE program you want to run.
Some of the programs included on the Visual Basic 6 For Dummies CD were developed prior to the release of Visual Basic 6. These programs are noted as working with Visual Basic 4 and/or Visual Basic 5 only. However, all software included on the CD had been tested and does install and run with Visual Basic 6. Please visit the manufacturer Web site for each program (as noted in the Readme file on the CD) to cheek for Visual Basic 6 updates.
||
Table A-2
ActiveX Folder Contents
Program - Crescent Internet ToolPak 4.1
What It Does - If you want to create Internet applications, try this demo version of Crescent Internet ToolPak, which helps VB developers create applications that can browse the World Wide Web, send e-mail, execute FTP file transfers, and more. The Crescent Internet ToolPak simplifies accessing the alphabet soup of Internet standards such as IMAP4, TCP/IP Server, and FTP proxy support.
Program - ElasticLight 6.0
What It Does - This demo version can make your programs automatically adjust to any screen resolution. So if you wrote your program on a super VGA monitor displaying an 800 x 600 screen resolution, but someone runs your program on a plain old VGA monitor at 640 x 480 resolution, your program isn't cut off by the screen. This ActiveX control also adjusts the size of objects on your form (such as command buttons and radio buttons) if the user resizes a form.
Program - HASHcipher
What It Does - If you need secrecy, try this demo version of HASHcipher. This ActiveX control uses the Secure Hash Standard (SHS), developed by the U.S. Government's National Security Agency (NSA), to provide security through authentication. The algorithm specified by the Standard, the Secure Hash Algorithm, (SHA-1) is considered by many cryptographers to be the strongest hash algorithm available today and can be used in any application that requires authentication of a file or message.
Program - InstallShield Express 2.02
What It Does - Provides a more sophisticated installation program than the feeble one included with Visual Basic. InstallShield is the most popular installation program that commercial developers everywhere use, so join the bandwagon and try this trial version of InstallShield Express today!
Program - True DBList
What It Does - The trial version of True DBList provides a set of customizable list and combo boxes for accessing information trapped in database files. Supports multiple columns; split group heading; multiple lines per record; alternate row color or style; in-cell graphics and text; data-sensitive color and font; and tons of other features that will make absolutely no sense to you until you try to torture yourself by accessing data with Visual Basic's built-in features.
Program - True DBInput
What It Does - The trial version of True DBInput provides dataaware ActiveX input controls for creating Visual Basic database applications. True DBInput includes scores of features to handle data entry and validation; intuitive customized display of dates, time, text, and numbers; plus other features to make sure your users don't type in invalid information into your database files.
Program - VBAssist 5.0
What It Does - The trial version of VBAssist contains various tools for helping you write Visual Basic programs faster and easier than before. With VBAssist you can quickly assign Tab Order to controls on a form, assign shortcut keys or accelerators to controls, place controls into exact positions on a form, along with other time-saving features.
Program - VBPartner
What It Does - This trial edition of VBPartner is an integrated add-in for Microsoft Visual Basic that includes 11 unique timesaving "Partners" that assist you with routine application design, error handling, and more by simplifying tedious development tasks.
Program - VSData
What It Does - This demo version of VSData enables your Visual Basic applications to access database files without using Visual Basic's own bulky database files that gobble up disk space and may offer way more features than your program needs.
Program - VSDirect
What It Does - This demo version of VSDirect provides routines for accessing Microsoft's DirectX technology for creating graphics, sound, and animation. Designed mostly for creating multimedia applications, VSDirect also includes routines for playing games over a modem link or network.
Program - VSDocX
What It Does - This demo version of VSDocX helps you create help files and printed documentation for your Visual Basic applications. Takes the hassle out of writing your program's documentation yourself, a task that most every programmer despises, as proven by the poor quality of computer manuals all over the world.
Program - VSFlex 3.0
What It Does - Visual Basic comes with an ActiveX control called MSFlexGrid, which is an older version of VSFlex. Like MSFlexGrid, this demo version of VSFlex lets you sort, merge, and group data in a grid. VSFlex also includes a string pattern matching ActiveX control for evaluating mathematical expressions or adding natural language capabilities to your programs.
Program - VS-OCX 6.0
What It Does - Provides notebook-style tabs along with a string parsing ActiveX control for advanced string manipulation. This demo version of VS-OCX also includes a more sophisticated screen resolution adapting control (similar to ElasticLight).
Program - VSReports
What It Does - VSReports enables your Visual Basic programs to create and print reports from Microsoft Access database files. By using this demo version of VSReports, you can avoid using Visual Basic's own bulky and cumbersome database reporting files.
Program - VSSpell
What It Does - Provides a spell-checker and a thesaurus for your Visual Basic applications. Perfect for creating the next generation of word processors to put Microsoft Word out of business.
** If You Have Problems (of the CD Kind)
Because computers are notoriously finicky, picky, and downright unreliable at crucial times in our lives, you may be one of the unfortunate few plagued with problems while trying to use the enclosed CD.
If your computer has trouble reading the files off the CD, you may have a faulty CD. Despite our best efforts, occasionally a CD decides to follow the "dark side of the Force," turns corrupt, and tries to frighten people into thinking their computer problems are actually their fault.
If your computer can read other CDs okay but not the enclosed Visual Basic CD, please call the Hungry Minds Customer Care phone number, at 800-762-2974, and request a replacement CD.
If you don't have Visual Basic 6.0, you can't load any of the sample programs from the CD, even if you have an older version of Visual Basic, such as version 5.0 or 4.0. So check your version of Visual Basic by choosing Help->About Visual Basic. A dialog box appears to show you which version of Visual Basic you have.
Author of Microsoft Office 2000
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** About the Author
At the time of this writing, the author's body temperature was 98.6 degrees Fahrenheit, and he was breathing normally. But beyond the physical characteristics, the author of this book has also has written over a dozen computer books including MORE Visual Basic For Windows For Dummies, Microsoft Office 97 For Windows For Dummies, and MORE Microsoft Office 97 For Windows For Dummies. When he's not writing computer books, he does what other computer authors usually do and writes articles for computer magazines instead. He writes a monthly column for Boardwatch magazine and has written articles for The Net magazine as well.
When not writing computer books, he often refers to himself in the third person singular form, such as in this current sentence. While such self-referential sentences tend to look authoritative and profound when appearing in print within a book's "About the Author" page, they tend to make people stare in a most peculiar way when spoken out loud in public. Not many people introduce themselves at parties as "Hi, his name is Wallace," and this author, of course, is no exception.
In his spare time, Wallace can often be seen watching the demolition crews knock down the buildings in San Diego that used to house his former employer. Wallace's only regret is that the demolition crews didn't start much earlier when his former boss and coworkers would still have been inside.
** Dedication
This book is dedicated to all the wonderful people I've met during my mad pursuit in the world of stand-up comedy:
Budd Friedman, for giving me my first national TV appearance on his show A&E's Evening at the Improv.
Mark Kuker, for offering delightfully strange, one-nighter gigs in Ruidoso and Deming (home of the annual duck races), New Mexico. Mark and his wife Amanda are two of the best people a comedian could ever hope to work for.
Steve Schirripa and Don Learned, for giving me my first Las Vegas gig at the Riviera Comedy Club, located in the Riviera Hotel & Casino. The next time you're in Las Vegas, drop by, see a show, and dump some money in the Riviera Casino slot machines. Who knows? Maybe you'll win enough money to buy yourself a new computer.
Patrick DeGuire and Dat Phan who helped me put together Top Bananas, our company devoted to booking the three of us in corporate comedy events around San Diego. Thanks also goes to Fred Burns, Leo (the man, the myth, the legend) Fontaine, Ron Clark, Dante, Frank Manzano, Chris (the Zooman) Clobber, Bob Zany, Tony Vicich, and George Hirschmann.
Final thanks go to Gene Perret, Linda Perret (and the rest of the whole happy Perret clan), Liz Sage, and everyone else who has helped me navigate my way through the minefield of inflated egos and hair-trigger temperamental personalities known as show business.
** Author's Acknowledgments
Nobody writes and publishes a book without the help of other people, and this book is no exception. It goes without saying (although I'm going to say it anyway) that two of the most important people responsible for this book are Matt Wagner and Bill Gladstone of Waterside Productions. Thanks guys. I'd give you more than your usual 15 percent cut but if I did, then I wouldn't have anything left over to pay for my groceries.
Two other people who deserve thanks include Brian Kramer of IDG Books Worldwide, Inc., and Allen Wyatt of Discovery Computing, Inc.
Next, I have to acknowledge Cassandra (my wife) along with Bo, Scraps, Tasha, and Nuit (my cats) for their support during the long hours I've spent glued to my computer instead of doing anything else around the house.
A final acknowledgment goes to the friendly people at Complete Design & Remodeling including Wes McKusick and Tom DeLisle who did all the remodeling of my new house while I just wrote them checks.
** Publisher's Acknowledgments
We're proud of this book; please send us your comments through our Online Registration Form located at www.dummies.com.
Some of the people who helped bring this book to market include the following:
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Hungry Minds, Inc.: John Kilcullen, CEO; Bill Barry, President and COO; John Ball, Executive VP, Operations & Administration; John Harris, CFO
Hungry Minds Technology Publishing Group: Richard Swadley, Senior Vice President and Publisher; Mary Bednarek, Vice President and Publisher, Networking and Certification; Walter R. Bruce Ill, Vice President and Publisher, General User and Design Professional; Joseph Wikert, Vice President and Publisher, Programming; Mary C. Corder, Editorial Director, Branded Technology Editorial; Andy Cummings, Publishing Director, General User and Design Professional; Barry Pruett, Publishing Director, Visual
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The publisher would like to give special thanks to Patrick J. McGovern, without whom this book would not have been possible.
++ CONTENTS AT A GLANCE
Reader's Note:
INTRODUCTION
PART I: CREATING A VISUAL BASIC 6 PROGRAM
Chapter 1: How Visual Basic Works
Chapter 2: Using the Visual Basic User Interface
Chapter 3: Designing Your First User Interface
Chapter 4: Writing BASIC Code
PART II: CREATING USER INTERFACES
Chapter 5: User Interface Design 101
Chapter 6: Forms and Buttons
Chapter 7: Boxes and Buttons for Making Choices
Chapter 8: Text Boxes for Typing and Showing Words
Chapter 9: Scroll Bars and Labels
Chapter 10: Pretty Pictures and Objects from Geometry
PART III: MAKING MENUS
Chapter 11: Creating and Editing Pull-Down Menus
Chapter 12: Submenus, Growing Menus, and Pop-Up Menus
Chapter 13: Dialog Boxes
PART IV: THE BASICS OF WRITING CODE
Chapter 14: Event Procedures
Chapter 15: Using Variables
Chapter 16: Responding to the User
Chapter 17: Math 101: Arithmetic, Logical, and Comparison Operators
Chapter 18: Strings and Things
Chapter 19: Defining Constants and Using Comments
Chapter 20: Killing Bugs
PART V: MAKING DECISIONS (SOMETHING YOU STOP DOING WHEN YOU GET MARRIED)
Chapter 21: The If-Then and If-Then-Else Statements
Chapter 22: The Select Case Statement
PART VI: GETTING LOOPY
Chapter 23: The Do While and Do-Loop While Loops
Chapter 24: The Do Until and Do-Loop Until Loops
Chapter 25: For Next Loops That Can Count
Chapter 26: Nested Loops and Quick Exits
PART VII: WRITING SUBPROGRAMS (SO YOU DON'T GO CRAZY ALL AT ONCE)
Chapter 27: General Procedures (Subprograms That Everyone Can Share)
Chapter 28: Passing Arguments
Chapter 29: Functions, a Unique Type of Subprogram
Chapter 30: Getting Some Class with Object-Oriented Programming
Chapter 31: Managing Files
PART VIII: DATABASE FILES AND PRINTING
Chapter 32: Creating Database Files
Chapter 33: Using Files from Database Programs You'd Rather Not Use
Chapter 34: Making Your Program Print Stuff
PART IX: THE PART OF TENS
Chapter 35: The Ten Visual Basic Topics That Didn't Fit Anywhere Else
APPENDIX A: ABOUT THE CD
++ Reader's Note:
This book has been scanned and edited by Jane Etherington. She has marked the beginning of each chapter with a double plus [++], The beginning of each section in the chapters with a double star [**] and each sub-heading with a single star [*]. These markings are to assist the reader in finding their way around the book more easily and rapidly
The reader may come across such mistakes as: The letter "I" for a number 1 or the letter "L" or any combination of these. Other mistakes may also have been missed. These have been corrected when detected, but inevitably, some may have been missed. It is our hope that these minor errors do not detract from the overall enjoyment that this book gives?
++ PART I - CREATING A VISUAL BASIC 6 PROGRAM
In this part ......
Writing your own program isn't hard. If you've always been curious about computer programming but wer intimidated by the hard-to-read books, less-than-useful softward, or obtuse and convoluted "explanations" from "experts," then this book is for you.
Rather than impress you with matematical proofs and theoretical background about computer programming, this book (with the help of Visual Basic) lets you jump right in and start finding out about programming on your own computer all by yourself.
So grab some snack foods, a few carbonated beverages, a comfortable seat, and get ready to program your computer and make it finally to what you want it to do.......
++ CHAPTER 1
HOW VISUAL BASIC WORKS
In This Chapter:
Looking at the Visual Basic development cycle
Creating a Visual Basic user interface
Understanding what BASIC code does
The whole purpose of writing a program is to make your computer do something useful. People often spend thousands of dollars for one of these machines, so it's important that the computer does something more than consume electricity and take up space on a desk.
Before you delve into the world of programming, keep two thoughts in mind. First, anyone can write a program. Programming is just a skill, much like swimming, sailing, or shoplifting. If you've ever taught yourself a new hobby or skill, you can teach yourself how to write a program without an extensive mathematics background or a fancy college degree that puts you in debt for the next ten years.
Second, the key to programming is defining exactly what you want your program to do. Defining what you want your program to do is half the battle (a battle that governments and large corporations routinely lose all the time). The other half of the battle involves taking the time to write your program and making sure it works correctly. (This part of the battle is another place where governments and large corporations fall flat on their faces on a fairly regular basis).
You can write a program to make your computer do anything you want, short of launching nuclear missiles at your next-door neighbor. (Of course, if you write a program for the Air Force's computers, you may even be able to do that).
** Writing a Visual Basic Program
There is no single correct way to write a program. Theoretically, you can use a million different ways to write a program correctly, just as you can travel from New York to Los Angeles a million different ways. Some people may fly, others may take a train or drive; the more adventurous may walk, hitchhike, or hijack a vehicle. Similarly, you can write the same program a million different ways. However, no matter how you write the program, the result can always be the same.
As a programmer, your job is to write a program that works correctly and is easy to use. If your program doesn't work, nobody can use it (although you often can sell a few thousand copies to unsuspecting individuals first). If your program isn't easy to use, nobody will want to use it, even if it works perfectly.
Testing whether your program works is usually simple enough. If your program is supposed to print mailing labels but erases the computer's hard disk instead, then your program obviously doesn't work correctly.
However, determining whether your program is easy to use is a bit more difficult. What you may consider easy to use may be almost impossible for someone else to understand.
To create programs that everyone can understand how to use, Visual Basic helps you to easily produce windows, pull-down menus, dialog boxes, and command buttons. These features are the same ones found in Windows 95/98/NT programs. Visual Basic helps you write programs that look and act like other programs on the market.
Making your program look and act like an existing program can help others learn your program faster. For example, most people can drive a Toyota or a Ford without any problems; the steering wheel and brakes always look and work the same way, even if the windshield wipers and horn may not. The same goes for programs. Pull-down menus contain a program's numerous commands, and you can always use the mouse to highlight and choose commands or objects. So while each program may work differently, they all look and work in similar ways.
* The Visual Basic Development Cycle
Before writing a Visual Basic program (or any program for that matter), get away from your computer and plan your program using an old-fashioned paper and pencil. After you know what you want your program to accomplish and how you want it to look, then you can start writing your program. Skipping this crucial first step is like building a house without blueprints. You can do it, but it will probably take you longer.
Writing a Visual Basic program requires nine steps - three steps fewer than those required to overcome and addictive habit. The first eight steps are what programmers call the development cycle. The ninth step is what programmers call job security.
1. Decide what you want the computer to do.
2. Decide how your program will look on the screen. (The appearance of your program is its user interface).
3. Draw your user interface using common parts such as windows, menus, and command buttons. (The parts of a user interface are objects or controls).
4. Define the name, color, size, and appearance of each user interface object. (An object's characteristics are its properties).
5. Write instructions in BASIC to make each part of your program do something. (BASIC instructions are commands).
6. Run your program to see if it works.
7. Cry when your program doesn't work perfectly. (Required).
8. Fix any errors (or bugs) in your program.
9. Repeat steps 6 through 8 over and over again until you get tired of searching for more bugs.
Although you don't have to memorize these nine steps, you do have to follow them. Shortcuts aren't an option. Trying to skip from Step 1 to Step 4 is like trying to start a car by using the gas pedal but forgetting to turn the ignition key. You can try it, but you're not goint to get anywhere.
Believe it or not, Step 1 is actually the hardest and most important step of all. After you know exactly what you want your program to do, it's just a matter of finding ways to do it. Persistence and creativity are helpful, as are lots of caffeine-laden beverages and plenty of sleepless nights in front of the computer screen.
* Why Programs Don't Work (Part I)
Writing a program that works 100 percent correctly all the time is mathematically impossible. First of all, if you write a program that works 100 percent correctly today, there's no way you can guarantee that it will work 100 percent correctly on future computer brands, models, processors, and accessories. As a result, you can never guarantee that your program will work correctly on all types of computers unless you exhaustively test every possible computer configuration in the world.
Second, not only do you have to test your program with the latest and greatest products (including the ones invented after you wrote your program), but you also have to consider the virtually infinite number of possibilities that your program must face during everyday use.
For example, your program needs to behae correctly if the user presses any key and then clicks the mouse anywhere on the screen. What if the user clicks the mouse by mistake while tapping a key? What if the user pounds the keyboard in frustration? What if another program happens to interfere with the computer's memory, thus affecting your program? What if..
(well, you get the idea).
Unless a programmer can plan for an infinite number of possible problems and situations that a program may face during its existence, then writing a program that works 100 percent of the time is impossible.
What's scarier is that this scenario holds true for every computer operating system in the world (such as Windows 95/98/NT). Therefore, you'll always be writing programs to run on an operating system that doesn't work 100 percent correctly either. This situation is like building a house on a foundation of quicksand and then wondering why your house keeps failing apart.
Because no one has an infinite amount of time to test an infinite number of possible problems, computer programs always (yes, always) will have bugs that keep them from working 100 percent correctly. That includes every program you write and every program that Microsoft's millionaire programmers may write. That's why when you write a program, set aside plenty of time for testing so you can kill any potentially fatal bugs before you give your program to someone else.
So the next time you're using a program that doesn't work right, now you'll know that ifs not your fault; it's the programmer's fault.
* Making a neat user interface
The user interface is what someone sees when your program is running. Every program has a user interface in one form or another. Some programs have elaborate, colorful windows, while other programs have a sparse appearance - as if the programmer were afraid that screen phosphor may be in short supply one day.
A Visual Basic user interface consists of forms and objects. A form is nothing more than a window that appears on the screen. Most Visual Basic programs have at least one form, although most programs likely will use several forms.
Objects are items that appear on a form, such as a command button, scroll bar, option button, or check box. An object lets the user give commands to your program. If you really wanted, you could create a program with only one form and no objects, but it wouldn't be very useful or interesting.
* Defining properties to make your user interface unique
After you create a form and draw some objects on it, the next step is to define the properties of each form and object. An object's properties determine the object's name, color, size, location, and appearance on the screen.
Different objects have different properties. Each time you draw an object on a form, Visual Basic assigns default property values, which define a generic object that no one can really use. If you want to customize an object, you need to define one or more properties for each object that your program uses. Chapter 3 provides a quick introduction to changing an object's properties.
* Writing BASIC code
When you're happy with the way your program looks, the next step involves writing BASIC commands (also known as code) to make your program actually work. (Don't worry. If you change your mind and want to edit the appearance of your user interface, you can go back and alter it at any time.)
The whole purpose of Visual Basic code is to tell objects on a form what to do when the user does something. For example, if the user clicks on an OK or Cancel command button, nothing happens unless you've written BASIC commands to tell your computer exactly what to do.
Any time a user presses a key, moves the mouse, or clicks the mouse button, it's called an event. Whenever an event occurs, your BASIC commands tell the computer, "Hey stupid, something just happened. Let's do something about it!"
Essentially, writing a Visual Basic program means drawing your user interface and then writing BASIC code to make it work. If you can handle these two steps without losing your mind, you can start writing your very own programs using Visual Basic. Chapter 4 provides a short introduction to writing real-life BASIC code.
** Why programs don't work (Part II)
Most programs are written by professional programmers who may have studied programming for years. Does this mean that every program they write will always work? Of course not. This is the world of computers where nothing works right, remember?
Besides the fact that the skill level among professional programmers can vary widely, professional programmers are often called upon to write programs for tasks that they don't understand themselves. For example, programmers may know nothing about accounting yet be hired to write a program to control a bank's electronic-fund-transfer system. Likewise, programmers with no skill or experience in flying may be hired to write a program to control the landing, takeoff, and flight of a 747 jumbo jet. Programmers with no knowledge of medicine may be required to write a program to control a medical instrument that administers doses of radiation to cancer patients. How can you work in a field where you have no experience and still get paid a lot of money? Easy, you become a programmer.
Hiring a programmer with no knowledge of the task she's trying to solve is like hiring a translator to translate Greek into French without that person's knowing how to read or write in either language. Given this paradox in the programming world, is it any wonder that planes crash, banks lose money, and hotels can't keep our reservations straight?
++ CHAPTER 2
USING THE VISUAL BASIC USER INTERFACE
In This Chapter:
Loading Visual Basic
Getting to know the Visual Basic user interface
Opening, closing, and moving windows around
Quitting Visual Basic
Before diving into the depths of Visual Basic programming, take a deep breath and examine the program's user interface. After all, if you don't know how to use the Visual Basic user interface, you won't be able to write your own Visual Basic programs.
To use Visual Basic, you need to know the following three functions:
How to load Visual Basic from Windows 95/98/NT
How to use Visual Basic to write your own programs
How to exit out of Visual Basic
** Loading Visual Basic
To load Visual Basic, just follow these simple steps:
1. Click on the Start button of the Windows 95/98/NT taskbar. A pop-up menu appears.
2. Click on Programs, click on the Microsoft Visual Basic 6 folder (or the Microsoft Visual Studio folder), and then click on Microsoft Visual Basic 6.
Visual Basic displays a New Project dialog box.
To load Visual Basic quickly, create a desktop shortcut by following these steps: Right-click on the Windows desktop, click on New, click on Shortcut, click on the Browse button, search for the VB6.EXE file, click on the Next> button, type Visual Basic 6, and click on the Finish button.
After the New Project dialog box appears, you have two options. You can
Start writing a brand new program.
Load an existing program so you can modify it.
* Starting a new program
After you have the New Project dialog box displayed, you have several choices as to the type of program you want to create. (The Learning and Standard Editions of Visual Basic do not display all the following types of programs in the New Project dialog box.)
Standard EXE: Creates a stand-alone program that you can copy, give away, or sell to others. Examples of stand-alone programs are Microsoft Word, Lotus 1-2-3, and Netscape Navigator. Stand-alone programs have an EXE file extension.
ActiveX DLL: Creates a file that has a .DLL file extension. ActiveX DLL files are not meant to be used by themselves. Instead, these types of files contain subprograms designed to function as building blocks when creating a stand-alone program.
ActiveX EXE: Creates a file that has an .EXE file extension. Unlike a stand-alone EXE file, an ActiveX EXE file is designed to work as an OLE server, which is nothing more than a program designed to share information with another program.
ActiveX Control: Creates a file that has an .OCX file extension. Unlike an ActiveX DLL or ActiveX EXE file, an ActiveX Control file usually provides both subprograms and a user interface that you can reuse in other programs.
ActiveX Document DLL: Creates a file that has a .DLL file extension. An ActiveX Document DLL file is designed to help you run programs on a Web site.
ActiveX Document EXE: Creates a file that has an .EXE file extension. An ActiveX Document EXE file can display a Visual Basic form within an Internet Web browser.
AddIn: Enables you to create an addin program specially designed to work with the Visual Basic user interface.
VB Application Wizard: Helps you create a skeleton Visual Basic standalone EXE program quickly and easily.
Data Project: Creates a program for devising a database report, which lets you see your database information in a pretty and organized way.
DHTML Application: New to Visual Basic 6, this program creates a DHTML (Dynamic HyperText Markup Language) document suitable for posting on a Web Site.
IIS Application: Creates a file for use with Microsoft Internet Information Server - another new Visual Basic 6 feature.
To choose one of these options from the New Project dialog box, just click on the option you want and click on OK.
If you choose File->New Project (or press Ctrl+N), Visual Basic displays a New Project dialog box that does not contain the Existing or Recent tabs.
Creating ActiveX, DHTML, IIS, and Data Project files is fairly advanced, so don't worry about such files until you figure out how to create a simple (Standard EXE) Visual Basic program first.
* Loading an existing program
Most of the time you are goint to want to load an existing program so you can modify it. To load an existing program, click on the Existing tab. Visual Basic displays folders so you can choose the specific Visual Basic program you want to load.
If you want to load a program that you've loaded in the past, click on the Recent tab. Visual Basic politely displays a list of all the programs you recently loaded. Just click on the Visual Basic program you want to edit and then click on Open.
If you choose File->Open Project (or press Ctrl+O), Visual Basic displays an Open Project dialog box that does not contain the New tab.
If you click on the File menu, Visual Basic kindly displays a list of programs that you last edited. By clicking on one of these program names, you can load the program without going through the Open Project dialog box.
Welcome to the Visual Basic User Interface
After you decide to start a new Visual Basic program or edit an existing one, the Visual Basic user interface appears in its full glory. Of course, before you can draw your program's user interface and write BASIC code, you have to know how to use the user interface of Visual Basic itself. The eight main parts of the Visual Basic user interface appear in Figure 2-2, although all parts don't necessarily have to be visible at any given time.
1. Pull-down menus: Provide access to every available Visual Basic command, although these menus can be confusing and intimidating to use.
2. Toolbar: Displays icons that represent the most commonly used Visual Basic commands - which may still be confusing and intimidating to use.
3. Toolbox: Displays the types of objects (such as a command button or check box) that you can draw on a form. The Toolbox may not list all possible objects you can draw on a form. To get a complete list of objects you can display in your Toolbox, press Ctrl+T.
4. Project explorer: Lists all the files that make up a single Visual Basic program.
5. Properties window: Displays the properties of the currently selected form or object.
6. Form layout window: Enables you to arrange the location where your forms appear on the screen.
7. Form: Provides a window where you can draw objects to design your program's user interface.
8. Immediate window: Enables you to debug your Visual Basic program.
When you're designing your user interface, you use the Toolbox to draw objects on a Form. After you draw your objects, the next step is to customize the appearance of each object by using the Properties window. Finally, after you're happy with the way your program's user interface looks, you can define the location of the interface on the screen by using the Form layout window.
To see more of the Visual Basic user interface, change your screen resolution by following these steps:
1. Click on the Start button on the Windows 95/98/NT taskbar.
2. Choose Settings->Control Panel. A Control Panel window appears.
3. Double-click on the Display icon. A Display Properties window appears.
As an alternative to Steps 1 through 3, just point the mouse cursor anywhere on your Windows 95/98/NT desktop, click the right mouse button, and click on Properties.
4. Click on the Settings tab.
5. Click on the Desktop area horizontal scroll bar and drag the slider toward the right.
The screen resolution (such as 800 x 600 pixels) appears underneath the horizontal scroll bar.
6. Click on OK.
* Closing and opening windows
If the Visual Basic user interface seems cluttered, don't be afraid to modify it. Because most parts of the Visual Basic user interface consist of windows, you may want to close them to give yourself more screen space. Then, when you need to use the interface windows again, just open them back up again.
To close a window in the Visual Basic user interface, click on the Close box of the window you want to close.
To open a window and make it appear in the Visual Basic user interface, choose View and then click on the window you want to open, such as the Code or Properties window.
* Resizing and moving your windows
Rather than close a window and make it disappear completely, you may prefer to resize or rearrange the window instead.
To resize a window, move the cursor over the edge of the window until the cursor turns into a doubIe-headed arrow. Next, hold down the left mouse button, drag the mouse, and release the left mouse button when the window is the shape you want the window to be.
To move a window, move the cursor over the title bar of a window, hold down the left mouse button. Drag the mouse and then release the left mouse button when the window is in the desired location.
* Docking your windows
With so many windows cluttering up the screen, you can see how easily the windows overlap one another and just get in the way. To help solve this problem, Visual Basic offers a docking feature.
Docking simply means that Visual Basic neatly stacks your windows to one side of the screen. To dock (or undock) a window, follow these steps:
1. Move the mouse cursor over the title bar of the window you want to dock (or undock).
2. Double-click the left mouse button. Visual Basic automatically docks (or undocks) your chosen window.
In case you don't want Visual Basic to dock (or undock) your windows at all, you can turn off the docking feature. To turn the docking feature on or off for a particular window, follow these steps:
1. Choose Tools->Options. An Options dialog box appears.
2. Click on the Docking tab.
3. Click on a check box to place a check mark in or remove a check mark from that check box. A check mark means that the docking feature is turned on for that particular window. An empty check mark means that the docking feature is (surprise!) turned off.
4. Click on OK.
** Quitting Visual Basic
No matter how much you may love using Visual Basic, eventually you need to turn off the computer and go to sleep (or at least pass out on the keyboard for an hour or two). To exit Visual Basic, use one of these three methods:
1. Choose File->Exit.
2. Press Alt+Q
3. Click on the Close box of the Visual Basic user interface window.
If you haven't saved the currently displayed Visual Basic program, Visual Basic displays a dialog box, giving you one last chance to save your work before the material is gone for good. Just click on Yes to save your work (or No to lose any changes you have made since the last time you saved the file).
As long as your computer hasn't crashed, Visual Basic smoothly exits and dumps you back to the Windows 95/98/NT desktop.
++ CHAPTER 3
DESIGNING YOUR FIRST USER INTERFACE
In this chapter:
Understanding the common parts of a user interface.
Drawing a user interface.
Changing the properties of your user interface.
As I discuss in Chapter 1, you must go through a number of steps in order to create a Visual Basic program from start to finish. While the earliest of these steps deal with designing and creating the user interface, this chapter focuses on the fundamental steps in writing a Visual Basic program:
Drawing the user interface
Defining the user interface properties
Writing BASIC code
So before you can write your first program in Visual Basic, you need to know how you can create a user interface.
** Common Parts of a User Interface
Despite the different varieties of user interfaces available, most graphical user interfaces share similar features such as displaying text or pictures in a window. A window can fill the entire screen or just part of it. Two or more windows can appear on the screen at the same time, either overlapping like cards or side by side like tiles. For some odd reason, Visual Basic calls a window a form.
When you first create a form, it's entirely blank. To make your form useful, you have to draw objects on the form. An object can be a command button, a text box, a picture, or an option button. The user communicates with your program by clicking, typing, or manipulating the objects displayed on a form.
By themselves, objects do absolutely nothing but look nice. To make them functional, you have to write BASIC code (Which you can read about in Chapter 4).
** A short history of user interfaces
In the old days of computers (back in the '50s), using a computer meant opening up the computer and rearranging some wires. Not only did you have to know how to program a computer to use it, but you also had to know how to connect the computer's wires together without electrocuting yourself in the process.
To use a computer in the '60s, you had to type commands to punch holes in cards and then feed the stack of punch cards into the computer. Because typing commands perfectly on cards wasn't something that most people considered exciting, using a computer was slow, tedious, and boring.
In the 70s, scientists connected a TV set to the computer and called the whole thing a computer terminal. For the first time,you could type a command directly into the computer and the computer could respond right away. This was the first attempt at creating a user interface that people could actually use and understand.
Of course, these first crude user interfaces consisted of nothing more than a blank screen and a blinking dot, called a cursor. To get the computer to do anything, you had to type the proper commands. Unfortunately, if you didn't know the right commands to type, the computer would refuse to work and make you feel stupid. Once again, using a computer became slow, tedious, and boring.
In a desperate attempt to make computers simpler to use, computer programmers soon invented something called a graphical user interface or GUI (pronounced "gooey"). Basically, a GUI displays menus and icons that the user can choose by clicking on commands with the mouse.
Apple Computer created the first commercial GUI when it introduced the Macintosh, but Microsoft quickly created its own GUI (dubbed Microsoft Windows). Unfortunately, GUls can still make a computer slow, tedious, and boring to use, so when this happens, blame the computer industry. This won't solve any problems, but at least you can make yourself feel better emotionally for a minute or two.
** Drawing objects with Visual Basic
To draw objects on a form, you need to use the Toolbox, which normally appears on the left side of the screen (unless you move it somewhere else). The Toolbox contains little drawings that represent the various objects you can draw on a form.
To draw any object on a form, you always have to follow these steps:
1. Click on the object in the Toolbox to tell Visual Basic what you want to draw on a form.
2. Move the mouse pointer onto the form where you want to draw the object.
3. Hold down the left mouse button and drag the mouse to draw your chosen object on the form.
As a fast way to draw an object on a form, just double-click on an Icon in the Toolbox and Visual Basic draws your chosen object in the center of the form automatically.
Designing your user interface is a lot like doodling. Although Part II of this book explains more about using specific objects and how they work, for now just remember two things: All programs need a user interface, and Visual Basic uses forms and objects to help you create a user interface quickly and easily.
* The two ways to create a user interface
When you want to create a user interface with Visual Basic, you can either:
Use the VB Application Wizard to create user interface for you automatically
Create your own user interface from scratch
So which way is better? If you need to create a Windows 95/98/NT program that offers standard File, Edit, Window, and Help pull-down menus, let Visual Basic create your user face for you automatically.
However, if you re just creating a simple program that doesn't need pull-down menus, creating your own user interface all by yourself is going to be easier.
Whichever way you decide to create a user interface, you can always add, delete, or modify your user interface at a later date.
* Drawing your first user interface from scratch
In case you don't feel like going through the following 14 steps, the enclosed CD-ROM has the HELLO1.VBP file stored in the Chapter 3 directory for you to examine and study.
To get acquainted with Visual Basic right away, the following are some steps you can use to create a real-life user interface from scratch:
1. In Windows 95/98/NT, start Microsoft Visual Basic if you haven't done so already. If Visual Basic is running already, then choose File->New Project.
Visual Basic displays a New Project dialog box, asking you what type of program you want to create.
2. Click on the Standard EXE Icon and click on Open.
Visual Basic displays a blank form titled Form 1. (If you want Visual Basic to create a user interface for you automatically, click on the VB Application Wizard icon at this step, but don't click on the icon now. The end of this chapter explains how to use the VB Application Wizard.)
3. Move the mouse cursor over the bottom right corner of the form (directly over the small rectangle, called a handle, that appears In the center of the right edge) so the mouse cursor turns into a left and right pointing arrow. Hold down the left mouse button and drag the mouse to make the form larger.
4. Choose View->Toolbox to make the Toolbox appear on the left side of the screen.
(Skip this step if the Toolbox is visible already.)
5. Click on the Command button Icon In the Visual Basic Toolbox.
6. Move the mouse over the form, then drag the mouse to draw a command button.
7. Click on the Option button icon in the Toolbox and draw an option button. Repeat this process two more times to draw three option buttons.
8. Click on the Image box icon and draw an image box. Repeat this process two more times to draw three image boxes.
9. Click on the Text box icon and draw a text box. Repeat this process two more times to draw three text boxes.
10. Choose File->Save Form 1 or press Ctrl+S. A Save File As dialog box appears, asking what you want to name the file. (You may want to click on the Save In list box to choose a specific folder to save your Visual Basic project and forms.)
11. Type HELLO and select Save. This action saves your form in a file called HELLO.FRM.
12. Choose File->Save Project. A Save Project As dialog box appears, asking what you want to name your project.
13. Type HELLO and click on Save. This action saves your entire Visual Basic project in a file called HELLO.VBP.
14. Choose File->Exit (or press Alt+Q) if you want to exit from Visual Basic.
Congratulations! You've just created a generic Visual Basic user interface. If your user interface looks a little less than impressive, this is because you haven't customized the user interface for your program.
To customize a Visual Basic user interface, you have to define the properties for each object on your forms.
** Defining the Properties of Your User Interface
Drawing your user interface creates the initial appearance of your Visual Basic program. To finish defining your program's user interface, you next have to define the properties for each object on your user interface.
But do you really need to define these properties? Yes and no. Visual Basic automatically sets default property values for all the objects on your user interface. However, these default values make your program look pretty ugly. So if you want to make your user interface more attractive, you must define the properties for your objects.
While each object typically has 10 to 30 different properties that you can change, you don't have to modify every single property. Most of the time, you just need to modify two or three properties of each object.
* What properties do
Before you change the properties of any object, you may want to know what the heck properties do in the first place. Essentially, properties define the characteristics of an object such as its name, size, shape, and color on the screen.
The name of an object is for your convenience only. Visual Basic automatically gives all objects boring names like Text1 or Command3. However, when you need to refer to specific objects, it's much easier if you have descriptive names for each object used by your program.
An object's size, location, and color define its appearance on the screen. The whole purpose of an object's appearance is to make your user interface look pretty.
* Changing property settings
You can change the property settings of an object at two separate times:
During design time
During run time
Design time is when you're drawing your user interface but before you actually run your program.
Most of the time, you want to change an object's property settings at design time. The most important property to change at design time is the name of the object.
Run time is when your program uses BASIC code to change an object's properties while your program is actually running. Of course, before your program can change an object's properties, you have to write BASIC code that tells your program exactly which object's properties you want to change.
Changing a property during run time enables you to create animation or display messages on the screen, such as error messages, program status messages, or warning messages alerting the user that the computer is going to blow up in ten seconds.
Changing property settings at design time:
Several properties are assigned to every object. To change the property of an object, make sure you have a form displayed within Visual Basic and then follow these steps:
1. Click on the object whose properties you want to change.
2. In the Properties window, click on the property that you want to change. If the Properties window is not visible, press F4 to make the window appear.
3. Type or choose a new setting for the property.
Simple, don't you thing? When you need to change multiple properties for one or more objects, this book displays a table similar to the following:
Object Property Setting
Form Name frmHello
Caption Hello, world!
The following steps detail what this table is telling you to do:
1. Click on the Form object.
2. Click on the Name property in the Properties window.
3. Type frmHello to change the value of the Name property.
4. Click on the Caption property in the Properties window.
5. Type Hello, world! To change the value of the Caption property.
Defining the properties of your first user interface:
In case you don't feel like following the next 17 steps, the enclosed CD-ROM has the HELLO2.VBP file stored in the Chapter 3 directory for you to examine and study.
To define the properties of your user interface, follow these steps:
1. In Windows 95/98/NT, start Microsoft Visual Basic.
Visual Basic displays the New Project dialog box. If Visual Basic is already running, choose File->Open Project.
2. Click on the Recent tab.
3. Click on the HELLO file (which is the HELLO.VBP file) and click on Open.
Visual Basic loads your HELLO.FRM form on the screen.
4. Click on Option 1 to highlight the option button, which causes little rectangles to appear around the edges of the option button.
5. Click on the Properties window, click on the Name property and type optGrin. Click on the Caption property and type I'm happy!
6. Click on the Image 1 image box. (This appears near the top of the form.)
7. Click on the Properties window, click on the Name property and type imgGrin.
8. Click on the Properties window and click on the three dots (...) in the Picture property. Visual Basic displays a Load Picture dialog box.
9. Open the Icons folder. (You may have to dig through the Graphics folder in your Visual Basic folder to find the Icons folder.) Next, open the Misc folder within the Icons folder. Visual Basic displays a Load Picture dialog box.
10. Double-click on the FACE03 icon. Visual Basic displays a really happy face in the image box.
11. Click on the Visible property and then click on the downward-pointing arrow and choose False.
12. Click on the Text1 text box.
13. Click on the Border Style property and then click on the downward-pointing arrow and choose 0-None.
14. Click on the Name property and type txtGrin.
15. Double-click on the Text property and then press Backspace to clear the Text property so no text appears.
16. Finish changing the properties for the rest of the objects according to Table 3-1.
17. Choose File->Save Project to save all your changes.
Congratulations! You just defined all the necessary properties for your first user interface.
||
TABLE 3-1
Properties to Change to Finish Designing Your User Interface
Object - Form
Property - Name; Caption
Setting - frmHello; Hello, world!
Object - Option 2
Property - Name; Caption
Setting - optSmile; I'm okay.
Object - Option 3
Propety - Name; Caption
Setting - optFrown; I'm sad
Object - Image 2
Property - Name; Picture; Visible
Setting - imgSmile; FACE02; False
Object - Image 3
Property - Name; Picture; Visible
Setting - imgFrown; FACE01; False
Object - Text2
Property - Name; Border Style; Text
Setting - txtSmile; 0-None; (Empty)
Object - Text3
Property - Name; Border Style; Text
Setting - txtFrown; 0-None; (Empty)
Object - Command1
Property - Name; Caption
Setting - cmdExit; Exit
** Test your newfound knowledge
1. What are the two common parts of almost every user interface?
a. The easy-to-use interface and the 500page manual that explains how easy the interface is to use
b. Menus that nobody can understand and commands that nobody can use
c. Useless icons that don't make any sense and text that doesn't explain anything
d. Forms and objects
2. How can you change property settings of an object?
a. You can't, Property settings have to want to change first.
b. Use constant threat and intimidation.
c. Use the Property window while designing the user interface (known as design time) or write BASIC code to change the properties while the program is running (known as run time).
d. Stick a magnet next to your monitor and watch the images warp and wreck your computer screen.
** Letting Visual Basic Create a User Interface Automatically
If you actually went through the steps to create a user interface from scratch, you can see how tedious and time-consuming this process can be. Because computers are supposed to save you time (so you have more time to play computer games), Visual Basic offers a VB Application Wizard.
The VB Application Wizard creates a skeleton program complete with pulldown menus, toolbars, and dialog boxes. If you want to create a program that requires a user interface similar to Microsoft Word, Excel, or PowerPoint, you can save time by letting the VB Application Wizard make your user interface instead.
To see how the VB Application Wizard works, follow these steps:
1. In Windows 95/98/NT, start Microsoft Visual Basic. (If you already have Visual Basic running, choose File->New Project.)
Visual Basic displays the New Project dialog box.
2. Double-click on the VB Application Wizard icon.
The Introduction window of the Application Wizard appears. This window asks for a profile to use. Profiles are skeleton settings so you can customize the VB Application Wizard. For now, just keep the profile option as (None).
3. Click on Next>. The Application Wizard - Interface Type window appears
4. Click on the Multiple Document Interface (MDI) option button and click on Next>.
The Application Wizard - Menus window appears. (If you want, click on a different option button for Step 4 to see what a Single Document Interface [SDI] and Explorer Style user interface look like.)
5. Click on Next> to accept the default selection.
The Application Wizard - Customize Toolbar window appears. By dragging and dropping icons, you can create your own toolbars.
6. Click on Next> to accept the default selection.
The Application Wizard - Resources window appears. Resource files can help you develop foreign language versions of your program. Rather than force you to retype your program's text (such as manus and dialog box captions) in different languages, a resource file stores the information in a separate file so you can just use a different resource file for each foreign language.
7. Click on Next> to accetp the default selection.
The Application Wizard - Internet Connectivity window appears. This is where you can specify whether you want your program to be able to access the Internet.
8. Click on Next> to accept the default selection. The Application Wizard - Standard Forms window appears.
9. Click on the About Box check box and then click on Next>.
The Application Wizard - Data Access Forms window appears. This is where you can specify what type of database files you want your program to use.
10. Click on Next> to accept the default selection. The Application Wizard - Finished! Window appears.
11. Click on the Finish button. Visual Basic displays an Application Created dialog box.
12. Click on OK.
13. Press F5 to run the skeleton program that the VB Application Wizard created for you. The program appears.
14. Choose Help->About. An About Project1 dialog box appears.
15. Click on OK.
16. Click on any icon on the toolbar or on any of the pull-down menus to see what happens.
17. Choose File->Exit when you're done playing around with this skeleton program.
18. Choose File->New Project. A dialog box appears, asking if you want to save your newly created user interface.
19. Click on No.
As you can see, the VB Application Wizard can quickly create a user interface that includes pull-down menus and a toolbar. However, you still need to customize the user interface and write BASIC code to make the program do anything worthwhile. The VB Application Wizard simply gives you a head start in creating your user interface.
++ CHAPTER 4
WRITING BASIC CODE
In This Chapter:
Discovering the basics about BASIC code
Choosing an object and writing a procedure
Writing BASIC code for your program
To have your computer do anything, you have to give it step-by-step instructions. If you skip a step or give unclear instructions, your computer doesn't know what to do. (Actually, the computer knows what to do - it just doesn't do what you want it to.)
Programmers call a single instruction a command. A typical BASIC command looks like the following:
Taxes = Income * FlatTaxRate
You call a series of commands code. A typical series of commands looks like this:
Income = 90000
FlatTaxRate = .95
Taxes = Income * FlatTaxRate
A collection of code that makes your computer do something useful (such as play a game, calculate your taxes, or display flying toasters on your screen) is called a program.
If you want to speak the language of programmers (even though programmers are notorious for never saying much of anything), you have to know programming etiquette.
You never write a program; you write code. Heaven forbid if you say, "Let me look at your series of commands." Cool programmers are likely to blush at your faux pas. Instead, you ought to say, "Let me look at your code."
** What Is BASIC Code?
To get your computer to do anything, you have to give the machine instructions that it can understand. Because you're using Visual Basic, you have to use the BASIC programming language.
Like all computer languages, BASIC has special commands called reserved keywords. Some examples of reserved keywords are as follows:
Loop Function Sub End
Do Integer Case If
Else Select Then For
BASIC code consists of nothing more than BASIC reserved keywords creatively strung together to form a program. Whenever the computer sees a reserved keyword, the computer automatically thinks, "Oh, this is a special instruction that I already know how to obey."
A program can be as short as a single reserved keyword or as long as several million reserved keywords. Short programs generally don't do anything more interesting than display something such as Hello, world! on the screen. Long programs usually do much more, but these programs are often as confusing to read as an IRS tax form.
Theoretically, you can write one long program consisting of a million or more reserved keywords. However, any programmer attempting to do so is likely to go insane long before completing the task.
* Writing a program one step at a time
To make programming easier, most programmers divide a large program into several smaller ones. After you finish writing each of the smaller programs, you paste the pieces together to make a complete program.
When you divide a large program into several smaller ones, these smaller programs are subprograms. In Visual Basic lingo, subprograms are event procedures (although some programmers may call them subroutines). Visual Basic also has special subprograms called functions, which you can read about in Chapter 29, and general procedures, which you can read about in Chapter 27.
Procedures tell each object on your form how to react to something that the user does. Each object can have zero or more procedures that tell the object how to respond to the user.
One event procedure may tell the computer what to do if the user clicks on an object (such as a command button) with the mouse. Another event procedure may tell the computer what to do if the user presses a certain key while the object is highlighted.
Not every object needs event procedures. The only objects that need event procedures are those that the user can click on or choose in some way, such as command buttons, check boxes, or radio buttons.
* Choosing objects and Events
Before you can write an event procedure for an object, you have to tell Visual Basic:
The name of the object to use
The event you want the object to respond to
Visual Basic gives you two ways to choose an object:
The simplest way is to click on an object (such as a command button) on your form and press F7 (or just double-click on the object in the first place) to switch to the Code window. Visual Basic then kindly displays the most likely procedure that you need for that object.
The second way is almost as easy. Each time you draw an object on a form, Visual Basic stores the name of that object in a list called the Object list. The Object list appears at the top of the Code window. To open this list, press F7. Next, scroll through the Object list until you find the object for which you want to write a procedure.
To choose an event to use, click on the Procedure list, which also appears at the top of the Code window next to the Object list. The Procedure list contains all possible events that you can write a procedure to respond to.
Just scroll through this list until you find the event for which you need to write a procedure. The most common event to use is the Click event.
After you choose an object from the Object list and an event from the Procedure list, Visual Basic displays the first and last lines of the procedure. You're now ready to start writing code for this procedure.
** Writing Visual Basic Event Procedures
Before you can write a Visual Basic event procedure for an object, you have to draw the object on a form first.
Next, you have to change the properties of each object to give them unique names you can remember. If you don't do this, you're stuck with the generic names that Visual Basic provides by default for everything, such as Option1 or Text3.
Do you know what happens if you write an event procedure for a particular object and then later change that object's name? Visual Basic gets confused and thinks that you've created a brand-new object, which means that the renamed object won't have any event procedures attached to it. So if you're going to rename an object, do this renaming before you write any event procedures for that object.
To write a Visual Basic event procedure, click on the object that you want to write an event procedure for and then open the Code window. (To open the Code window, press F7, choose View->Code, or double-click on the object.) After the Code window appears, you can start typing your procedure or code.
But wait! Visual Basic doesn't just display a blank window. Visual Basic automatically types Private Sub, which is followed by the object's name, an underscore, an event (such as Click), and a set of parentheses ( ) that may be empty or that may contain stuff inside of it. (You can find out more about the stuff inside of parentheses in Chapter 28).
If you click on an object named cmdExit and then open the Code window, Visual Basic displays the following:
Private Sub cmdExit_Click( )
End Sub
The first line of this Visual Basic procedure begins with Private Sub, which is short for a subprogram that belongs exclusively to a specific object. In this case, the subprogram belongs to the object named cmdExit.
Next, Visual Basic types your object's name. If you forgot to change the name property of your object, Visual Basic uses a default name such as Text2. Otherwise, Visual Basic displays your object's name (for example, cmdExit).
Following your object's name is an underscore, which separates your object's name from its event. An event is something the user does to communicate with the computer. In this example, the event is Click, which means that the user clicked the mouse on the object named cmdExit.
Next comes an empty set of parentheses. Sometimes, the parentheses contain data that the subprogram uses from the routine that called this subroutine. An empty pair of parentheses says that this subprogram does not need any special data passed to it from another part of your program.
Translating this procedure into English, the first line means, "This is a subprogram for the object named cmdExit, and the subprogram tells the computer what to do if the user clicks on the cmdExit object."
The last line of this Visual Basic event procedure consists of two words: End and Sub. This line tells the computer, "This is the end of all the commands that belong in this subprogram." Rather than type all that, Visual Basic uses the simpler
End Sub
Right now, this Visual Basic procedure does nothing. To make the procedure do something, you have to add commands between the first line and the last line. Before you start adding commands, you need to know what BASIC commands (also known as code) can do.
** Test your newfound knowledge
1. What are reserved keywords?
a. Words you say when you have a reservation in a fancy restaurant where water costs $25 a glass.
b. What shy people want to say.
c. Special instructions that every programming language has.
d. Words that you waritto sayto the face of someone you don't like.
2. How can you write a large program without losing your mind?
a. Divide the program into subprograms, otherwise known in Visual Basic as procedures (such as event or general procedures).
b. Watch others write the program and then steal their homework.
c. You mean I actually have to write a program?
d. If you're thinking of writing a large program, you've probably already lost your mind.
* What can BASIC code do?
BASIC code is generally used to do the following:
Calculate a result
Modify the properties (appearance) of another object
If you want to calculate the number of people who live in wooden cabins, subscribe to Soldier of Fortune, and own cats, Visual Basic can calculate this as long as you provide all the necessary data.
After you calculate a result, you probably want to show the result on the screen. To do so, you have to modify the properties of an object on your user interface. For example, if you want to display a message on the screen, you first need to draw a text box object on a Visual Basic form.
You then have to name this text box with something such as txtMessage. Finally, to display anything in this text box, you have to modify the Text property of the txtMessage text box, such as:
txtMessage.Text = "This is hard to explain."
This command displays the message "This is hard to explain" in the txtMessage text box on the screen.
Of course, Visual Basic code can't change all the properties of an object. You can only change some properties (such as the object's name) during design time by using that object's property window.
* How a Visual Basic event procedure works
In Visual Basic, the instructions in an event procedure run only when a specific event occurs, such as when the user clicks on an object. The same set of instructions can run over and over again each time the user clicks on an object. The only time a Visual Basic program ends is when an object's procedures specifically tell the program to end.
As an example, look at the simplest Visual Basic procedure that is necessary for every program to stop completely. For a simple "Hello, world!" program, this procedure looks like the following:
Private Sub cmdExit_Click( )
Unload Me
End Sub
The code in this procedure contains a Visual Basic reserved keyword called Unload and a variable called Me. The Me variable represents the form, so this procedure tells Visual Basic, "Unload the currently displayed form from memory." Because this program only consists of one form, this command effectively stops the program from running.
This procedure runs only when the user clicks on the cmdExit object. If you look at the cmdExit object on your user interface, you see that the object is a command button labeled Exit.
As an alternative to using the command Unload Me to end aVisual Basic program, you can also use the command End instead, like in the following:
Private Sub cmdExit_Click( )
End
End Sub
So which method should you use? The End command forces your program to stop running immediately, which can be like stopping your car by crashing it into a wall. For a gentler, kinder approach to stopping your program, Microsoft wholeheartedly recommends that you use the Unload Me command instead.
If you run this program, the following is what happens:
1. Visual Basic displays your user interface on the screen, including a command button named cmdExit. This name does not appear on the user interface. Instead, the command button's Caption property appears, which is the word Exit.
2. Clicking on the Exit button causes Visual Basic to ask, "Hey, what's the name of this object on which the user just clicked?"
3. In a huff, Visual Basic quickly notices that the Exit button's name is cmdExit.
4. Then Visual Basic asks, "Are there any instructions here that tell me what to do if the user clicks on the cmdExit object?" Happily, Visual Basic finds the Private Sub cmdExit_Click ( ) procedure.
5. Visual Basic then examines the first instruction of the cmdExit_Click ( ) procedure. In this case, the instruction is Unload
Me, which tells Visual Basic to unload the form. Because this is the only form in the program, this effectively stops the program.
6. Visual Basic stops running the program and removes it from the screen. Naturally, all this happens in the blink of an eye, and your computer looks as though it's responding instantly.
** Writing BASIC Code for your First Visual Basic Program
For those who just like jumping right into the program without typing all the BASIC code, load the HELLO.VBP program from the enclosed CD-ROM in the back of the book.
Because experience is always the best teacher, the following steps show you how to write real-life BASIC code that you can use to impress your friends. You can find out how to write BASIC code for the HELLO.VBP program in Chapter 3.
Don't worry about understanding everything you're typing. The purpose of this exercise is just to show you how simple creating a program in Visual Basic can be.
1. Start Microsoft Visual Basic if you haven't already done so. (Or choose File->Open Project.)
Visual Basic displays a New Project or Open Project dialog box.
2. Click on the Recent tab, click on HELLO, and click on Open.
If the HELLO.FRM form does not appear on the screen, click on the frmHello form in the Project Explorer window and click on the View Object icon. (You can skip this step if you already have the form displayed on the screen from Chapter 3)
3. Click on the optGrin option button displayed in the upper-left corner of the form. (This is the button that says "I'm happy!")
4. To Open the Code window, press F7 or choose View->Code.
5. Type in the Private Sub optGrin_Click( ) procedure so that it looks like the following:
Private Sub optGrin_Click( )
imgFrown.Visible = False
imgSmile.Visible = False
imgGrin.Visible = True
txtSmile.TEXT = ""
txtGrin.TEXT = "I'm going,to Disney World!"
txtFrown.TEXT = ""
End Sub
6. Click on the downward-headed arrow in the Object list box at the top of the Code window and then choose the optSmile object.
Visual Basic displays an empty Private Sub optSmile_Click() procedure.
7. Type in the Private Sub optSmile_Click( )procedure so that it looks like the following:
Private Sub optSmile_Click( )
imgFrown.Visible = False
imgSmile.Visible = True
imgGrin.Visible = False
txtSmile.TEXT = "Hello, world!'
txtGrin.TEXT = ""
txtFrown.TEXT = ""
End Sub
8. Click on the downward-pointing arrow in the Object list box at the top of the Code window and then choose the optFrown object.
Visual Basic displays an empty Private Sub optFrown_Click( ) procedure.
9. Type in the Private Sub optFrown_Click( ) procedure so that It looks like the following:
Private Sub optFrown_Click( )
imgFrown.Visible = True
imgSmile.Visible = False
imgGrin.Visible = False
txtSmile.TEXT = ""
txtGrin.TEXT = ""
txtFrown.TEXT = "Good-bye, cruel world."
End Sub
10. Click on the Object list at the top of the Code window and then choose the cmdExit object.
Visual Basic displays an empty Private Sub cmdExit_Click( ) procedure.
11. Type in the Private Sub cmdExit_Click( )procedure so that it looks like the following:
Private Sub cmdExit_Click( )
Unload Me
End Sub
12. Press F5 to run your program or choose Run->Start.
If you typed everything correctly, Visual Basic displays your user interface on the screen.
13. Click on the option button next to the label "I'm sad."
Visual Basic displays a face on the screen, along with the message Good-bye, cruel world.
14. Click on the option button next to the label "I'm okay."
Visual Basic displays a smiley face on the screen, along with the message Hello, world!.
15. Click on the option button next to the label "I'm happy!"
Visual Basic displays a really happy face on the screen, along with the message - I'm going to Disney World!.
16. Click on the Command button labeled Exit.
Visual Basic quits running your program and returns you to Visual Basic.
You finally completed the Hello, World! Example. Now, you can see how you can use Visual Basic to create a friendly user interface quickly and easily
++ PART II
CREATING USER INTERFACES
In this part ..........
A user interface allows other people to use your program. The clumsier the user interface, the harder your program is to use. So if you make your user interface simple and logical, you can bet that more people will be able to use your program.
This is the fun part of the book. You aren't forced to type any bizarre code, comprehend arcane commands, or memorize ridiculous keystrokes. In this part of the book, you get to doodle on your computer screen while actually finding out how to write your own programs at the same time.
++ CHAPTER 5
USER INTERFACE DESIGN 101
In this chapter .......
Detailing tips for creating a user interface
Drawing objects on the user interface
Moving, deleting, and copying objects
Making objects unavailable
To make oneself understood to the people, one must first speak to their eyes.
Napoleon Bonaparte
First of all, nobody really wants to use your program. Most people would rather play at the beach, watch TV, or make out. However, people do want the results that your program can produce. If they could get these same results by other means with less work, they would. But because they can't, they're willing to use your program.
This means that people really want your program to read their minds and then magically do all their work for them automatically. Because that's not possible, the best you can hope for is to make your program as easy to use as possible. If a completely incompetent moron (your boss) can use your program, then most other people are going to be able to use your program as well.
** Before You Create Your User Interface
Creating a user interface doesn't mean just slapping together some pretty pictures in a colorful window and hoping that the user can figure out how your program works. Your program's user interface must make your program easy to use. To help you create a user interface, here are some points to keep in mind.
* Know your user
Before designing your user interface, ask yourself who is going to use your program. Are your typical users data-entry clerks who understand computers, or managers who understand only paper procedures and are learning to use a computer for the first time?
When you decide who your users are, design your user interface so that it mirrors the way the users already work, regardless of whether the user interface seems totally inefficient or alien to anyone else. Accountants readily accept spreadsheets because the row-and-column format mimics green sheets of ledger paper. Likewise, typists prefer word processors because a word processor mimics a blank sheet of paper.
But imagine if all word processors looked like spreadsheets with rows and columns. Any typist trying to use this kind of word processor would quickly feel lost and confused (although accountants may feel right at home with such a word processor).
The more a programmer understands the user, the more likely the interface is going to be used and accepted. The only person the user interface really has to satisfy is the user.
* Orient the user
Not surprisingly, people get lost wandering through today's supermalls, which contain multiple levels and two different time zones. How do you feel when you have no idea where you are and no idea where you can go from your current position?
This feeling of helplessness is the reason why lost kids cry uncontrollably and confused computer users curse under their breath. (This is also the reason why malls install directories with the big red X that says, "You are here.")
A good user interface must orient people so that they know where they are in your program and how to get out if they want. Some user interfaces display a message at the bottom of the screen, such as "Page 2 of 5." In this case, the user knows exactly how many pages are available for viewing and which page currently appears on the screen.
Your user interface is a map to your program. Make sure that your user interface shows just enough information to orient users but not too much to confuse them.
* Make the choices obvious
In addition to letting users know where they are in a program, a good user interface must also make all choices obvious to the user. If your user interface displays "Page 4 of 25" at the bottom of the screen, how can the user know what to do to see the next or previous page? One solution may be to show forward- and backward-pointing arrows in each bottom corner of the page. Another solution may be Next Page and Previous Page buttons.
As long as your program shows the user which options are available next and which keys to press or where to click the mouse, the user feels a sense of control and confidence when using your program.
* Be forgiving
The key here is useful feedback. If your program takes an arrogant attitude and displays scolding messages like "File MPR.DLL missing" whenever the user presses the wrong key or clicks the mouse in the wrong area, the user may feel intimidated if your program doesn't explain what the error message means and how he or she can avoid the error in the first place.
So be kind. Have your program hide or dim any buttons or menu commands that are unavailable to the user. If the user does press the wrong key or click the mouse in the wrong area, have your program display a window and explain what the user's options are. Users love a program that guides them, which means you can spend a lot less time answering phone calls for technical support.
* Keep it simple
Most programs offer users two or more ways to choose a specific command. You can click on a button, choose a command from menus, or press certain keystroke combinations (Ctrl+F2, for example). Of these three methods, clicking directly on the screen is the easiest procedure to remember and pressing bizarre keystroke combinations is the hardest.
Make sure that commonly used commands can be accessed quickly through a button or a menu. Not all commands must be or need to be accessed through a keystroke combination.
Although keystroke combination commands are faster to use, these commands are harder to learn initially. Make keystroke combinations easy to remember whenever possible. For a Save command, Ctrl+S is easier to remember than something totally abstract like Shift+F12. People can easily remember that S stands for Save, but who has any idea what F12 represents?
** Designing Your Visual Basic User Interface
When you write a Visual Basic program, you first have to design the user interface. Essentially, a Visual Basic user interface consists of objects that you place on the screen and arrange in some semblance of organization so that the screen looks pretty.
The common elements of a Visual Basic user interface consist of the following:
Forms (also known as windows)
Buttons (such as command buttons and radio buttons)
Boxes (such as text boxes and check boxes)
Labels
Pictures (such as icons and graphics)
To design your user interface, follow these steps:
1. Create a form.
2. Choose the object you want to draw from the Toolbox. (See Table 5-1.)
3. Draw the object on the form.
* TABLE 5-1
Tools in the Visual Basic Toolbox
Tool Name - Pointer
What this Tool Does - Selects objects
Tool Name - Picture box
What this Tool Does - Draws a box to display graphics
Tool Name - Label
What this Tool Does - Drawa a box to display text
Tool Name - Text box
What this Tool Does - Draws a box that can display text and let the user type in text
Tool Name - Frame
What this Tool Does - Groups two or more objects together
Tool Name - Command button
What this Tool Does - Draws a command button
Tool Name - Check box
What this Tool Does - Draws a check box
Tool Name - Option (or radio) button
What this Tool Does - Draws a radio button
Tool Name - Combo box
What this Tool Does - Draws a combo box
Tool Name - List box
What this Tool Does - Draws a list box
Tool Name - Horizontal scroll bar
What this Tool Does - Draws a horizontal scroll bar
Tool Name - Vertical scroll bar
What this Tool Does - Draws a vertical scroll bar
Tool Name - Timer
What this Tool Does - Places a timer on a form
Tool Name - Drive list box
What this Tool Does - Draws a drive list box that displays all the disk drives available
Tool Name - Directory list box
What this Tool Does - Draws a directory list box that displays a directory on a particular disk drive
Tool Name - File list box
What this Tool Does - Draws a file list box that displays files in a specific directory
Tool Name - Shape
What this Tool Does - Draws a geometric shape such as a circle or a square
Tool Name - Line
What this Tool Does - Draws a line
Tool Name - Image box
What this Tool Does - Draws a box to display graphics
Tool Name - Data control
What this Tool Does - Draws a control to link a program to a database file
Tool Name - OLE
What this Tool Does - Draws a box to insert an OLE object
* Drawing an object
To draw any object, follow these steps:
1. Click on the icon in the Visual Basic Toolbox that represents the object you wan to draw (command button, picture box, label, and so on).
2. Move the mouse to the place on the form where you want to draw the object.
The cursor turns into a crosshair shape.
3. Click and drag the mouse to where you want to draw your object and then release the mouse button.
If you want to draw an object on a form in a hurry, double-click on the object's Icon in the Toolbox. For example, if you want to draw a command button quickly, just double-click on the command button icon in the Toolbox, and Visual Basic draws the command button for you automatically.
* Using the Properties Window
After you draw an object on a form, the next step is to define the properties for that object. To define an object's properties, you have to use the Properties window. In case the Properties window doesn't appear on your screen, you can make the window appear by using one of the following three commands:
Press F4.
Choose View ->Properties Window.
Click on the Properties Window Icon in the toolbar.
The Properties window can display properties in two ways:
Alphabetical: Lists an object's properties alphabetically from A to Z.
Categorized: Organizes properties into categories, such as properties that affect an object's appearance, behavlor, or position on the screen.
After you open the Properties window, follow these steps to view the properties of any object in your Visual Basic program:
1. Choose one of these methods to open the Project Explorer window.
Press Ctrl+R.
Choose View->Project Explorer.
Click on the Project Explorer icon in the toolbar.
2. Click on a form name in the Project Explorer window that contains the object whose properties you want to examine and then click on the View Object lcon.
3. Click on the downward-pointing arrow of the Object list box in the Properties window and choose the object whose properties you want to examine.
4. Double-click on the property name that you want to change.
If you click on an object displayed on a form, the Properties window automatically displays all the properties for that object.
* Naming objects
Every object you draw has a Name property, which Visual Basic uses to identify the object. (That's the same reason your parents gave you a name - so people don't say, "Hey, you!" all the time to get your attention.) If you click on the Alphabetic tab in the Properties window, the Name property appears as (Name) at the top of the list.
Every Visual Basic object must have a unique name. If you try to give the same name to two different objects, Visual Basic complains and refuses to let you make such a horrid mistake.
When you create an object, Visual Basic automatically gives your object a boring, generic name. For example, the first time you create a command button, Visual Basic names the button Command1. The second time you create a command button, Visual Basic names this new button Command2, and so on.
The name of an object never appears on the screen. Names can be up to 40 characters long, but they cannot contain punctuation marks or spaces. You can name your objects anything you want, but Microsoft recommends that Visual Basic programmers all over the world use Visual Basic three-letter prefixes, as shown in Table 5-2. If everyone in the world uses these prefixes, it makes modifying another person's Visual Basic program easier to read.
To change the name of an object, follow these steps:
1. Click on the object that you want to name.
Handles appear around the object. (To name a form, click anywhere on the form, but do not click on any objects on the form.)
2. Open the Properties window.
To do so, press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.
3. Double-click on (Name) and type a new name.
* TABLE 5-2
SUGGESTED PREFIXES WHEN NAMING OBJECTS
Object - Check box
Suggested Prefix - chk
Example Name - chkYourZipper
Object - Combo box
Suggested Prefix - cbo
Example Name - cboBLT
Object - Command button
Suggested Prefix - cmd
Example Name - cmdOpenSesame
Object - Data
Suggested Prefix - dat
Example Name - datFanOverThere
Object - Directory list box
Suggested Prefix - dir
Example Name - dirTree
Object - Drive list box
Suggested Prefix - drv
Example Name - drbLikeMad
Object - File list box
Suggested Prefix - fil
Example Name - filDocuments
Object - Form
Suggested Prefix - frm
Example Name - frm1040Tax
Object - Frame
Suggested Prefix - fra
Example Name - fraGroupedButtons
Object - Horizontal scroll bar
Suggested Prefix - hsb
Example Name - hsbTemperature
Object - Image
Suggested Prefix - img
Example Name - imgPeace
Object - Label
Suggested Prefix - lbl
Example Name - lblFakeName
Object - Line
Suggested Pefix - lin
Example Name - linBorder
Object - List box
Suggested Prefix - lst
Example Name - lstCandidates
Object - Menu
Suggested Prefix - mnu
Example Name - mnuHamandEggs
Object - Picture box
Suggested Prefix - pic
Example Name - picPrettyPictures
Object - Radio button
Suggested Prefix - opt
Example Name - optStation101
Object - Shape
Suggested Prefix - shp
Example Name - shpUpOrShipOut(circle, square, oval, rectangle, rounded rectangle, and rounded square)
Object - Text box
Suggested Prefix - txt
Example Name - txtReadStuffHere
Object - Vertical scroll bar
Suggested Prefix - vsb
Example Name - vsbMoneyWasted
* Creating captions for objects
In addition to a name, most (but not all) objects also have a caption. An object's caption appears as text on the screen.
By default, an object's caption and name are the same until you change them. So the moment you draw a check box on a form, the check box's caption is something dull like Check1, and the check box's name is also Check1.
The caption for a form appears in the title bar of that form. The caption for an object (such as a command button, label, or text box) appears directly on that object while a caption for a check box or radio button usually appears to the right of the check box or option button.
Captions are meant to help the user figure out how to use your program. A caption can be blank or up to 255 characters long, including spaces, punctuation marks, and four-letter words. The following are valid captions:
Hello
Hello, buddy!
Do I really know what I'm doing?
* To change the caption of an object, follow these steps:
1. Click on the object whose caption you want to change so that black handles appear around the object. (To select a form, click anywhere on the form, but do not click on any objects on the form.)
2. Open the Properties window (by pressing F4).
3. Double-click on the Caption property and type a new caption. Notice that Visual Basic displays your caption on the screen as you type.
* Adding hot keys to a Caption:
Besides looking pretty and displaying information to the user, captions can also be used to create hot keys so that the user can choose an object without having to click on the object with the mouse.
To add a hot key to a caption, you have to put the ampersand character (&) into an object's caption. You may be wondering, "Why the heck do I want to use an ugly symbol like that?" The answer is to give users yet another way to choose an object on your user interface.
For example, to push a command button, users can either:
Click on the command button with the mouse.
Press Tab until the command button appears highlighted and then press the spacebar or Enter (real obvious, huh?).
However, if you use the ampersand in the command button's caption, the user can push that button by pressing Alt plus whatever letter the ampersand is in front of. For example, if a command button has a caption of &Exit, it appears in the command button with the E underlined, as in Exit. To push this button, you can simply press Alt+E. If the command button has a caption of E&xit, however, the caption appears in the command button with the x underlined, as in Exit, and you can use Alt+X to press this command button.
* Changing the font of an object's caption:
When you create a caption, Visual Basic displays your caption on the screen in a plain type style. For you creative types who want to make your captions look spiffier, you can change the font, type style, and size of captions to give them more pizzazz.
Fonts are different ways to display text. Normally, Visual Basic uses the MS Sans Serif font, but you can use any font stored in the memory of your computer. (MS Sans Serif is similar to the Helvetica font, and the Visual Basic MS Serif font is similar to the Times Roman font.)
To change the font of a caption, follow these steps:
1. Click on the object whose caption you want to modify.
2. Open the Properties window (by choosing View->Properties Window).
3. Double-click on the Font property.
Visual Basic displays a dialog box of all the fonts you can use.
4. Click on the font you want and click on OK.
Visual Basic immediately changes the font of the caption.
Fonts give you a chance to be creative, but they can also disorient the user, especially if you choose bizarre fonts that don't look like anything normally found in nature. To avoid confusion, let Visual Basic use the default font of MS Sans Serif, unless you have a really good reason to use a different font.
if you choose a really bizarre font, that font may not exist on other people's computers. When using fonts, try to use the most common ones found on all computers or else your program's captions may look really weird.
* Changing the size of an object's caption:
You can also change the size of your caption by making it smaller or larger to fit inside your object. Depending on the font you choose, Visual Basic gives you a variety of font sizes to choose from.
For example, if you choose the MS Sans Serif font, Visual Basic gives you the following choices of font sizes:
8
10
12
14
18
24
Obviously, the more font sizes you use, the odder your captions look. The best approach is to use one size to avoid confusing the user any more than you have to.
To define the font size of your captions, follow these steps:
1. Click on the object whose font size you want to modify.
2. Open the Properties window (by clicking on the Properties Window icon on the toolbar).
3. Double-click on the Font property. Visual Basic displays a dialog box.
4. Choose the font size you want to use and click on OK.
* Changing the type style of an object's caption:
If changing the font and size of your captions isn't enough excitement for one day, Visual Basic also lets you change the type style of your captions. The number of available type styles depends on the font you're using for your caption.
For example, if you choose the MS Sans Serif font, Visual Basic gives you the following choices of type styles:
Regular
Italic
Bold
Bold Italic
Underline
Strikeout
If you want, you can even combine two or more type styles for extra emphasis.
To set different font styles, follow these steps:
1. Click on the object whose caption you want to modify.
2. Open the Properties window (by pressing F4).
3. Double-click on the Font property. Visual Basic displays a dialog box.
4. Click on the font style you want to use and click on OK. Visual Basic immediately changes the caption's appearance.
* Changing the background and foreground colors of captions:
Captions normally appear in boring black, white, and shades of gray. To make your captions stand out more colorfully, you can change the background and foreground colors. The BackColor property of an object represents the object's background color, and the ForeColor property represents the foreground color.
Unlike other types of objects, command buttons have only a BackColor property. The BackColor property simply changes the color that surrounds the caption when the command button is highlighted.
To change the color surrounding an object's caption, follow these steps:
1. Click on the object whose background color you want to change.
2. Open the Properties window (by pressing F4).
3. Double-click on the BackColor (or ForeColor) property and click on the Palette tab. Visual Basic displays a color palett
4. Click on the color you want. Visual Basic instantly obeys.
* Moving objects on the screen
Objects can appear anywhere on a form. Visual Basic provides two ways to define the position of an object on the form:
Use the mouse.
Change the Left and Top properties in the Properties window.
To change the position of an object using the mouse, follow these steps:
1. Click on the object you want to move so that black handles appear around the object.
2. Position the mouse over the object (not over one of the object's handles). Then hold down the left mouse button and move the mouse to where you want the object to appear.
3. Release the mouse button.
In case you haven't figured out by now what is happening, when you create and place an object for the first time, that position is where Visual Basic displays the object. Use the mouse whenever you want to move an object quickly without regard to exact placement on the screen.
For more precise measurements when moving an object, use the Properties window and type in values for the Left and Top properties.
For objects, the Left property measures the distance from the left edge of the form to the left edge of the object. The Top property measures the distance from the top of the form to the top of the object.
For forms, the Left property measures the distance from the left edge of the screen to the left edge of the form. The Top property measures the distance from the top of the screen to the top of the form.
To change the position of an object using the Properties window, follow these steps:
1. Click on the object you want to move so that black handles appear around the object.
(If you want to change the position of a form, click anywhere on the form so that handles appear around the edges of the form.)
2. Open the Properties window (by pressing F4).
3. Double-click on the Left property and type a new value.
4. Double-click on the Top property and type a new value.
* Deleting objects off the face of the earth
Sometimes you draw an object and then decide that you don't need it after all.
To delete an object, follow these steps:
1. Click on the object you want to delete.
2. Press Delete or choose Edit->Delete.
If you press Ctrl+Z right after you delete an object, you can undelete the object you previously deleted.
* Copying objects because you're too tired to draw new ones
After you draw an object that is the exact size you need, you may want to make a copy of the object rather than create a new one and go to the trouble of resizing it.
To copy an object, follow these steps:
1. Click on the object you want to copy.
2. Press Ctrl+C, choose Edit->Copy, or click on the Copy Icon.
3. Press Ctrl+V, choose Edit->Paste, or click on the Paste icon.
Visual Basic displays a dialog box that asks whether you want to create a control array. If you know what a control array is and want to create one, click on Yes; otherwise, click on No. Visual Basic displays a copy of your object in the upper-left corner of the form.
A control array lets you create two or more objects that share the same name. That way, two or more objects can share the same event procedure. If none of this makes any sense to you, just pretend this paragraph doesn't exist or refer to the latest edition of MORE Visual Basic For Dummies, which explains what control arrays are and why you might want to use them.
4. Move this copy of the object anywhere on your screen.
* Selecting more than one object to move, copy, or delete
Before you can move, copy, or delete any object, you have to select the object by clicking on the object. However, if you want to move, copy, or delete more than one object at the same time, you have two choices:
Use the mouse to select multiple objects.
Click on multiple objects while holding down Ctrl or Shift.
To use the mouse to select multiple objects, follow these steps:
1. Position the mouse at the upper-left corner of the group of objects you want to select. (But don't position the mouse directly over any of the objects you want to select.)
2. Hold down the left mouse button while you drag the mouse to the lower-right corner of the group of objects you want to select.
Visual Basic displays a dotted line around all the objects you select.
3. Release the mouse button.
Visual Basic displays a gray rectangle around all the objects you select.
To click on multiple objects while holding down Ctrl or Shift, follow these steps:
1. Click on the first object that you want to select.
Visual Basic displays black handles around the object.
2. Point to the second object that you want to select.
3. Press Ctrl or Shift while you click on the second object.
Visual Basic displays gray rectangles around this object and each of your previously selected objects.
4. Repeat Steps 2 and 3 until you select all the objects you want.
* Changing the size of objects
After creating an object, the next step is to define the object's size, which is a topic most men tend to exaggerate. Visual Basic provides two ways to change the size of an object:
Use the mouse.
Change the Height and Width properties in the Properties window.
To change the size of an object using the mouse, follow these steps:
1. Click on the object that you want to resize. Little black handles appear around the edges of the object.
2. Move the mouse to the edge of the object until the mouse pointer turns into a double-headed arrow.
3. Hold down the mouse button and drag the mouse. When the object is in the shape you want, release the mouse button.
To change the size of a form using the Properties window, follow these steps:
1. Click on the object you want to resize.
2. Open the Properties window (by pressing F4).
3. Double-click on the Height property, type a new value, and press Enter.
4. Double-click on the Width property, type a new value, and press Enter.
Use the mouse method when the exact size of your object isn't crucial. Change the Height and Width properties manually when you want absolute precision or when you feel like being picky about details that nobody else cares about.
* Defining the TabIndex property of your objects
The TabIndex property determines the order in which Visual Basic highlights buttons when the user presses either the Tab, up-arrow, down-arrow, right-arrow, left-arrow, or Shift+Tab keys.
The Tab, down-arrow, and right-arrow keys highlight the object with the next-highest TabIndex value.
The Shift+Tab, up-arrow, and left-arrow keys highlight the object with the next-lowest TabIndex value. To highlight option buttons, you can use only the up, down, right, and left arrow keys, but not the Tab or Shift+Tab keys.
The spacebar or Enter key selects a highlighted object.
Some objects, such as image boxes and menu objects, don't have a TabIndex property, so you can't highlight them by pressing any keys.
An object with a TabIndex property set to 0 appears highlighted as soon as your program runs. If the user presses Tab, the object with a TabIndex property of 1 is highlighted next, and so on.
The first object that you create has a TabIndex property of 0. The second object that you create has a TabIndex property of 1, and so on.
The only way to highlight an object stored inside a frame (see Chapter 6) is to press Tab or to click on the object with the mouse. After an object inside a frame is highlighted, pressing the up-, down-, left-, and right-arrow keys only highlights other command buttons or objects in that frame.
Although most users use a mouse to highlight and select objects, some users may not. For those rare instances, the keyboard is the only way these behind-the-times people can select the objects on your user interface.
To change the TabIndex property of an object, follow these steps:
1. Click on the object that you want to modify.
2. Open the Properties window (by pressing F4).
3. Double-click on the TabIndex property and type a number (such as 1 or 4).
Whenever you change the TabIndex property of a button, Visual Basic automatically renumbers the TabIndex of your other buttons. Thus, you can never assign two buttons identical TabIndex values.
If you have created lots of objects, you can set the TabIndex properties for them quickly and easily by following these steps:
1. Click on the object that you want highlighted last (the object that Is to have the highest TabIndex property value).
2. Open the Properties window (by pressing F4).
3. Click on the TabIndex property and type 0.
4. Click on the object that you want highlighted second to last.
5. Repeat Steps 2 through 4 until you set the TabIndex properties for all your objects to 0.
If you follow these steps, the last object you click on has a TabIndex of 0, the second-to-last object you click on has a TabIndex of 1, and so on.
For some reason if you don't want the user to be able to highlight an object by pressing the tab key, you can set that object's TabStop property to False.
* Dimming objects
If you don't want the user to press a particular object (such as a command button, a check box, or an image box), you can dim the object. A dimmed object tells the user, "Sometimes you can click on this object but not right now. So there."
To dim an object, follow these steps:
1. Click on the object that you want to dim.
2. Open the Properties window (by pressing F4).
3. Click on the Enabled property and set it to False.
A dimmed object doesn't do anything, so if you want to undim it during runtime events, you eventually have to use BASIC code.
To give you a sneak preview of the incredible power of BASIC code, here's how BASIC undims and dims a button. To undim a button, set the button's Enabled property to True. The following example undims a command button named cmdExit:
cmdExit.Enabled = True
To dim a button using BASIC code, set the button's Enabled property to False. The following example dims a command button named cmdExit:
CmdExit.Enabled = False
You can dim and undim buttons using BASIC code while your program is running. That way, you can dim and undim buttons in response to whatever the user is doing (typing, moving the mouse, pounding helplessly on the keyboard, and so on).
* Making objects invisible
Rather than dimming an object (which essentially taunts the user because the object is there but unavailable), you can make objects disappear completely.
To make an object disappear, follow these steps:
1. Click on the object you want to disappear.
2. Press F4 or select Properties from the View menu to open the Properties window.
3. Click on the Visible property and set it to False.
You can also make an object disappear using BASIC code. To do so, set the object's Visible property to False. The following example makes a command button named cmdNew disappear:
CmdNew.Visible = False
Like dimmed objects, invisible objects are useless unless you can make them visible once in a while. To make an object appear again, you have to use BASIC code to set the object's Visible property to True. The following example makes a command button named cmdNew appear:
CmdNew.Visible = True
* Adding Tooltips to an Object
Despite the standard user interface of Windows 95/98/NT, many people may still have no clue what the different objects of your program actually do. Rather than click on these unmarked objects at random (and risk wrecking their data), most people sit paralyzed with fear and wind up never using many parts of a program at all.
To overcome this hesitation, all of your objects can display tooltips. A tooltip is nothing more than brief text that explains what a particular object does. The tooltip remains invisible until the user moves the mouse cursor over the object and leaves the mouse hovering there for a few seconds. Then your Visual Basic program can spring to life and display the tooltip text.
To create tooltip text for your objects, follow these steps:
1. Click on any object on your form.
2. Open the Properties window (by pressing F4).
3. Double-click on the ToolTipText property.
4. Type the brief explanation that you want to appear whenever the user leaves the mouse cursor over the object.
* Doing Caption Changes for Yourself
The following sample program lets you change the caption on the form by typing a new caption in a text box and by clicking on the cmdCaption command button. Just create a user interface according to Table 5-3 and see for yourself the amazing power of Visual Basic.
TABLE 5-3
Properties to Change for CAPTION.VBP
Object - Form:
Property - Caption
Setting - The Incredible Changing Caption
Property - Height
Setting - 3885
Property - Width
Setting - 4680
Object - Lable1:
Property - Name
Setting - lblHeadline
Property - Caption
Setting - This caption can be changed by clicking on the command button below
Property - Height
Setting - 600
Property - Left
Setting - 240
Property - Top
Setting - 360
Property - Width
Setting - 4000
Object - Label2:
Property - Caption
Setting - Type a new caption here:
Property - Height
Setting - 300
Property - Left
Setting - 120
Property - Top
Setting - 1680
Property - Width
Setting - 1935
Object - Text1
Property - Name
Setting - txtCaption
Property - Height
Setting - 495
Property - Left
Setting - 2280
Property - Multiline
Setting - True
Property - Text
Setting - (Empty)
Property - Top
Setting - 1560
Property - Width
Setting - 1935
Object - Command1
Property - Name
Setting - cmdCaption
Property - Caption
Setting - Change Caption
Property - Height
Setting - 495
Property - Left
Setting - 1200
Property - ToolTipText
Setting - Click here to change the caption
Property - Top
Setting - 2640
Property - Width
Setting - 2175
For those of you who don't like typing, you can examine the CAPTION.VBP program on the enclosed CD-ROM.
Private Sub cmdCaption_Clock( )
LvlHeadline.Caption = txtCaption.Text
End Sub
When you run this program, just type a new caption in the text box and click on the command button labeled Change Caption. Visual Basic immediately displays your newly typed text in the top label.
++ CHAPTER 6
FORMS AND BUTTONS
In This Chapter:
Creating forms and drawing borders
Choosing the order in which forms are displayed
Creating buttons
The main part of a user interface is a window, which Visual Basic calls a form. A Visual Basic program needs at least one form, but most programs use two or more forms.
For example, a typical program uses one form to display a list of command buttons on which to click. If the user clicks on a command button, a second form appears displaying information such as names, addresses, and telephone numbers of people who owe you money.
The CD-ROM at the back of this book contains a simple Visual Basic program that shows how to change the background of a form. Feel free to experiment with this program and modify it so you can see how you can easily change the appearance of a form.
** Creating a Form
Visual Basic provides two ways to create a form for your program:
Create a blank form
Use a form template
Creating a blank form is handy when you want to design the appearance of the form all by yourself. As a quicker way of creating forms, you can use a form template, which provides pre-designed forms for displaying a Tip of the Day, Web Browser, or Splash Screen window in your program.
If you use a form template, you still have to customize the form for your program, but at least you won't have to waste time drawing objects on the form from scratch.
Form: Creates a blank form.
Dialog: Creates a skeleton of a dialog box with an OK and Cancel command button.
About Dialog: Displays information about your program.
Log in Dialog: Forces users to type a User ID and password before they can use your program.
Options Dialog: Displays a dialog box with tabs, allowing your program to offer options for customizing your program.
Splash Screen: Displays the name of your program along with a logo whenever your program loads.
Tip of the Day: Provides tips for using your program.
ODBC Log In: Displays options for connecting your program to a database.
Web Browser: Adds a Web browser to your program.
VB Data Form Wizard: Automatically creates a form to display and edit database information.
* To create a form for your program, follow these steps:
1. Choose Project->Add Form (or click on the Add Form Icon on the toolbar and click on form).
An Add Form dialog box appears.
2. Click on an icon in the Add Form dialog box, such as the Form (which adds a blank form), Web Browser, or Splash Screen, and click on Open.
* Saving Forms
After you create a form, you may want to save the form (if not for religious purposes, then at least for practical ones) so that you don't have to create the form all over again later. To save a form, Visual Basic gives you two choices:
Choose File->Save Form
Press Ctrl+S
If you have two or more forms displayed on the screen and you want to save changes to all your forms at once, choose File->Save Project or click on the Save Project icon. This command automatically saves every file (listed in the VBP project file) that makes up your entire Visual Basic program.
Get into the habit of periodically saving your forms. If your computer fails, the power goes out (but doesn't get your hard drive), or terrorists raid your home and riddle your computer with bullets, you lose only the changes that you've made since you last saved the form.
* Viewing different forms
Most Visual Basic programs that you create need two or more forms. To keep multiple forms from cluttering up your screen, Visual Basic kindly displays only one form at a time.
So if you want to switch to a different form to look at, follow these steps:
1. Choose one of the following to switch to the Project Explorer window:
Choose View->Project Explorer
Press CtrL+R
Click on the Project Explorer icon in the toolbar
2. Look for the Forms folder. If a plus sign appears to the left of the Forms folder, click on the plus sign.
Visual Basic displays a list of all the forms that make up your program.
3. Click on the form that you want to view and then click on the View Object icon in the Project Explorer window (or just double-click on the form name).
* Positioning a form with Form Layout window
The Form Layout window lets you decide where your forms initially appear on the computer screen when the program actually runs. In case you can't see the Form Layout window, choose one of the following:
View->Form Layout window
Click on the Form Layout window icon on the toolbar
The Form Layout window displays your forms within a tiny computer screen. To move a form's position, follow these steps:
1. In the Form Layout window, move the mouse cursor over the form that you want to move. The mouse cursor turns into a four-way pointing arrow.
2. Hold down the left mouse button and drag the mouse to move the form to its new position.
3. Release the left mouse button.
No matter where your form may appear on the screen while you're editing it, the Form Layout window controls the real position of your form when your program runs.
For a quick way to adjust the position of a form within the Form Layout window, right-click on a form, highlight Startup Position, and then choose an option from the pop-up menu. As another alternative, you can change the StartUpPosition property directly within the Properties window.
* Drawing borders around forms
Borders make forms look pretty, and they also give nations something to argue about. By changing the BorderStyle property, forms can have one of six types of borders.
In addition to making your forms look pretty, each border style also affects whether the user can move or resize the form.
The 0 - None style doesn't display a border around your form, but any objects your form contains can still be seen. A user cannot move, resize, or minimize this type of form.
The 1 - Fixed Single style displays a Control box, title bar, and Close box. Users can move, but not resize, this type of form.
The 2 - Sizable style is the default style. This style displays a Control box, a title bar, Minimize and Maximize buttons, and the Close box. Users can move, resize, and minimize or maximize this form.
The 3 - Fixed Diallog style displays a Control box, a title bar, and a Close box. Users can move this form but cannot resize, minimize, or maximize this form style.
The 4 - Fixed ToolWindow style displays a title bar and Close box. Users can move this form but cannot resize, minimize, or maximize this style of form.
The 5 - Sizable ToolWindow style displays a title bar and Close box. Users can move and resize this form.
To change the borders around your form using the Properties window, follow these steps:
1. Click anywhere on the form where there is not an object.
2. Open the Properties window (by pressing F4).
3. Click on the BorderStyle property.
4. Choose one of the following:
0 -None
1 - Fixed Single
2 - Sizable
3 - Fixed Dialog
4 - Fixed ToolWindow
5 - Sizable ToolWindow
* Minimizing and maximizing forms
Forms can cover part of the screen or the entire screen. Any form that hogs the whole screen is considered maximized. At the other extreme, forms can be shrunk and displayed on the screen as !cons. A shrunken form is considered minimized. Any form that just covers part of the screen is considered normal. Anyone who thinks that computer programmers invent too many definitions is also considered normal.
If you define a BorderStyle of 0 - None, 1 - Fixed Single, 3 - Fixed Dialog, 4 -Fixed ToolWindow, or 5 - Sizable ToolWindow, you cannot minimize or maximize a form.
To display a form as normal, minimized, or maximized while your program runs, follow these steps:
1. Click anywhere on the form. (Do not click on any objects on the form.)
2. Open the Properties window (by pressing F4).
3. Click on the WindowState property in the Properties window.
4. Choose one of the following:
0 - Normal
1 -Minimized
2 - Maximized
You can also give the user the option of minimizing or maximizing a form. To do so, your form needs to display Minimize and Maximize buttons.
To display Minimize and Maximize buttons on a form, follow these steps:
1. Click anywhere on the form where there are no objects.
2. Open the Properties window (by pressing F4).
3. Click on the MinButton (or the MaxButton) property in the Properties window and choose True or False. Depending on the border style you choose for your form, the default setting for the MinButton and MaxButton settings may be True or False.
* Removing forms
Occasionally, you may decide you don't want a particular form as part of your Visual Basic program after all. To remove a form from a Visual Basic project, follow these steps:
1. Choose one of the following to switch to the Project Explorer window:
Choose View->Project Explorer
Press Ctrl+R
Click on the Project Explorer Icon in the toolbar
2. Click on the form you want to remove.
3. Choose Project->Remove or right-click on the form name and choose Remove.
If you remove a form that you've previously saved, the form still exists on your hard disk; it just isn't part of your Visual Basic project anymore. To physically remove all traces of a form out of existence, use Windows Explorer and delete the form file.
** The Control Box
Users can move or resize a form by using the mouse. To minimize or maximize a form, simply click the Minimize or Maximize button. For those users who haven't caught up with current technology or refuse to use a mouse, each form also provides a control box, as well.
If you choose a BorderStyle of 0 - None, 4 - Fixed ToolWindow, or 5 - Sizable ToolWindow, you won't see a control box icon on your forms.
To activate the control box menu while a program is running, click on the control box icon with the mouse pointer or press Alt+spacebar. A menu appears, which lets the user move, resize, minimize, or maximize a form.
* Removing a control box
A control box appears on all forms. But if you think that the box looks really ugly and want to remove it, follow these steps:
1. Click anywhere on the form (except where you find objects on the form).
2. Open the Properties window (by pressing F4).
3. Click the ControlBox property in the Properties window.
4. Choose True or False.
Depending on the border style you choose for your form, the default value may be True or False.
* Displaying icons as the control bar on forms
Icons are special graphics symbols with the ICO file extension. Normally, Visual Basic displays the control box of a form with a default icon that looks like a sail flapping in the wind. If you don't like this icon, you can customize the icon. To change an icon with the Properties window, follow these steps:
1. Click anywhere on the form, but not on any objects on the form.
2. Open the Properties window (by pressing F4).
3. Double-click on the Icon property in the Properties window. Visual Basic displays a Load Icon dialog box.
4. Choose the Icon you want to use and click on Qpen.
* Choosing Which Form Visual Basic Displays First
When your program runs, the first form your program displays is generally the first form you've created. To make another form appear first, follow these steps:
1. Choose Project->ProjectName Properties (where ProjectName Is the name of your Visual Basic project). Visual Basic displays a ProSect Properties dialog box.
2. Click on the General tab. Visual Basic displays the Project Properties dialog box.
3. Click on the downward-pointing arrow of the Startup Object list box to display a list of all the forms for your project.
4. Choose the form you want to display first and click on OK.
** Defining Units of Measurement on a Form
The reason to have a form is so that you can have a place onto which you can put objects. You may notice, therefore, that every form always displays a grid to help you align objects.
By default, each form uses a unit of measurement called a twip, which sounds like something Elmer Fudd says. ("I'm tying stwing acwoss this path so that wabbit will twip ove wit. Eh, eh, eh, eh, eh.")
In case you actually care, 1,440 twips equal 1 inch. If you don't like using twips as your preferred unit of measurement, Visual Basic offers seven options:
Twips (1,440 twips = 1 inch)
Points (72 points = 1 inch)
Pixels (The number of pixels that equals 1 inch depends on your monitor's resolution.)
Characters (A character is 1/6 inch high and 1/12 inch wide.)
Inches (1 inch = 1 inch - amazing, don't you think?)
Millimeters (25.4 mm = 1 inch)
Centimeters (2.54 cm = 1 inch)
Hard-core programmers may be happy to know that Visual Basic even lets you create your own customized coordinate system. If you're thinking about doing that, you probably need to read a book with a title like Visual Basic For Hard-Core Programming Geniuses instead of this book. For the rest of us, the seven available units of measurement in Visual Basic are more than sufficient.
* To change your form's grid scale, follow these steps:
1. Click anywhere on the form. (Do not click any objects on the form.)
2. Open the Properties window (by pressing F4).
3. Click on the ScaleMode property.
4. Click on the downward-pointing arrow to display a list of all the different measurement units you can use.
5. Select the unit of measurement you want your form to use.
** Pushing Your Buttons
Pushing a button is a simple task that anyone can do. Even children can push buttons, which gives them the power to throw a hot dog in a microwave oven and shout with glee when the meat explodes before their eyes.
Everyone uses buttons. Your disk drive probably has a button that you press to eject a floppy disk. Your monitor has a button to turn the screen's power on and off. Even your mouse has a button (or two or three).
Because buttons are so familiar and easy to use, programs often display buttons on the screen that you can push with a mouse. Instead of forcing you to wade through various menus to find the right command, buttons conveniently display your options right before your eyes. All you have to do is figure out which button you want to press.
Buttons are a feature of nearly every program. Therefore, the rest of this chapter is all about making, modifying, and pushing your own buttons.
* Types of buttons
Essentially, a button is nothing more than an area on the screen that the user can click with the mouse. When pushed (clicked on), a command button rushes off and performs a command. (That's why they call them command buttons.) Visual Basic lets you create two types of buttons: command buttons and image buttons.
A command button displays a caption or an optional icon. This caption can be as unimaginative as OK, Cancel, or Quit. Or the caption can represent a particular command, such as Erase File, Next Screen, or Lose Mr. Johnson's Airline Reservation.
Command buttons often appear in dialog boxes where the program displays a message, such as, "Do you really want to erase your IRS tax files to avoid criminal prosecution?" The available choices may be Yes and No.
Unlike a command button that can display both captions and icons, an image button can display only a picture.
The advantage of image buttons is that they can be smaller than command buttons. The disadvantage is that unless the user knows which command each image button represents, the user has no idea how to use your image buttons.
Because image buttons aren't as self-explanatory as command buttons, a good idea is to type a brief description of the image button's purpose in the ToolTipText property (see Chapter 5).
For example, if the only way to exit your program is to click an image button that shows an open door, people may not understand the connection and may wind up turning off their computer to exit your program instead. (And then they probably aren't going to use your program again, either.)
* Creating buttons
To create a command button, follow these steps:
1. Click on the Command Button Icon In the Visual Basic Toolbox.
2. Move the mouse to the place on the form where you want to draw the command button.
3. Press and hold the left mouse button and drag the mouse to form a command button box.
4. Release the left mouse button to complete the operation. Visual Basic displays a boring label like Command1 in the command button.
Although command buttons are the easiest for users to see and click on, you may want to use an image button to give users the option of clicking on a picture to create multimedia programs. To create an image button, follow these steps:
1. Click on the Image Button !con in the Visual Basic Toolbox.
2. Drag the mouse to the place on the torm where you want to draw the image button.
3. Open the Properties window (by pressing F4).
4. Double-click on the Picture property. Visual Basic cheerfully displays a Load Picture dialog box.
5. Load any graphics file (bitmap, icon, or metafile). Visual Basic displays this graphic as your image button.
After you create a command or image button, you still have to write BASIC code to make the button do something when the user clicks on it.
* Displaying pretty icons on command buttons
Although most command buttons simply display a caption, such as OK, Cancel, Yes, No, or Blame It On Your Parents, you also can display icons on command buttons. Pictures can help clarify a command button's purpose. For example, rather than just having a command button display a caption that says "OK," you can also display a green traffic light picture on the command button.
To make a command button display an icon, you can define the following properties:
Style (Choose 1 - Graphical)
Picture (Defines the icon that appears on the command button)
DownPicture (Defines the icon that appears when the user clicks on the command button)
DisabledPicture (Defines the icon that appears when the command button is dimmed)
The DownPicture and DisabledPicture properties are optional if you want to display an icon on a command button.
To display a picture on a command button, follow these steps:
1. Click on the command button on the form that you want to display a picture.
2. Open the Properties window (by pressing F4).
3. Click on the Style property, click on the downward-pointing arrow, and choose 1 - Graphical.
4. Double-click on the Picture property. Visual Basic displays a dialog box.
5. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
6. Double-click on the DownPicture property to define a picture to appear when the user clicks on the command button.
7. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
8. Double-click on the DisabledPicture property to define a picture to appear If the command button appears dimmed.
9. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
* Changing the size of an image button
Visual Basic lets you freely draw an image box of any size that you want; however, if your graphics image is too big for your image button, the graphics image appears cut off.
To make your graphics image change size when you change the image button's size, you need to change the value of the image box Stretch property to True. (The default value of the Stretch property is False.)
For example, the image box in the upper-left corner has the Stretch property set to False, so no matter how large you make the image box, the graphics image remains the same size. The other three image boxes have their Stretch properties set to True; therefore, the graphics image adjusts in size when you change the size of the image box.
* Creating a default command button
The default command button is the one button that users can choose by pressing the Enter key right away as soon as the command buttons appears on the screen. A default button is the most likely choice for the user to choose.
For example, if the user gives a command to launch nuclear missiles at another nation, a dialog box may pop up asking, "Wouldn't you rather play a nice game of chess?" If the default button was Yes, the user can mindlessly hit Enter and save the world from nuclear destruction.
Only command buttons can be default command buttons. Image buttons can never be default command buttons because Visual Basic says so.
You can use two ways to create a default command button:
Set the command button's TabIndex property to zero. This highlights the command button designated as the default button.
Set the Default property on the default command button to True. This works only if no other object has a TabIndex value of zero.
To create a default command button using the TabIndex property, follow these steps:
1. Click the command button that you want to be the default button.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar).
3. Click on the TabIndex property and type 0.
If no other command buttons on your form have a TabIndex property of zero, you can create a default button by setting the Default property of a command button to True.
To create a default command button using the Default property, follow these steps:
1. Click on the command button that you want to be the default button.
2. Open the Properties window (by pressing F4).
3. Click the Default property and set the value to True.
4. Make sure that no other objects on the form have a TabIndex of zero.
What happens if one command button has a TabIndex of zero but another command button has a Default property that is set to True? The default command button is the one with the TabIndex of zero. So there.
* Defining the Cancel button
When users bang on Esc, they usually want to cancel their last command or exit out of the program. Any button that lets the user do this needs to be designated the Cancel button. Only one command button can be defined as the Cancel button. (Another good name for the Cancel button is the Panic button, but that destroys the image of programming as a fine science rather than an incoherent art.)
To create a Cancel command button, follow these steps:
1. Click the command button that you want to be the Cancel button.
2. Open the Properties window (by pressing F4).
3. Click the Cancel property, set the value to True, and change the button's caption to "cancel" or something similar.
* Grouping command buttons
Occasionally, you may want to group related command buttons on the screen to give the illusion of organization.
To create a group of command buttons, follow these steps:
1. Click on the Frame Icon in the Visual Basic Toolbox.
2. Move the mouse to where you want to draw the frame.
3. Hold down the mouse button and move the mouse to draw a frame.
4. Click on the Command Button icon in the Visual Basic Toolbox.
5. Move the mouse inside the frame to where you want to draw a command button.
6. Hold down the mouse button and move the mouse to draw your command button Inside the frame.
7. Repeat Steps 4 through 6 until you draw all the command buttons you need or until you decide that the time has come to do something else.
After you draw a command button inside a frame, the button remains forever trapped inside the frame. When you move the frame, all command buttons inside move along with their host frame.
You cannot create a command button outside a frame and then try to move the button inside a frame, so don't bother trying.
If a command button exists outside of a frame, click on it, choose the Copy or Cut command, click inside the frame so the frame's handles appear, then choose the Paste command. This lets you copy or cut a command button from outside a frame and put it inside the frame.
As a final modification for grouped command buttons, set the TabStop properties of all grouped buttons to False. Next, set the TabStop property of the first command button to True. That way, if a lame user presses Tab, only the first command button in a group becomes highlighted.
To turn off the TabStop properties for a group of buttons, follow these steps:
1. Click any command button within the frame, except for the first command button.
2. Open the Properties window (by pressing F4).
3. Click on the TabStop property and set the value to False.
4. Repeat Steps 1 through 3 until the TabStop property for all the command buttons (except the first command button) has been set to False.
5. Go to the kitchen and reward yourself with a Twinkle for your good deed.
* Test your newfound knowledge
1. Why do command buttons have names and captions?
a. So you have twice as many chances to call them a four-letter word.
b. The name is a bad word you can call the command button, and the caption is there so that you can write a funny punch line.
c. The name identifies the command button, and the caption is what actually appears on the screen.
d. Because Visual Basic says so, and any product sold by Bill Gates can't be wrong because he's a billionaire,
2. Why do you want to group buttons together in a frame?
a. To keep them from escaping into the wild.
b. So related commands are easy to find on the screen.
c. To make understanding what your program is supposed to do harder for anyone.
d. No reason, except to cause more confusion to people trying to learn how to program a computer for the first time.
++ CHAPTER 7
BOXES AND BUTTONS FOR MAKING CHOICES
In this chapter:
Creating check boxes and radio buttons
Creating list boxes and combo boxes
Sorting items in a list box or combo box
Making listed items look pretty
In school, multiple-choice tests were always easier than essay tests because you could substitute guessing for thinking and still get a decent grade. However, students aren't the only ones who don't want to think if they can avoid doing so. Most users are the same way - they want choices clearly laid out in front of their eyes. That way, they can make wild guesses and be on their merry way.
Visual Basic provides several ways to offer choices to users: check boxes, radio buttons (also called option buttons), list boxes, and combo boxes. Check boxes let users choose one or more options. Radio buttons let users choose only one option. List boxes and combo boxes offer users multiple choices.
The CD contains a Visual Basic program that shows how check boxes, radio buttons, list boxes, and combo boxes work. Feel free to modify the source code and see what happens.
** Creating Check Boxes and Radio Buttons
Check boxes get their name from those silly questionnaires that ask, "Check all that apply," as in:
Why do you want to work here? (Check all that apply.)
need the money.
want to participate in employee theft.
want a place where I can steal more office supplies.
need a safe place to hide from the police.
Radio buttons get their name from those old AM car radios that let you push a button to change stations quickly. Just as you can listen to only one radio station at a time, radio buttons let you choose only one option at a time. The following is an example of radio buttons.
What is your sex? (Choose only one.)
Male
Female
Ex-male (surgically a female)
Ex-fernale (surgically a male)
* Aligning your boxes and buttons
Check boxes and radio buttons are usually left-aligned, which means they look like the following:
This is left-aligned.
For some odd reason known only to those few programmers who actually use aligning options, you can also right-align check boxes and radio buttons.
This is right-aligned.
* To left-align or right-align a check box or radio button, follow these steps:
1. Click on the check box or radio button that you want to align.
2. Open the Properties window. (Press F4, choose View->PropertiesWindow, or click on the Properties Window Icon on the toolbar.)
3. Click on the Alignment property and set the value to 0 - Left Justify or 1 - Right Justify.
* Grouping check boxes
Check boxes rarely appear by themselves. Usually, two or more check boxes huddle together like frightened farm animals. The best way to isolate groups of check boxes is to use a frame. Frames visually separate different groups of check boxes. To create a group of check boxes, follow these steps:
1. Click on the Frame Icon In the Visual Basic Toolbox.
2. Move the mouse to where you want to draw the frame.
3. Hold down the left mouse button and move the mouse to draw a frame.
4. Click on the Check Box icon In the Visual Basic Toolbox.
5. Inside the frame, move the mouse to where you want to draw a check box.
6. Hold down the left mouse button and move the mouse to draw your check box.
7. Repeat Steps 4 through 6 until you've drawn all the check boxes you want inside your frame or until you decide the time for a break has arrived.
As a final modification to a group of check boxes, set the TabStop properties of all grouped check boxes to False. Next, set the TabStop property of the first check box to True. That way, when someone presses the Tab key, only the first check box in that frame becomes highlighted.
To turn off the TabStop properties for a group of check boxes, follow these steps:
1. Click on any check box or radio button within the frame, except the first check box.
2. Open the Properties window. (In other words, press F4, choose View->PropertiesWindow, or click on the Properties Window icon on the toolbar.)
3. Click on the TabStop property and set the value to False.
4. Repeat Steps 1 through 3 until the TabStop property has been set to False for all but one of the check boxes.
* Grouping radio buttons
If the radio buttons on a form are not grouped inside a frame, Visual Basic assumes that all radio buttons appearing on the same form belong to the same group. Thus, even if two radio buttons have nothing in common with each other but they appear on the same form, only one of the radio buttons can be chosen at any time.
If you need to display two or more groups of radio buttons, you have to group them within a frame. Otherwise, Visual Basic lumps all the radio buttons in a single group, which means that only one radio button can be chosen at any given time.
* To create a group of radio buttons in a frame, follow these steps:
1. Click on the Frame icon in the Visual Basic Toolbox.
2. Move the mouse to where you want to draw the frame.
3. Hold down the left mouse button and move the mouse to draw a frame.
4. Click on the Radio Button icon in the Visual Basic Toolbox.
5. Inside the frame, move the mouse to where you want to draw a radio button.
6. Hold down the left mouse button and move the mouse to draw your radio button.
7. Repeat Steps 4 through 6 until you've drawn all the radio buttons you want inside your frame or until you have to go to the bathroom.
After you draw a radio button inside a frame, the button remains trapped inside that frame forever. When you move the frame, all radio buttons inside move along with their host frame.
* Adding icons to check boxes and radio buttons
Normally, check boxes and radio buttons display only a caption; but if you want to get fancy, you also can display icons on your check boxes and radio buttons, too.
* To make a check box or radio button display an icon, you must change the following properties as explained below:
Style: Choose 1 - Graphical
Picture: Defines the icon that appears on the check box or radio button
DownPicture: Defines the icon that appears when the user clicks on the check box or radio button
DisabledPicture: Defines the icon that appears when the check box or radio button is dimmed
For some odd reason, Visual Basic makes check boxes and radio buttons with pictures appear like command buttons. Make sure that you take this strange appearance into account, or else both you and your users may mistake a check box or radio button for a command button instead.
* To display a picture on a check box or radio button, follow these steps:
1. Click on the check box or radio button on the form that you want to display a picture.
2. Open the Properties window. (Press F4, choose Viewt->PropertiesWindow, or click on the Properties Window icon on the toolbar.)
3. Click the Style property, click the downward-pointing arrow, and choose 1 - Graphical.
4. Double-click on the Picture property. Visual Basic displays a dialog box.
5. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
6. Double-click on the DownPicture property to define a picture to appear when the user clicks on the command button.
7. Click on the picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
8. Double-click on the DisabledPicture property to define a picture to appear if the command button appears dimmed.
9. Click on the Picture you want to use. (You may have to dig through the Graphics folder to find a picture to use.) Click on Open.
** Offering More Choices with List Boxes and Combo Boxes
When you have only a few choices, check boxes and radio buttons work nicely. If you have ten or more choices, however, bombarding the user with a screen full of check boxes or radio buttons can be intimidating and ugly. To present many choices to the user in a clear and helpful manner, Visual Basic provides two alternatives to check boxes and radio buttons: list boxes and combo boxes.
List boxes display long lists of options from which users can choose. If users want to choose something that isn't on the list, too bad. They can't.
Combo boxes also display long lists of options for the user to choose. The difference is that combo boxes also let the user type a choice if the selection the user wants cannot be found on the list. Figure 7-4 shows an example of a list box and a combo box. Notice that the combo box displays items only if you click on the down arrow; the list box always displays items.
is a combo box.
* Creating list boxes and combo boxes
List boxes are like fast-food menus. You can choose only what's on the menu because the folks working there don't know how to handle special requests. Combo boxes are like fancy restaurants where you have a choice of ordering off the menu or saying, "I know this is a vegetarian restaurant, but I want the cook to grill me a steak anyway."
* To create a list box, follow these steps:
1. Click on the List Box !con in the Visual Basic Toolbox.
2. Move the mouse to the place on the form where you want to draw the list box.
3. Hold down the left mouse button and move the mouse to draw the list box. Visual Basic displays one list box with a dull caption such as List3.
4. Repeat Steps 1 through 3 until you've drawn all the list boxes you need.
* To create a combo box, follow these steps:
1. Click on the Combo Box icon In the Visual Basic Toolbox.
2. Move the mouse to the place on the form where you want to draw the combo box.
3. Hold down the left mouse button and move the mouse to draw the combo box. Visual Basic displays one combo box with a dull caption such as Combo1.
4. Repeat Steps 1 through 3 until you've drawn all the combo boxes you need.
* Combo box styles
A combo box lets you type a choice or select one from the displayed list. For added variety, three styles of combo boxes are available:
Drop-down combo box (Style 0, the default)
Simple combo box (Style 1)
Drop-down list box (Style 2)
The drop-down combo box lets users type an item. If users have no idea what to type, they can click the downward-pointing arrow to the right of the combo box, and the combo box politely displays a list of possible choices. Visual Basic always creates this type of combo box unless you change the Visual Basic Style property.
The simple combo box always displays the list on the screen but also gives users the choice of typing an item.
Unlike the other two combo box styles, you must draw the full size of a simple combo box style so the user can see the items stored in the combo box.
The drop-down list box is actually a list box and always displays a range of choices, but you can't type anything of your own.
If you don't want the user to type anything into a combo box, set the combo box's Style property to 2 - Dropdown List.
At this point, you may be asking, "Wait a minute. Why do I want to create a combo box and then turn it into a stupid list box?" Unlike ordinary list boxes, a drop-down list box doesn't display the list on the screen until the user clicks on the arrow to the right of the box. This type of list box is useful when you need to conserve screen space.
* To define the style for a combo box, follow these steps:
1. Click on the combo box that you want to change. (This assumes that you've already created the box.)
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Click on the Style property.
4. Click on the arrow in the Settings Box to display your list of choices. (Hey, what do you know? The Settings Box is an example of a dropdown list box!)
5. Click on the combo box style you want.
** Test your newfound knowledge
1. What is the main difference between a check box and a radio button?
a.You can choose one or more check boxes but only one radio button.
b. Radio buttons tune in to your favorite radio station, but check boxes are places where you save canceled checks.
c. I don't know. Aren't you supposed to be the teacher with all the answers?
d. Everything is one, man. Like, the answer is all in your point of view.
2. What is the major difference between a list box and a combo box?
a. A combo box gives you a choice of typing an item or choosing one from a displayed list. A list box forces you to choose an item from a displayed list.
b. A list box is spelled L-I-S-T, but a combo box is spelled C-O-M-B-O.
c. Combo boxes are cooler than list boxes because a combo box tends to be more confusing to the average user.
d. No difference. In fact, two out of three French chefs think that they both taste exactly like butter.
** Adding Items to List Boxes and Combo Boxes
After you create your list box or combo box, you have to fill up your box with items. (Otherwise, you have no real point in creating a list box or combo box, now do you?) Visual Basic gives you two ways to add items to a list box or combo box:
Use the List property in the Properties window.
Use BASIC code.
** To add items to a list box or a combo box using the List property, follow these steps:
1. Click on the list box or combo box that you want to add items to.
2. Open the Properties window. (Press F4, choose View->PropertiesWindow, or click on the Properties Window Icon on the toolbar.)
3. Double-click on the List property. A drop-down text box appears.
4. Type the first item you want to appear in your list box or combo box. Next, press Ctrl+Enter. Repeat this step for each item you want to add.
5. Press Enter.
When typing items in the List property, make sure that you press CtrL+Enter in between each item. The moment you press Enter, Visual Basic assumes you're done typing items in the List property.
If you want to use BASIC code to add items to a list box or combo box, the secret BASIC command to use is AddItem. So if you want to add the item "Pick me" to a list box named 1stCommands, here is the magic BASIC code that you use to do so:
1stCommands.AddItem "Pick me"
You can add items to a list box or combo box any time your program is running, but the most common time is when the form containing the list box or combo box first loads.
To add items to a list when a form loads, follow these steps:
1. Click on the form in the Project Explorer window.
2. Click on the View Code !con.
3. Click in the Object list box and choose the form name. Visual Basic displays the following procedure:
Private Sub Form_Load()
End Sub
4. For each item you want to display in a list box or combo box, use the AddItem secret command.
For example, if you had a list box named 1stToDo and a combo box named cboHideln, the Sub Form-Load () procedure may look like the following:
Private Sub Form_Load()
1stToDo.AddItem "Call stockbroker"
1stToDo.AddItem "Make airline reservations"
1stToDo.AddItem "Act normally until noon"
1stToDo.AddItem "Steal $250,000"
1stToDo.AddItem "Fake headache"
1stToDo.AddItem "Leave work early"
1stToDo.AddItem "Go to airport"
cboHideln.AddItem "Acapulco"
cboHideIn.AddItem "Rio de Janeiro"
cboHideIn.AddItem "Paris"
cboHideIn.AddItem "Tokyo"
cboHideIn.AddItem "New York"
cboHideIn.AddItem "Bangkok"
End Sub
This procedure adds these items to the 1stToDo list box and to the cboHideln combo box whenever the first form of your program loads.
** Highlighting default items
The purpose of list boxes and combo boxes is to provide users with choices. To make choosing items even more mindless and thus more efficient from the user's point of view, combo boxes can display default items. (With list boxes, the first item is the default item.) A default item is the item that the computer assumes the user wants unless instructed otherwise. (For a default item to be the least likely choice doesn't really make much sense.)
** To create a default item for a combo box, follow these steps:
1. Click on the combo box for which you want to assign a default Item.
2. Open the Properties window. (Press F4, choose View->PropertiesWindow, or click on the Properties Window icon on the toolbar.)
3. Click on the Text property.
4. Type the item that you want to appear as the default item in this combo box.
If you set a combo box's Style property to 2 - Dropdown List, you can define a default item by using BASIC code to set the ListIndex property to a value such as 0 (to make the first item the default item):
CboHideIn.ListIndex = 2
When a user clicks on the combo box, the default item is highlighted.
If you don't define a default item, Visual Basic displays the combo box's generic name (Combo1, Combo2, and so on) as the default item. Because this looks very ugly and amateurish, you should always define a default item for your combo boxes.
** Sorting Items In a list box or combo box
The order that you add items to a list box or combo box is the order in which the items appear. For a little variety, Visual Basic lets you sort items in two ways:
Alphabetically
Any way you want
When Visual Basic sorts a list alphabetically, the list is sorted without regard to whether items are capitalized or not. For example, Visual Basic considers "Your Momma" and "YOUR MOMMA" to be identical.
To sort items in a list box or combo box alphabetically, follow these steps:
1. Click on the list box or combo box in which you want to display items alphabetically.
2. Open the Properties window. (Press F4, choose View->PropertiesWindow, or click on the Properties Window icon on the toolbar.)
3. Click on the Sorted property and set the property to True.
Visual Basic always sorts items with the A's on top and the Z's at the bottom. You cannot sort items in descending order, with the Z's on top and the A's at the bottom (unless, of course, you flip your monitor upside down).
If alphabetic sorting isn't what you want, you have to sort items one-by-one yourself. Visual Basic assigns an index number (which is just an ordinary number such as 1 or 3) to each item in a list.
The first item in a list is assigned an index number of 0, the second item is assigned an index number of 1, the third item is assigned an index number of 2, and so on. (If you've ever been in a European elevator where the ground floor is labeled 1, the first floor is labeled 2, and the second floor is labeled 3, you are going to recognize the confusing way that Visual Basic assigns index numbers.)
To put an item at the top of a list, you still have to use the magic Add Item BASIC command in the following way:
cboHideIn.AddItem "Harare", 0
If you omit the index number, Visual Basic adds items to a list in one of two ways:
If the Sorted property of the list box or combo box is False, the item goes to the bottom of the list.
If the Sorted property of the list box or combo box is True, the item goes in the correct alphabetic order.
If you set the Sorted property of a list box or combo box to True and add items by using index numbers, Visual Basic adds the item according to the index number and does not sort the newly added items alphabetically.
** Removing items from a list box or combo box
Adding items and sorting them may make your lists look nice, but wiping out an item to satisfy that destructive urge that everyone experiences once in a while is more fun.
Visual Basic gives you two ways to remove an item from a list:
Use the RemoveItem BASIC command to remove items one at a time.
Use the Clear BASIC command to wipe out an entire list at once.
To use the Remove Item BASIC command, you have to know the index number of the item you want to remove. For example, to remove the item with an index number of 5 that's located in a list box named 1stToDo, use the following BASIC command:
1st ToDo.Removeltem 5
To use the Clear BASIC command to wipe out an entire list in a single blow, you need the name of the list box or combo box that contains the list you want to kill. To wipe out the entire contents of a combo box named cboHideln, use the following BASIC command:
CboHideIn.Clear
Before using the Clear BASIC command, make sure that you really want to wipe out an entire list.
** Displaying check boxes in your list boxes
When you display items in a list box, you can also display them as check boxes. By using check boxes, you can check off items in a list box such as to-do lists, grocery lists, or lists of New Year's resolutions that you never plan to complete.
* To display check boxes in a list box, follow these steps:
1. Click on the list box where you want check boxes to appear.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Click on the Style property and choose 1 - Checkbox.
To figure out which item a user may have chosen in a checked list box, you have to use BASIC code. Remember that items in a list box are assigned a number. The top item is given an index number of 0, the second from the top a number of 1, the third from the top 2, and so on. Knowing this, you have to write an event procedure that tells you which item was just checked (or unchecked). To do this, you have to use the event procedure attached to your list box such as:
Private Sub List1-ItemCheck(Item As Integer)
GlobalVariable = Item
' This will print the item checked (or unchecked)
Print Item
End Sub
The moment someone checks (or unchecks) an item in a checked list box, the ItemCheck event procedure runs and the Item variable tells you which item was checked (or unchecked). So if the top item is checked (or unchecked), the ItemCheck event procedure assigns the number 0 to Item.
Then you have to assign your own variable to Item so you can tell the other parts of your program which list box item got checked (or unchecked). From there, you have to write BASIC code to figure out what to do if someone checks or unchecks an item.
** Creating Multiple Column List Boxes
For aesthetic purposes - or just because you're bored and want to goof around - you can dispIay multiple columns in a list box. Visual Basic provides three types of multiple columns, depending on the value defined by the Column property:
Value 0: A single column list with vertical scrolling (the default appearance of list boxes)
Value 1: A single column list with horizontal scrolling (but no vertical scrolling)
Any value greater than 1 in number: A multiple column list of two or more columns with horizontal scrolling (but no vertical scrolling)
* To change the way a list box displays items in columns, follow these steps:
1. Click on the list box that you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Click on the Columns property and type 0, 1, or any number larger than 1.
** Making Listed Items Look Pretty
To spice up your lists and make them look a little less like boring shopping lists, Visual Basic lets you change the font, type style, and size of your list's items.
Fonts are different ways to display text. Normally, Visual Basic uses the MS Sans Serif font, but you can use any font stored in the memory of your computer. (MS Sans Serif is similar to the Helvetica font, and the Visual Basic MS Serif font is similar to the Times Roman font.)
* To change the font of items that appear in a list box or combo box, follow these steps:
1. Click on the list box or combo box whose font you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Double-click on the Font property. Visual Basic displays a Font dialog box.
4. Click on the font you want and click on OK. Visual Basic immediately changes the font in the list box or combo box.
Be careful when you use fonts. Novices often get carried away and use so many bizarre fonts that all semblance of normality is lost. Unless you have a really good reason to use different fonts, let Visual Basic use its default font of MS Sans Serif.
You can also change the size of your items, making them smaller or larger. However, the larger the type size is, the larger your list box or combo box must be to show the entire item at once.
The larger the font size, the more the items in your list boxes and combo boxes stand out. Usually, the best method is to use one size for all your list boxes or combo boxes to avoid confusing the user any more than you have to.
* To define the font size, follow these steps:
1. Click on the list box or combo box whose font size you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Double-click on the Font property. Visual Basic displays a Font dialog box.
4. Click on the font size you want and click on OK.
Visual Basic immediately changes the font size of the list box or combo box.
Besides changing the font and size, you can also change the font style and display text in bold, italics, underline, or strikeout.
* To set any one or more font styles, follow these steps:
1. Click on the list box or combo box whose fonts you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar).
3. Double-click on the Font property. Visual Basic displays a Font dialog box.
4. Click on the font style you want and click on OK.
Visual Basic immediately changes the font style of the list box or combo box.
The more attractive you make your list boxes and combo boxes, the more likely the user is going to notice the boxes, at least (if not use them). Just remember that you want to make your program easy to use, not a work of art. If you want to get creative, take up finger painting. If you want to create useful programs and make millions of dollars, make your programs easy, fun, and simple to use.
++ CHAPTER 8
TEXT BOXES FOR TYPING AND SHOWING WORDS
In This Chapter:
Creating text boxes
Filling text boxes and hiding passwords
Using different fonts, sizes, type styles, and colors
Despite the growing acceptance of icons and graphical user interfaces, not all choices can always be offered through command buttons, radio buttons, or combo boxes. Sometimes your program may need to display a word, sentence, paragraph, or entire novel on the screen. And sometimes the user may want to type in a good word or two as well.
So what's the solution? Combo boxes work with words or short phrases, but if your program needs to display a chunk of text or if the user needs to type in a substantial amount of information, a text box can make your job a whole lot easier.
Text boxes have two purposes in life:
To show text on the screen
To let the user type text into the program
Text boxes are among the most flexible programming objects because you can display instructions in a text box and the user can type a reply using ordinary words. If you use enough text boxes in your programs, you may help increase literacy among our population today.
The Visual Basic program enclosed on the CD-ROM provides a simple example of using a text box for accepting a password. Dig into the source code and play around. At the very least, you may mess up the program but still learn something in the process.
** Creating a Text Box
Text boxes are like miniature word processors but can display only one font, one size, and one type style (such as bold or italics). So if you want to display multiple fonts in a text box, give up that thought right now because you can't.
When a user types text in a text box, the following keys work:
Delete: Erases the character to the right of the cursor
Backspace: Erases the character to the left of the cursor
Shift+Arrow: Highlights a block of text
Ctrl+Left arrow: Moves the cursor one word to the left
Ctrl+Right arrow: Moves the cursor one word to the right
Home (or Ctrl+Home): Moves the cursor to the beginning of the line
End (or Ctrl+End): Moves the cursor to the end of the line
Shift+any movement key (such as Home): Highlights text
F11 or F12: Doesn't do a thing and is about as useful on your keyboard as wisdom teeth in your mouth
To create a text box, follow these steps:
1. Click on the Text Box Icon in the Visual Basic Toolbox.
2. On the form, move the mouse where you want to draw the text box.
3. Hold down the left mouse button and draw the text box. Visual Basic displays your text box with default text inside, such as Text1.
4. Repeat Steps 1 through 3 until you've drawn all the text boxes you need or until you find something else to do.
** Putting pretty borders around text boxes
Normally, Visual Basic displays a single line around a text box, defining the boundaries of that text box. If you want to keep your users guessing where a text box is, you can remove this border.
* To change the borders around a text box, follow these steps:
1. Click on the text box whose border you want to change.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Click on the BorderStyle property and choose one of the following:
0 -None
1 - Fixed Single
** Displaying wordes in a text box
After you create a text box, the next step is to put some text in the box. By default, Visual Basic displays the text box's name in the text box, such as Text1.
Changing the Text property of a text box does not affect the Name property of the text box. So if you want your text box to display something more exciting than Text1, you have to change the Text property.
The Text property can contain anything from a blank line (which means your text box appears empty) to ordinary text, to a mass of incomprehensible text that resembles a typical computer manual.
* To change the Text property of a text box, follow these steps:
1. Click on the text box whose Text property you want to change.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Double-click on the Text property (or just click once on the Text label) and type whatever text you want to appear in your text box.
If you want to change the contents of your text box while your program is running, you have to use BASIC code such as:
txtMessage.Text = "This will now appear in the text box."
Whenever you change the Text property of a text box, the new contents of the text box completely wipe out the previous contents of the text box.
** Aligning text in a text box:
To make a text box look nice and organized, Visual Basic can align the text as left-justified, right-justified, or centered.
* To align text in a text box, follow these steps:
1. Click on the text box whose text you want to align.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Click on the MultiLine property and set the value to True. If the MultiLine property is set to False, Visual Basic ignores any changes you make to the Alignment property.
4. Click on the Alignment property and choose one of the following:
0 - Left Justify
1 - Right Justify
2 - Center
Changing the Alignment property doesn't align the text until you first change the text box's MultiLine property to True.
** Word-wrapping text boxes:
In addition to displaying text that the programmer typed into the Text property, a text box also lets users type in their own text. By default, a text box is pretty stupid at handling text. If you type text into a text box, the text box cheerfully displays the text as one huge line that scrolls endlessly out of sight. To make a text box wrap words within the box's boundaries like in a word-processor program, you have to set the text box's MultiLine property to True.
* To set the MultiLine property to True for a text box, follow these steps:
1. Click on the text box where you want to use word-wrapping.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Click on the MultiLine property and set the value to True.
When a text box has a MultiLine property of True, the text box word-wraps text within the boundaries of the text box. If you change the width of a text box while your program is running, the text box automatically word-wraps the text within the new size of the text box. Now, don't you think that what computers can do nowadays is amazing?
** Adding horizontal and vertical scroll bars in text boxes
Word-wrapping is a fine way to display text within a text box, but if the text box isn't tall enough, the text box can't display all the text you may type in. To solve this problem, you may also have to add horizontal or vertical scroll bars.
Be careful! If you add a horizontal scroll bar to a text box, that turns off all word-wrapping. When a text box uses horizontal scroll bar, the only way a user can type on the next line is to press Enter.
Adding a vertical scroll bar lets users type and display more text than the text box can display. With vertical scroll bars, users can press PageUp or PageDown to display text in a text box.
* To add scroll bars to a text box, follow these steps:
1. Click on the text box in which you want to add scroll bars.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Click on the ScrollBars property and choose one of the following:
0 - None
1 -Horizontal
2 - Vertical
3 - Both (horizontal and vertical scroll bars)
Vertical and horizontal scroll bars work only if you have set the text box's MultiLine property to True. (Otherwise, there's no point in having scroll bars remain if you can't display more than one line of text.)
** Making a Password Text Box
In case you work for the CIA, FBI, NSA, DIA, IRS, or any organization that spends lots of money, buries itself in secrecy, and hides behind a three-letter acronym, you may be interested in the ability of Visual Basic to create special password text boxes.
Rather than display ordinary text, password text boxes mask any text you type into a text box with a single character, such as an asterisk (*).
To create a password text box, you need to define the character that the text box displays when someone types in text.
* To create a password text box, you need to follow these steps:
1. Click on the text box that you want to turn Into a password text box.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window Icon on the toolbar.)
3. Click on the MultiLine property and set the value to False.
Always set the MultiLine property of a password text box to False, otherwise the password text box can't mask any typed characters. This is the Visual Basic way of saying that passwords can't be so long that they require two or more lines to write.
4. Click on the PasswordChar property and type the masking character, such as an asterisk.
The masking character can only be a single character.
** Limiting the Length of Text
To prevent people from getting too wordy, you can set the maximum length of text for a text box. This way, people can't type rambling essays about what they did last summer in your text boxes.
To define the maximum number of characters that a text box can accept from the user, change the MaxLength property. If the user tries to type any characters beyond the MaxLength limit, Visual Basic beeps and accepts no more.
(Alas, Visual Basic doesn't have a minimum length property. For ordinary text boxes, this isn't a problem, but if you're creating a password text box, you can bet that at least one bozo is going to choose a one-letter password that some hacker can easily guess.)
* To define the maximum length of characters that a text box accepts, follow these steps:
1. Click on the text box whose maximum character length you want to define.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Double-click on the MaxLength property and type any number greater than zero.
A value of zero effectively means that no limit to the number of characters a user can type in a text box exists.
** Changing Fonts, Sizes, and Type Styles
Visual Basic normally displays text in a text box using the MS Sans Serif font, but if you want to exercise your creativity, you can choose any font stored in the memory of your computer. (The MS Sans Serif font is similar to the Helvetica font, and the Visual Basic MS Serif font is similar to the Times Roman font.)
* To change the font used in a text box, follow these steps:
1. Click on the text box whose font you want to change.
2. Open the Properties window.
3. Double-click on the Font property. Visual Basic displays a font dialog box.
4. Click on the font you want and click on OK. Visual Basic immediately changes the font in the text box.
Be careful when using fonts. Beginners often get carried away and choose really bizarre fonts that confuse more than they clarify. Unless you have a really good reason to use a different font, just use the default font, MS Sans Serif, for most of your work.
You can also change the size of your text, making your text smaller or larger. Of course, the larger the type size of your text, the larger your text box must be to show the entire text. The larger the font size, the less text you can display. To avoid confusing the user any more than necessary, use one font size for all your text boxes.
* To define the font size for a text box, follow these steps:
1. Click on the text box whose font size you want to modify.
2. Open the Properties window.
3. Double-click on the Font property. Visual Basic displays a font dialog box.
4. Click on the font size you want and then click on OK. Visual Basic immediately changes the font size of the text displayed in your chosen text box.
Changing the font and type size of text can be fun so Visual Basic also gives you additional ways to change the appearance of your text to display bold, italic, or strikeout.
* To choose a font style, follow these steps:
1. Click on the text box whose font that you want to modify.
2. Open the Properties window.
3. Double-click on the Font property. Visual Basic displays a font dialog box.
4. Click on the font style you want and click on OK. Visual Basic immediately changes the font style of your chosen text box.
** Test your newfound knowledge
1. Give two uses for text boxes.
a.To store letters from your Scrabble game and to contain words that may win you a million dollars on Wheel of Fortune.
b. To display text on-screen and to let users type text into a program.
c. To store all the computer books that you buy but never read, and to make cardboard forts that your children can hide in.
d.To use as a litter box and to give your cat something to read.
2. If a text box has the PasswordChar property set to * (asterisk) and the MaxLength property set to 10. what happens?
a. I have to flip back through the pages of this book to find the answer, so wait while I do that.
b. I'm not sure, but whatever happens must be important because this question is listed here.
c. This defines the secret password that is needed to break into the Pentagon's computers.
d. The text box accepts a maximum of 10 characters and displays an asterisk in place of an actual typed character.
** Coloring Text Boxes
If you loved the idea of writing in different colors with crayons when you were a kid, then you are going to love the idea of coloring your text boxes using Visual Basic.
Normally, Visual Basic displays text in black against a white background. For more creativity, you can change the foreground and background colors of your text boxes.
The color inside the text box (background color) is defined by the BackColor property. The color of the text itself (foreground color) is defined by the ForeColor property.
To change the background or foreground color of a text box, follow these steps:
1. Click on the text box whose background or foreground color you want to modify.
2. Open the Properties window.
3. Double-click on the BackColor or ForeColor property and click on the Palette tab. Visual Basic displays a color palette.
4. Click on the color you want. Visual Basic immediately changes the color of your chosen text box.
By changing the color of your text boxes, you can highlight certain information and make it easier for the user to see (or ignore). Just remember that too many colors can be distracting and that some people may be colorblind, which means they won't experience the full effect of your program if they can't see the text displayed in certain colors. Remember, use colors sparingly.
++ CHAPTER 9
SCROLL BARS AND LABELS
In This Chapter:
Creating scroll bars
Moving the scroll box
Making labels
Aligning and word-wrapping text
Not all choices in life can be divided into neat categories like check boxes, radio buttons, or list boxes. Sometimes users may need to make choices that require a wide range of gradual adjustments.
Think of adjusting the volume on a stereo. If the only three choices you have are soft, medium, and loud, you can't adjust the volume to your taste. That's why most stereos let you turn a knob or press a button that gradually adjusts the volume higher or lower.
For minute measurements, or for moving through long lists of information, use scroll bars. Although text boxes, forms, and list boxes have built-in scroll bars, you can create separate scroll bars on your own.
The CD-ROM contains a Visual Basic program showing how a vertical scroll bar works. Study the source code and modify it to see what happens.
** Creating Scroll Bars
Visual Basic lets you create two types of scroll bars: horizontal scroll bars and vertical scroll bars. Horizontal scroll bars point left and right, just like the fast-forward and rewind buttons on your VCR or tape player. Vertical scroll bars point up and down, just like the volume control on some stereos.
To create a scroll bar, follow these steps:
1. Click the Horizontal or Vertical Scroll Bar Icon In the Visual Basic Toolbox.
2. Move the mouse cursor on the form to where you want to draw the scroll bar.
3. Hold down the left mouse button and move the mouse to draw the scroll bar, then release the left mouse button.
4. Repeat Steps 1 through 3 until you draw all the scroll bars you feel like making.
If you double-click on the Horizontal or Vertical Scroll Bar icon in the Visual Basic Toolbox, Visual Basic creates a scroll bar on a form right away.
* Setting scroll bar maximum and minimum values
Scroll bars are actually graphical representations of numeric values. The value of a scroll bar can range from -32,768 to 32,767. These numerical values can represent anything you want, such as measurements or quantities.
By default, Visual Basic sets the maximum value to 32,767 and the minimum value to 0. On horizontal scroll bars, the maximum value is represented when the scroll box is at the rightmost position on the scroll bar. The minimum value is represented when the scroll box is at the leftmost position on the scroll bar.
On vertical scroll bars, the maximum value is represented when the scroll box is at the bottommost position on the scroll bar. The minimum value is represented when the scroll box is at the topmost position on the scroll bar.
Obviously, the default values of 32,767 and 0 may be too extreme for most programs. To define a smaller range of values, you have to change the scroll bar's Max and Min settings.
To change the Max and Min settings for a scroll bar, follow these steps:
1. Click on the scroll bar whose Max and Min values you want to change.
2. Open the Properties window.
3. Double-click (or click once) on the Max property and type a new value.
4. Double-click (or click once) on the Min property and type a new value.
If the Min value is larger than the Max value, the scroll bar acts topsy-turvy In this case, the scroll bar represents the maximum value at the leftmost or topmost position and the minimum value at the rightmost or bottommost position.
* Where does the scroll box appear in my scroll bars?
By default, Visual Basic assigns scroll bars the value defined by the Min property (such as 0). This means that if your Max and Min values are positive, the scroll box always appears in the topmost position in a vertical scroll bar and the leftmost position in a horizontal scroll bar.
By default, the scroll box represents the minimum value of the scroll bar. However, if you want your scroll bars to display a default value of something other than the minimum value, you have to change the scroll bar's Value property.
To change the scroll bar's value, follow these steps:
1. Click on the scroll bar whose value you want to change.
2. Open the Properties window.
3. Click on the Value property and type a new value. Visual Basic dutifully changes the scroll bar's value while you watch.
* Moving the scroll box
The scroll box represents the current value of the scroll bar. To move the scroll box, users can do any of the following:
Drag the scroll box within the scroll bar.
Click on the scroll arrows at each end of the scroll bar.
Click in the area between the scroll box and each scroll arrow.
Each time the user clicks the scroll arrows, the scroll box moves a certain distance. By default, this distance is 1. Therefore, if your Min value is 0 and your Max value is 12, you have to click on the scroll arrow 12 times to move the scroll box from one end of the scroll bar to the other.
To modify the distance the scroll box moves when the user clicks on a scroll arrow, follow these steps:
1. Click on the scroll bar that you want to modify.
2. Open the Properties window. (Press F4, choose View->Properties Window, or click on the Properties Window icon on the toolbar.)
3. Double-click (or click once) on the SmallChange property and type a new value.
Likewise, each time the user clicks on the scroll bar shaft (in the area between the scroll box and the scroll area), the scroll box moves a certain distance. By default, this distance is 1, which means that if the Min value is 0 and the Max value is 5, you have to click five times to move the scroll box from one end of the scroll bar to the other.
To modify the distance the scroll box moves when the user clicks on the scroll shaft, follow these steps:
1. Click on the scroll bar that you want to modify.
2. Open the Properties window.
3. Double-click (or click once) on the LargeChange property and type a new value.
The values for LargeChange and SmallChange can vary between 1 and 32,767. The smaller the value, the smaller the distance the scroll box moves. The larger the value, the larger the distance the scroll box moves.
Here's some information you may never use: You can set LargeChange or SmallChange to values greater than the Max value. This just means that when the user clicks your scroll bar, the scroll box immediately jumps to one end of the scroll bar.
** Test your new found knowledge
1. When can you use a scroll bar?
a.When you want to give the illusion of complexity.
b. When the user needs to choose a range of values.
c.When you need to rewrap toilet paper that your cat unrolled onto the floor.
d.When nothing else seems to work, and you've run out of ideas on how to make your program easier to use,
2. What are the three ways to move the scroll box within a scroll bar?
a. Press the arrow keys, flip the mouse upside down, or unplug the computer.
b. Telekinesis, verbal threats, or pushing the scroll box with your finger.
c. The scroll box never moves. This is a trick question, right?
d. Drag the scroll box with the mouse, click the scroll arrows, or click the scroll bar shaft inside the scroll bar,
** Creating Labels
For pure decoration, you can sprinkle labels on any of your Visual Basic forms. Labels simply identify the objects on your form. In real life, you see labels all the time, such as the label MEN or WOMEN on a restroom door, FIRE EXTINGUISHER over a fire extinguisher in a public building, or POWER next to your monitor's on-and-off button. Labels simply call your attention to something you may otherwise overlook.
Both text boxes (see Chapter 8) and labels can display text on-screen. The main difference is that the user can modify the text inside a text box but can't modify text inside a label.
Psst, although users can't modify text in a label, BASIC code can modify the text inside a label. In this way, your labels can display changing messages to the user, such as "Sorry, that option isn't available at this time," "Now printing page 2 of 9," or "What are you, stupid or something?".
* To create a label, follow these steps:
1. Click on the Label icon in the Visual Basic Toolbox.
2. Move the mouse cursor on the form to where you want to draw the label.
3. Hold down the left mouse button and move the mouse to draw the label. Release the left mouse button.
Visual Basic draws a label with a boring caption, such as Label2.
4. Repeat Steps 1 through 3 as often as necessary for each label you want to draw.
If you double-click on the Label icon in the Visual Basic Toolbox, Visual Basic creates a label on a form right away.
* Putting pretty borders around labels
Normally, labels don't display any visible borders. However, you may want to put a boundary around a label to make the label easier to see. Visual Basic gives you two choices for label borders: a fixed single line or nothing at all.
To create a border around a label, follow these steps:
1. Click on the label whose border you want to change.
2. Open the Properties window.
3. Click on the BorderStyle property and choose one of the following:
0 -None
1 - Fixed Single
* Changing the size of labels
The size of a label on the screen determines the length of the caption (text) that the label can display. If a label is too small, part of the caption is cut off.
Note that the label caption (the text) doesn't change in size when you change the size of a label. To change the size of a label's caption, you have to change the Font property.
You may not know how much room your captions need, but to keep adjusting the width and height of your labels can be a real pain. Because computers are good at doing things that people don't want to do, Visual Basic can take care of this mundane task.
Visual Basic automatically can adjust the size of a label to fit any caption you stick inside of the label. Such automatic adjusting labels are perfect for displaying messages whose length may vary.
An automatic adjusting label grows or shrinks horizontally to match the leength of the label's caption. So if you have a really long caption, the label cheerfully expands in size and disappears off the right side of the screen. To make an automatic adjusting label word-wrap text, you have to set the label's WordWrap property to True.
To create an automatic adjusting label, follow these steps:
1. Click on the label that you want to automatically adjust to the size of its caption.
2. Open the Properties window.
3. Click on the AutoSize property and choose True.
The advantage of automatic adjusting labels is that you can use BASIC code to give a label captions of various sizes without ever worrying that a particular caption isn't going to fit. The disadvantage is that you don't have control over the label's maximum size. If you're not careful, a label can get too big and cover other parts of your user interface.
* Aligning text within a label
To make your label captions look nice and organized, Visual Basic offers three options for aligning captions.
Left-justified
Right-justified
Centered
To align a caption in a label, follow these steps:
1. Click on the label whose caption you want to align.
2. Open the Properties window.
3. Click on the Alignment property and choose one of the following:
0 - Left Justify
1 - Right Justify
2 - Center
* Word-wrapping labels
If you set a label's AutoSize property to True, the label expands horizontally as long as you keep stuffing it with text. However, if you want a label to expand vertically instead, you have to set both the label's AutoSize and WordWrap properties to True.
Setting both the AutoSize property and the WordWrap property to False means that long captions may be cut off at the bottom if your label isn't tall enough.
Setting AutoSize to False and WordWrap to True has the same effect as setting both AutoSize and WordWrap to False.
Setting AutoSize to True and WordWrap to False means that the label expands horizontally to fit an entire caption. However, the label shows only one line of the caption.
Setting both AutoSize and WordWrap to True means that the label grows or shrinks vertically to fit a caption.
To set the WordWrap property for a label to True, follow these steps:
1. Click on the label in which you want to use word-wrapping.
2. Open the Properties window.
3. Click on the WardWrap property and set the value to True.
When creating labels that adjust to the size of a caption, be careful that your labels don't accidentally grow over and cover up other parts of your user interface. Otherwise, you may really confuse someone who is trying to use your program.
++ CHAPTER 10
PRETTY PICTURES AND OBJECTS FROM GEOMETRY
In This Chapter:
Creating picture boxes and image boxes
Making the picture boxes and image boxes fit their contents
Creating geometric shapes
Altering the appearance of lines, circles, and other shapes
You can sprinkle labels and pictures on any of your Visual Basic forms. AIthough pictures can make your forms look nice, they also can be an actual part of your program.
For example, a picture on the front of a road map showing a smiling gas station attendant with the label "Always Trust Your Car to the Man Who Wears the Star" is superfluous and decorative. However, a road map using pictures to show city highways and major side streets can be integral and necessary.
If you sprinkle plenty of labels and pictures in your user interface, your program is going to be easier to use and understand. After all, that's the purpose of creating a user interface in the first place.
** Creating Pictures
Visual Basic provides two ways to display pictures on the screen:
In a picture box
In an image box
Use a picture box to display graphics or to group buttons together. Use an image box to display graphics or to create image buttons. To create a picture box or image box, follow these steps:
1. Click on the Picture Box or Image Box icon in the Visual Basic Toolbox.
2. Using the mouse, hold down the left mouse button on the form and drag to where you want to draw the picture box or Image box, and draw the box by dragging the mouse.
3. Repeat Steps 1 and 2 until you draw all the picture boxes or Image boxes you need.
If you double-click on the Picture Box or Image Box icon in the Visual Basic Toolbox, Visual Basic draws the picture box or image box on the form for you automatically. After drawing a picture box or image box, its size will likely change depending on the size of the graphic image you want to put in it.
* Displaying pictures in picture boxes or image boxes
After you create a picture box or an image box, putting a picture in the box is only natural. (Why else do you create the box?) Picture boxes and image boxes can display three types of graphics images:
Bitmap files: (BMP or DIB file extensions) Consist of patterns of dots, or pixels, which are the types of files created by paint programs, such as Microsoft Paint. If you enlarge a bitmap image, the image tends to look grainy and ugly.
Icon files: (ICO file extensions) Special kinds of bitmap files with a maximum size of 32 x 32 pixels.
Metafiles: (WMF file extensions) Images created by lines and geometric shapes that most people have forgotten about since high-school geometry. These types of files are created by draw programs, such as CorelDRAW.
To load a picture in a picture box or image box, follow these steps:
1. Click on the picture box or image box into which you want to load a graphics file.
(This assumes that you've already drawn the picture box or image box on a form. Otherwise, draw the picture box or image box as explained in the Creating Pictures section.)
2. Open the Properties window.
3. Double-click on the Picture property in the Properties window. Visual Basic displays the Load Picture dialog box.
4. Click on the picture file you want to load. Visual Basic instantly loads the picture in the picture box or image box.
In addition to loading pictures by using the Properties window, you also can load and remove pictures while your program is running (during runtime). To load a picture into a picture box or image box, use the following LoadPicture command:
ImgGreeting.Picture = LoadPicture("c:\graphics\martian.bmp")
The LoadPicture command specif ies the exact drive, directory, and file to store in the Picture property of a picture box or an image box.
To remove a picture from a picture box or image box during run time, use the LoadPicture command as follows:
ImgGreeting.Picture = LoadPicture("")
This statement essentially loads a blank image into the Picture property of a picture box or image box.
* Putting nifty borders around picture boxes and image boxes
To define the edge of your picture box or image box, Visual Basic can display a border. By default, neither type of box displays a border. To change the border around a picture box or image box, follow these steps:
1. Click on the picture box or image box whose border you want to change.
2. Open the Properties window.
3. Click on the BorderStyle property and choose one of the following:
0 - None
1- Fixed Single
** Changing the Size of Picture Boxes or Image Boxes
Generally, the size of picture boxes or image boxes has no effect on the size of the graphics image that the picture box or image box displays, with two exceptions:
Metafile graphics always change size to fit within a picture box or an image box.
If an image box's Stretch property is set to True, bitmap and !con graphics change size to fit within the image box.
* Changing the size of graphics images
Bitmap and icon graphics appear in their original size no matter what the size of the picture box or image box (unless the Stretch property is set to True). Therefore, if you create a huge picture box but load in a tiny bitmap graphics image, all you see is a tiny bitmap graphics image with lots of empty space around the image.
Unlike bitmap or icon graphics, metafiles expand or shrink to fill an entire picture box or expand to their original size to fill an entire image box. To change the size of a metafile, just change the size of the picture box or image box holding the metafile.
If you use a picture box, you can never (and I mean never) change the size of bitmap or icon graphics. On the other hand, if you use an image box, you can change the size of bitmap or icon graphics by changing the image box's Stretch property. To change the size of bitmap or icon graphics in an image box, follow these steps:
1. Click on the image box whose Stretch property you want to change.
2. Open the Properties window.
3. Click on the Stretch property and set the value to True.
After an image box has the Stretch property set to True, you can adjust the size of bitmap or icon graphics just by changing the size of the image box. (Isn't it amazing what $2,000 computers can do?)
* Automatically changing the size of picture boxes
If you're too busy to bother creating and adjusting the size of your picture boxes, let Visual Basic do such formatting automatically. All you need to do is set the AutoSize property to True. The moment you load the bitmap or icon graphics image into a picture box, the picture box immediately shrinks or expands to fit tightly around the graphics image, just like shrink-wrap around a box of floppy disks. To make a picture box automatically adjust its size around a graphics image, follow these steps:
1. Click on the picture box whose AutoSize property you want to change.
2. Open the Properties window.
3. Click on the AutoSize property and set the value to True.
** Coloring Picture Boxes
Visual Basic usually displays a plain gray background in picture boxes. If your graphics images fill up the entire picture box, the background color is irrelevant. But if the graphics image isn't big enough, the background color can be seen.
The background color is defined by the BackColor property. With a tasteful background color, you can highlight your graphics and make them more colorful.
To change the background color of a picture box, follow these steps:
1. Click on the picture box whose background color you want to change.
2. Open the Properties window.
3. Double-click on the BackColor property in the Properties window and then click on the Palette tab.
4. Click on the color you want. Visual Basic instantly obeys.
** Test your newfound knowledge
1. Which lets you change the size of a bitmap or icon graphic: a picture box or an image box?
a. Neither. Bitmaps and icon graphics are perfect just the way they are, and to believe that you can change anything for the better is blasphemous.
b. A picture box, because I don't have the slightest idea what the difference is.
c. An image box, but only if you set the Stretch property to True.
d. This question is too hard. I need a nap.
2. What are the three types of graphics files you can load in a picture box or image box?
a. Bitmaps, icons, and metafiles.
b, Bitmaps, graffiti, and forgeries of famous paintings.
c. Bitmaps, centerfolds, and home videos.
d. Bigfoot, UFOs, and grainy pictures of Elvis walking through downtown Manhattan.
** Lines. Circles, and Other Nightmares from Geometry
What performs absolutely no useful function except a decorative one? If you answered, "The vice president of the United States," you're close, but the real answer is the parts of a user interface that make the interface look more attractive.
The prettier something looks, the friendlier people feel toward the pretty something, which explains why physically attractive people can go through life without having to pay their own bills. So if you make your user interface pretty, the likely consequence is that more people are actually going to try to use your user interface.
Visual Basic provides seven objects for adding visual makeup to your user interface. These objects are as follows:
Lines
Squares
Rectangles
Ovals
Circles
Rounded rectangles
Rounded squares
* Creating lines
Lines are useful for underlining or separating items on the screen. To create a line, follow these steps:
1. Click on the Line Icon in the Visual Basic Toolbox.
2. On the form, move the mouse to where you want the line to start.
3. Hold down the mouse button and move the mouse to where you want the line to end.
4. Release the mouse button.
If you double-click on the Line icon in the Visual Basic Toolbox, Visual Basic draws a line on the form for you automatically.
* Creatinig circles and rectangles
Circles and rectangles can enclose and separate items on the screen. Or they can be an excuse for doodling on company time when the boss thinks that you're really writing a program. To create a circle or rectangle, follow these steps:
1. Click on the Shape icon in the Visual Basic Toolbox.
2. On the form, move the mouse to where you want the top-left corner of the circle or rectangle to appear.
3. Hold down the left mouse button and move the mouse to where you want the bottom-right corner of the circle or rectangle to end.
4. Release the mouse button. At this point, Visual Basic displays a rectangle on the screen. If that's what you want, stop right here; otherwise, continue to Step 5.
5. Open the Properties window.
6. Click on the Shape property and click on the arrow in the Settings Box. Visual Basic displays a list of shapes to choose from:
0 - Rectangle
1 - Square
2 - Oval
3 - Circle
4 - Rounded Rectangle
5 - Rounded Square
7. Click on the shape you want to create.
* Changing the color of lines and other shapes
Visual Basic usually draws lines, circles, and rectangles by using a solid black line. Although boring ol' black is okay for most purposes, sometimes a little color can spice up your user interface. You define the color of a line by using the BorderColor property. To change the line color of a line or shape, follow these steps:
1. Click on the line or shape whose line color you want to change.
2. Open the Properties window.
3. Double-click on the BorderColor property in the Properties window and click on the Palette tab.
4. Click on the color you want. Visual Basic instantly obeys.
* Changing the thickness of titles
Lines can be from 1 to 8,192 in thickness. (The numbers are relative and not related to an actual unit of measurement.) Any line thicker than 100, however, tends to look like a fat sausage on the screen. To change the thickness of a line, follow these steps:
1. Click on the line or shape whose line thickness you want to change.
2. Open the Properties window.
3. Double-click on the BorderWidth property and type a new value. Visual Basic immediately changes the thickness of your line.
4. Marvel at the wonder of technology and how one day you are going to be able to tell your children, "When I was going to school, we had to draw lines using an Etch-A-Sketch. You kids have line drawing so easy with computers and everything."
* Changing the appearance of lines, circles, and rectangles
Visual Basic usually draws lines, circles, and rectangles with a solid line. Although a solid line is easier to see, you may want to create special effects that look like perforations on a page or Morse code. Visual Basic provides the following seven line styles.
Transparent
Solid (the default)
Dash
Dot
Dash-Dot
Dash-Dot-Dot
Inside Solid
If the thickness of a line is greater than 1, the only BorderStyle settings you can use are 1 (Solid) and 6 (Inside Solid). If you use a different BorderStyle setting, nothing happens and you may think that Visual Basic is broken.
To confuse matters even more, the appearance of a line is determined by the BorderStyle property. Normally, you think of a border as something surrounding an object. But in the Visual Basic world of twisted logic, the BorderStyle defines the appearance of a line. To change the appearance of lines by themselves or lines that make up your circles or rectangles, follow these steps:
1. Click on the line, circle, or rectangle whose appearance you want to change.
2. Open the Properties window.
3. Click on the BorderStyle property and choose one of the following:
0 - Transparent
1 - Solid
2 - Dash
3 - Dot
4 - Dash-Dot
5 - Dash-Dot-Dot
6 - Inside Solid
If you choose any style from 2 to 5, set the BorderWidth property to 1. Otherwise, Visual Basic displays the BorderStyle you selected as a solid line.
* Changing the size and position of lines
When you create a line, try to draw the line as the exact size you need. (What's the point of drawing a long line when you know that you really need a short one?) Visual Basic provides two ways to change the size and position of a line:
You can use the mouse.
You can change the X1, X2, Y1, and Y2 properties in the Properties window.
The mouse is the quickest and sloppiest way to change the size and position of a line. But if you insist on using the mouse, follow these steps:
1. Click on the line that you want to change.
Visual Basic displays a blue rectangle at each end of the line. These rectangles are called handles. (Because clicking on a single line can be an exercise in frustration, you can also move the mouse above the line, hold down the mouse button, move the mouse below the line, and release the mouse button.)
2. Move the mouse over one of these handles until the mouse pointer turns into a crosshair.
3. Hold down the mouse button and move the mouse to adjust the line. When the line takes the shape you want, release the mouse button.
If you prefer not to soil your hands by touching the mouse, you can use a more refined method favored by people of distinction everywhere: Use the Properties window. To change the size of a line using the Properties window, follow these steps:
1. Click on the line you want to change.
2. Open the Properties window.
3. Double-click on the X1 property and type a new value.
4. Double-click on the Y1 property and type a new value.
5. Double-click on the X2 property and type a new value.
6. Double-click on the Y2 property and type a new value.
* Changing the size of circles, rectangles, and other shapes
Happily, changing the size of circles, rectangles, and other shapes is much easier than changing the size of a line. You can use the mouse or the Properties window to change the size of a shape. To change the size of a shape using the mouse, follow these steps:
1. Click on the shape you want to change. Visual Basic displays black handles around the shape.
2. Move the mouse over one of these handles until the mouse pointer turns into a double arrow.
3. Hold down the mouse button and move the mouse. When the object is the shape you want, release the mouse button.
For those who prefer to use a keyboard at the expense of ease and convenience, you can change the size of a shape also by using the Properties window. To change the size of a shape by using the Properties window, follow these steps:
1. Click on the shape you want to change.
2. Open the Properties window.
3. Double-click on the Height property and type a new value.
4. Double-click on the Width property and type a new value.
** Filling Shapes with Colours and Pretty Patterns
The inside of a shape is usually empty, blank, and boring. For more excitement than most people's hearts can handle, you can change the color and pattern of the inside of a shape. Visual Basic provides eight patterns that you can use to fill the inside of a shape. The pattern is defined by the FillStyle property. The color of the pattern is defined by the FillColor property.
To define the fill pattern of a shape, follow these steps:
1. Click on the shape whose inside pattern you want to change.
2. Open the Properties window.
3. Click on the FillStyle property and choose one of the following:
0 - Solid
1 - Transparent
2 - Horizontal Line
3 - Vertical Line
4 - Upward Diagonal
5 - Downward Diagonal
6 - Cross
7 - Diagonal Cross
To change the color of a shape's fill pattern, follow these steps:
1. Click on the shape whose pattern color you want to change.
2. Open the Properties window.
3. Double-click on the FillColor property in the Properties window and click on the Palette tab.
4. Click on the color you want. Visual Basic obeys instantly.
** Changing the Background Color of Shapes
In addition to changing the color of the fill pattern inside a shape (fill color) and the line color that makes up a shape (border color), you can also change a shape's background color (back color). Confused?
Before you can change the background color of a shape, you must set the shape's BackStyle property to Opaque. (The default is Transparent, which means that the shape is invisible. If the shape is invisible, changing the color isn't going to do a thing.)
To change the background color of a shape, follow these steps:
1. Click on the shape whose background color you want to change.
2. Open the Properties window.
3. Click on the BackStyle property and set the value to Opaque.
4. Double-click on the BackColor property in the Properties window and click on the Palette tab.
5. Click on the color you want. Visual Basic instantly obeys.
** Try Changing an Object's Size for Yourself
The following sample program lets you change a circle's thickness by using the horizontal scroll bar. To see for yourself, create three objects with the following property settings.
If you don't feel like creating the program listed below, just load and run the SHAPE.VBP file off the enclosed CD-ROM.
Object - Form
Property - Caption
Setting - The Shrinking/Growing Circle
Object - Shape1:
Property - Name
Setting - shpCircle
Property - Height
Setting - 1455
Property - Left
Setting - 1440
Property - Shape
Setting - 3 (Circle)
Property - Top
Setting - 1080
Property - Width
Setting - 1695
Object - HScroll1:
Property - Name
Setting - hsbCircle
Property - Height
Setting - 255
Property - Left
Setting - 720
Property - Max
Setting - 20
Property - Min
Setting - 1
Property - Top
Setting - 360
Property - Width
Setting - 3255
Double-click on the horizontal scroll bar and type the following in the Code window:
Private Sub hsbCircle-Change()
shpCircle.BorderWidth = hsbCircle.Value
End Sub
To run the program, press F5. Then click on the horizontal scroll bar and watch the circle grow before your eyes. Amazing! Astound your friends! Be the hit of your next cocktail party! Visual Basic reveals it all!
++ PART III
MAKING MENUS
In this part ....
Pull-down menus are a fancy way to organize all the options available in your program. That way, if users want to do something with your program, they just have to choose the right pull-down menu and pick the appropriate command.
This part of the book shows you how to make pull-down menus in your own programs. Believe it nor not, making your own menus is actually simple. (The hard part is making your program actually work the way you want it to, which is something that even eludes the grasp of the Microsoft programmers).
++ CHAPTER 11
CREATING AND EDITING PULL-DOWN MENUS
In This Chapter:
Creating menus and menu titles
Adding separator bars
Using shortcut keys and check marks
Dimming or making menu commands disappear
Generally, every menu bar contains the following menu titles: File, Edit, Window, and Help. The File menu appears on the far left, the Edit menu appears next, the Window menu appears next to last, and the Help menu appears last. In between the Edit and Window menus are menu titles unique to a particular program.
Every menu consists not only of menu titles but also menu commands. The menu titles appear at the top of the screen in a menu bar, and the menu commands appear in pull-down menus.
** The Basic Elements of a Menu Bar
Before creating menus, decide how many menu titles your program needs and where each command belongs in your menu titles.
Visual Basic can create menus for you if you use the VB Application Wizard to help you create your program (as I explain in Chapter 3).
The File menu needs to contain commands directly related to file operations, such as opening, closing, saving, and printing files, as well as quitting the program so that you can go to the kitchen and get something to eat.
The Edit menu, needs to contain commands related to editing (duh), such as Undo (and Redo), Cut, Copy, Paste, Clear, and Select All.
The Window menu needs to contain commands related to opening, closing, arranging, and switching among different windows.
The Help menu needs to contain commands for getting help from the program. Typical help commands include a table of contents to the help system, an alphabetical index, propaganda about product support, and a useless About command that displays information the programmers think looks cute on the screen.
Any other menus you sandwich between the Edit and the Window menu titles need to clearly organize the type of commands hidden underneath.
For example, many word-processing programs have a Tools menu title that displays commands for grammar checking, hyphenation, macro creation, and other commands that 99 percent of the working population of America is never going to use.
If your menu titles are unique to your particular program (in other words, they're not the standard Edit or Window menu titles found on other programs), try to make your menu titles descriptive - that way users will have a better idea where to find a specific command.
** Making Menus for Your User Interface
To create and edit menus, you have to open the Menu Editor window. (If you use the VB Application Wizard, Visual Basic can create standard pull-down menus for you automatically, but you still need to use the Menu Editor window to edit your menus.)
In the name of freedom and confusion, Visual Basic provides three ways to display the Menu Editor window:
Press CtrI+E.
Choose Tools->Menu Editor.
Click on the Menu Editor icon on the toolbar.
You can create one set of pull-down menus for each form. So if your program contains two forms, you can have a completely different menu for each form. Of course, having multiple menus may confuse users, but if you're a typical programmer who generally doesn't care what users think, this isn't going to bother you one bit.
The Menu Editor window is where you define everything to create your menus. The first two things you have to define for all your menu titles are their names and their captions.
* Naming menus
Every menu titile and menu command has a caption and a name. The caption is what appears on the screen. The name never appears on the screen; you use the name to identify which menu command the user chooses.
Captions can be up to 40 characters long, including numbers, spaces, punctuation, and the underscore character (_). Of course, the longer your caption is, the more space the caption is going to gobble up on the screen.
Because captions appear on the screen, you can use an ampersand (&) in your captions, such as &File or T&able. Why do you want to do such a silly thing? An ampersand in front of any letter makes that letter underlined in the caption.
When a letter is underlined in a menu title, users can pull down that menu by pressing and holding down the Alt key and pressing whatever letter is underlined. If a menu title is named &Window, the caption appears on the screen as Window, and users can pull the menu down by pressing Alt+W. Offering this option can help users choose menu titles by using these keyboard shortcuts rather than using the mouse.
When the ampersand underlines a letter in a menu command caption, users can choose the caption simply by typing the underlined letter without pressing Alt. So if a menu command caption is named &New, the caption appears on the screen as New. Users can choose that caption by first pulling down the menu and then pressing N.
Names, like captions, can be up to 40 characters long, including numbers and the underscore character. Because names never appear on the screen, you can make them as long as you want until you reach the magic number of 40.
Unlike captions, names cannot include spaces, punctuation, or words that the editors at IDG Books Worldwide, Inc., deem offensive and, hence, may harm sales of this book.
For menu names, Visual Basic recommends that the name begins with mnu, as in the following examples:
mnuFile
mnuWindow
mnuFileOpen
Visual Basic doesn't care whether you use uppercase or lowercase consistently. If you really want to, you can use the following names for menus:
MNufiLEmNuwINDow
MNUfileOPEN
Such names are not only hard to read, but the scattered casing also makes you look illiterate. So for consistency (and to protect your image), the best method is to adopt Microsoft's style and stick with this style whenever you use Visual Basic.
To identify menu commands that appear under certain menu titles, include the menu title as part of a menu command's name. For example, if the menu title File is named mnuFile, menu commands (such as the Open, Save, and Exit commands) that appear in the File menu should have names like mnuFileOpen, mnuFileSave, and mnuFileExit.
* Making menu tittes
Creating pull-down menus for your Visual Basic programs is a two-step process:
First you create the menu titles that appear in the menu bar.
Next you create the menu commands that appear under each menu title.
To create menu titles that appear in the menu bar at the top of a form, follow these steps:
1. Click on the form to which you want to add menu titles.
2. Open the Menu Editor window by pressing CtrI+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. In the Caption text box of the Menu Editor window, type the menu title that you want to have appear on the screen, including any ampersands.
As you type, Visual Basic displays your caption in the Menu control list box.
4. Press Tab to move the cursor to the Name text box.
5. Type your menu name, beginning with mnu followed by the menu caption itself, such as mnuFile or mnuFiIePrint.
You can mix uppercase and lowercase, but for consistency with Visual Basic programmers around the world, stick with the style mnuFileExit where you use uppercase letters to identify separate words such as File and Exit.
6. Press Enter or click on Next to create the next menu title.
7. Repeat Steps 3 through 6 until you create all the menu titles that you want to have appear at the top of the screen in the menu bar.
8. Click on OK.
Visual Basic displays your menus at the top of the form.
* Adding and deleting menu tittes and commands
Creating menu titles is fairly straightforward. Unfortunately, nothing in life is permanent, and that can include your menu titles. For that reason, Visual Basic gives you the option of adding or deleting menu titles.
To add another menu title to a form, follow these steps:
1. Click on the form to which you want to add another menu title.
2. Open the Menu Editor window by pressing CtrI+E, choosing Tools->Menu. Editor or clicking on the Menu Editor icon on the toolbar.
3. Click on the menu title that you want to appear to the right of your new menu title.
4. Click on Insert.
Visual Basic pushes the previously highlighted menu title down and highlights a blank line.
5. Click on the Caption text box and type your new menu title caption, such as &Tools or Forma&t.
6. Press Tab to move the cursor to the Name text box and type your new menu name (such as mnuTools or mnuFormat) and then press Enter.
7. Click on OK.
To delete a menu title, follow these steps:
1. Click on the form from which you want to delete a menu title.
2. Open the Menu Editor window by pressing CtrI+E, choosing Tools->Menu Editor, or clicking on the Menu Editor Icon on the toolbar.
3. Click on the menu title you want to delete.
4. Click on Delete. Visual Basic deletes the highlighted menu title.
5. Click on OK.
When you delete a menu title or menu command, any BASIC code you've written for that particular menu command still exists, so you'll have to delete this code as well.
** Creating Menu Commands under Menu Titles
After you create the menu titles that appear in the menu bar at the top of a form, the next step is to create the commands to appear underneath each menu title.
In the Menu control list box, all flush left items are menu titles that appear in the menu bar. Indented items are menu commands that appear below a menu title.
To create menu commands, follow these steps:
1. Click on the form to which you want to add menu commands.
2. Open the Menu Editor window by pressing CtrI+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
Visual Basic obediently opens the Menu Editor window.
3. Click underneath the menu title where you want to display the menu commands. For example, if you want to put menu commands underneath the File menu title, click underneath the File menu title.
4. Click on Insert.
5. Click on the Caption text box and type the menu command's caption, such as &Save or &Print.
6. Press Tab to move the cursor to the Name text box.
7. Type the menu command's name, such as mnuFiIeSave or mnuFilePrint.
8. Click on the right-arrow button to indent the menu command. This indentation shows you that an item is a menu command and not a menu title. (How's that for similar but confusing terms?)
9. Click on OK.
** Moving Menu Titles and Commands
When you create pull-down menus, you can always edit and change them at a later time. Visual Basic gives you four ways to move your menu titles and commands around:
Up
Down
Indent right
Indent left
In the Menu Editor window, Visual Basic provides four arrow buttons that enable you to move items up, down, right, or left. Moving an item up or down in the Menu control list box simply rearranges that item's position on the menu bar or in a pull-down menu.
To move an item up or down in the Menu control list box, follow these steps:
1. Click on the form containing the menu titles or commands that you want to rearrange.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. Click on the item in the Menu Editor window that you want to move up or down.
4. Click on the up-arrow button to move the item up, or click on the down-arrow button to move the item down.
5. Click on OK when you're done goofing around
When within the Menu Editor window, indenting an item to the right turns a menu title into a menu command. Likewise, while within this window, indenting an item to the left turns a menu command into a menu title.
To indent an item left or right in the Menu control list box, follow these steps:
1. Click on the form containing the menu titles or commands that you want to rearrange.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor Icon on the toolbar. Visual Basic obediently opens the Menu Editor window.
3. Click on the item that you want to indent left or right.
4. Click on the right-arrow button to indent the Item to the right, or click on the left-arrow button to indent the item to the left.
5. Click on OK
** Making Menus Pretty
Pull-down menus conveniently list commands where users can (hopefully) find them. To make your menus even easier to use, Visual Basic also lets you separate menu commands with separator bars, display check marks next to currently used menu commands, add shortcut keys so users don't have to use your pull-down menus at all, and dim or remove menu items altogether.
* Putting separator bars in menus
Separator bars are lines in a pull-down menu that divide groups of commands. Generally, separator bars group related items so that users can find the command they want.
To create a separator bar, follow these steps:
1. Click on the form containing the menus to which you want to add separator bars.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. Click on the item in the Menu Editor window that you want to appear directly below the separator bar.
4. Click on Insert so that Visual Basic displays an empty line.
If necessary, you may have to click on the right- or left-arrow buttons to make the separator bar appear on the same level as the items the bar is dividing.
5. Click on the Caption text box, type a hyphen (-), and press Tab to move the cursor to the Name text box.
6. Type any name you want to identify the separator bar.
Ideally, the name should include part of the menu title, such as mnuFileBar1 or mnuEditBar3.
7. Click on OK to close the Menu Editor window.
* Test your newfound knowledge
1. Why are pull-down menus so usefull?
a. They hide commands so that users can't find them.
b.They make programs easier to use by organizing related commands in easy-to-find menus.
c. Pull-down menus are useful for confusing people while giving them the illusion that it's their fault for not knowing how to use your program in the first place.
d. If pull-down menus are so useful, how come people still need to buy 400-page books to teach them how to use these stupid programs?
2. How can you create and edit pull-down menus in a Visual Basic program?
a. Press Ctrl+E, choose Tools->Menu Editor, or click on the Menu Editor icon on the toolbar to open the Menu Editor.
b. Copy someone else's program and hope that they don't notice.
c. To create pull-down menus, you need to earn a four-year degree studying C++ and Java.
d.Visual Basic can create pull-down menus?
* Assigning shortcut keys
After a while, you are likely to get tired of using pull-down menus every time you want to pick a command. For commonly used commands, a good idea is to assign these commands to shortcut keys, such as Ctrl+S to choose the Save command or Ctrl+X to choose the Cut command. Such a shortcut key lets the user give a command without wading through multiple pull-down menus.
Shortcut keys appear on menus next to the commands they represent. In this way, users can quickly discover the shortcut keys for all your menu commands.
To assign a shortcut key to a menu command, you have to use the Menu Editor window again. Although you may want to make up your own shortcut keys, Visual Basic lets you choose from only a limited list of possible keys.
Visual Basic doesn't let you assign the same shortcut keys to different commands. If you try to, Visual Basic scolds you with an Error dialog box.
To assign shortcut keys to menu commands, follow these steps:
1. Click on the form containing the menus to which you want to add shortcut keys.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor Icon on the toolbar. Visual Basic displays the Menu Editor window.
3. Click on the menu command for which you want to assign a shortcut key.
4. Click on the down-arrow button in the Shortcut list box.
Visual Basic displays a list of possible keystroke combinations you can use.
5. Scroll through this list until you find the right keystroke combination.
Ideally, you want to choose keystroke combinations that are easy to remember, such as Ctrl+S for the Save command or Ctrl+X for the Cut command. Visual Basic displays your choice in the Menu control list box.
6. Click on OK.
Now when you click on your pull-down menus, shortcut keys appear next to some of the commands. Because you haven't written any BASIC code to tell these commands what to do, nothing happens if you press any of the shortcut keys.
* Putting check marks next to menu commands
Check marks, which appear next to items on a menu, visually show that the items already have been selected. These check marks are often useful in identifying which font, type style, or size is currentIy in use.
If you want to make any default choices in your pull-down menus, you can have check marks appear when your program runs.
Check marks can appear next to menu commands only and riot menu titles. If you try to put a check mark next to a menu title, Visual Basic screams and displays the error message - Can't put check mark here.
To add check marks to menu commands, follow these steps:
1. Click on the form containing the menus to which you want to add check marks.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. Click on the menu command from the Menu Editor window that you want a check mark to appear next to.
4. Click on the Checked check box.
5. Click on OK.
If you put check marks next to your menu commands, you're going to want to remove the check marks eventually. To do this, you have to use (gasp!) BASIC code.
To remove a check mark that's next to a menu command, just set the command's Checked property to False. The following example removes a check mark from a menu command named mnuFont12:
mnuFont12.Checked = False
To add a check mark using BASIC code, just set the menu command's Checked property to True. The following example adds a check mark next to a menu command named mnuFontHelvetica:
MnuFontHelvetica.Checked = True
* Dimming menu commands
Sometimes using certain commands doesn't make sense. For example, until you select a block of text, having the Cut or Copy commands as options is pointless. To prevent users from choosing menu commands that aren't available, you can dim the commands. That way, the commands still appear in the menus, but the user can't choose them.
To dim a menu item, follow these steps:
1. Click on the form containing the menu commands that you want to dim.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor lcon on the toolbar. Visual Basic cheerfully displays the Menu Editor window.
3. Highlight the menu item that you want to dim.
4. Click on the Enabled check box to remove the check mark.
5. Click on OK.
If you dim a menu command, eventually, you're going to want to undim the command. To do so, you have to use BASIC code. To undim a menu command, set the command's Enabled property to True. The following example undims a menu command named mnuEditCut:
MnuEditCut.Enabled = True
To dim a menu command while your program is running, use BASIC code. Just set the menu command's property to False. The following example dims a menu command named mnuEditCopy:
MnuEditCopy.Enabled = False
* Making menu commands disappear
Rather than dim a menu command, you can make the command disappear. For example, some programs remove all menu titles except File and Help from the menu bar until the user opens or creates a file. (After all, displaying an Edit menu when you have nothing to edit is pointless.)
To remove a menu item, follow these steps:
1. Click on the form containing the menu commands you want to make invisible.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. From the Menu Editor window, highlight the menu item that you want to make invisible.
4. Click on the Visible check box to remove the cheek mark.
5. Click on OK.
After you make a menu command invisible, eventually you're going to have to make the command visible. To do so, you have to use BASIC code. To make a menu command visible, set the command's Visible property to True. The following example makes a menu title named mnuEdit visible:
mnuEdit.Visible = True
To make a menu command invisible while your program is running, use BASIC code and set the menu item's property to False. The following example makes a menu title named mnuTools disappear:
mnuTools.Visible = False
Just remember that all these fine points of beautifying your menus make your program easier to use and give your program that professional look. As any professional programmer can tell you, if a program looks good, users assume that any error they come across must be their fault. And that's the real reason programmers spend so much time creating a user interface - so that users aren't going to blame the programmers when the program fails catastrophically.
++ CHAPTER 12
SUBMENUS, GROWING MENUS, AND POP-UP MENUS
In This Chapter:
Creating submenus
Dynamically growing menus
Creating pop-up menus
A typical menu bar displays a list of menu titles at the top of the screen. Selecting one of the menu titles displays a pull-down menu.
Unfortunately, a menu bar can hold only a limited number of menu titles, and a pull-down menu can hold only as many commands as can appear on the screen simultaneously. So what happens if you write a killer application that requires more commands than can possibly appear on the menu bar or in multiple pull-down menus? The solution is to use submenus (or to redesign your program).
** Creating Submenus
Submenus are often used to bury a command several layers deep within a series of pull-down menus. If organized properly, submenus clearly show the relationship between various topics. If organized improperly, your program is going to look just as confusing as the most popular commercial programs that millions of people are forced to use every day.
For example, many programs have a Format menu title. Under this Format menu may be commands such as TypeStyle, Font, and Size. Choosing Font often displays a submenu listing all the possible fonts available.
Visual Basic lets you create up to four levels of submenus. Although this number of submenus can be handy, most programs use only one level of submenus to avoid burying commands so deeply that no one can find them again. Rumor has it that the Watergate tapes, Jimmy Hoffa's body, and the location of the Holy Grail reside somewhere in a well-known program's submenus.
Whenever a menu item displays an arrowhead symbol, the symbol indicates that a submenu exists for that item. When you create submenus, Visual Basic displays this arrowhead symbol automatically.
The Menu Editor window is the only place where you can define submenus. Any item that appears flush left appears as a menu title in the menu bar. Items indented once appear on the pull-down menus, items indented twice appear on the first submenu level, items indented three times appear on the second submenu level, items indented four times appear on the third submenu level, and items indented five times appear on the fourth and last submenu level.
To create submenus, follow these steps:
1. Click on the form on which you want to create submenus.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. In the Menu Control list box, highlight the menu item that you want to make into a submenu.
4. Click on the right-arrow button to indent the item.
5. Click on OK.
Each level of indentation (submenu level) is represented by four dots in the Menu Control list box.
If you want to move submenus up a level (such as from submenu level 3 to submenu level 2), you can. Just follow these steps:
1. Click on the form containing the submenus that you want to modify.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. In the Menu Control list, highlight the menu item box that you want to move up a level.
4. Click on the left-arrow button to indent the item to the left.
5. Click on OK when you finish playing around.
Rather than using multiple levels of submenus, most of the really cool programs use dialog boxes. (See Chapter 13 to find out all about dialog boxes.) A dialog box lets users make multiple choices all at once instead of making choices one at a time through many submenu levels. While submenus are fine for choosing a few options, dialog boxes are better for choosing lots of options.
Then again, what's the point of offering submenus if even Microsoft recommends against doing so? This is just one of the many ways that Microsoft gives you the freedom to write hard-to-use user interfaces so that your programs can never pose a real threat to Microsoft's own programs.
* Changing Menu Captions While Your Program Is Running
In certain cases, changing the caption of a menu command while the program is running is necessary. The most common menu command that changes is the Undo command in the Edit menu. After choosing the Undo command, some programs may toggle Undo to display the Redo command.
To change a menu caption, you have to use BASIC code. Just find the name of the menu item that you want to change and set the item's Caption property to a new caption. The following example changes the mnuEditUndo caption to Redo:
mnuEditUndo.Caption = "Redo"
The following example changes the mnuEditUndo caption back to Undo:
mnuEditUndo.Caption = "Undo"
When changing menu captions, you can use the ampersand (&) symbol to display a menu command hot key. (You can read more about hot keys in Chapter 11.) For example, the following code changes the mnuEditUndo caption to Undo where the capital letter U appears underlined:
mnuEditUndo.Caption = "&Undo"
** Designing Dynamically Growing Menus
If you use many Windows-based programs (such as Microsoft Word), you may notice an odd feature: Each time you load a program, the File menu displays a list of the last four or five files you worked on. If you ever open two or more windows in the same program, you may notice that the Window menu also lists the names of the files currently open.
To create a dynamically growing menu, you have to create empty spaces in your menu. To do this, use the Menu Editor window and create BASIC code to add items to make the items visible.
To create a dynamically growing menu, follow these steps:
1. Click on the form where you want to create a dynamically growing menu.
2. Open the Menu Editor window by pressing Ctrl+E, choosing Tools->Menu Editor, or clicking on the Menu Editor icon on the toolbar.
3. Click below the last menu command under the menu title to which you want to add new items.
For example, if you want to add new items under the File menu, click below the last menu command under the File menu title.
4. Click on Insert to add the item.
5. Click on the right-arrow button to indent the empty lines so that they appear as menu commands below the menu title.
6. Leave the Caption text box empty and press Tab.
7. In the Name text box, type the same name for each of these empty lines, such as mnuFileMRU.
8. Type 0 in the Index text box for the first empty line.
Each time you add another empty line, increase the number in the Index text box by 1. You want the first empty line Index text box to contain 0, the second to contain 1, the third to contain 2, and so on.
9. Click in the Visible check box to clear the check box.
10. Press Enter.
11. Repeat Steps 3 through 10 until you create four or five empty lines below the last menu title in the Menu Editor Control list box. Make sure that each empty line has a different number in its Index text box.
12. Click on OK.
The preceding steps create a dynamically growing menu, but to actually add items to this menu, you have to use BASIC code. The following example shows how to add an item to give the illusion of a dynamically growing menu:
Private Sub Form_Load()
mnuFileMRU(O).Caption = "&1 C:\VB\HELLO.VBP"
mnuFileMRU(O).Visible = True
End Sub
The second line of the preceding Form_Load event procedure sets the caption of the first empty line to 1 C:\VB\HELLO.VBP. The third line makes this item visible on the pull-down menu.
** Creating Pop-Up Menus
Pop-up menus (also called context menus) are often used to quickly display a list of commands on the screen. Any menu or submenu can appear as a popup menu. Pop-up menus are usually programmed to appear when the user presses the right mouse button.
To create a pop-up menu, you have to use the BASIC command PopupMenu. The following example displays the mnuEdit menu as a pop-up menu when the user clicks the right mouse button:
Private Sub Form_MouseDown(Button As Integer, Shift As _ Integer, X As Single, Y As Single)
If Button = 2 Then ' Right mouse button pressed _
PopupMenu mnuEdit ' Pops up the mnuEdit menu
End If
End Sub
Instead of using the number 1 to represent the left mouse button and the number 2 to represent the right mouse button, you can use the variables vbPopupMenuLeftButton and vbPopupMenuRightButton, respectively.
To create a pop-up menu, follow these steps:
1. Click on the form containing the menu that you want to turn into a pop-up menu.
2. Press F7, choose View->Code, or click on the View Code Icon In the Project Explorer window.
The Code Editor window appears.
3. Click in the Object list box and choose Form.
4. Click in the Procedure list box and choose MouseUp. Visual Basic displays an empty Private Form_MouseUp procedure.
5. Type the following code below the Private Sub Form_MouseDown statement and above the End Sub statement:
If Button = vbPopupMenuRightButton Then
PopUpMenu (type the menu name here such as mnuEdit)
End If
If for some odd reason you want to make a menu pop up by pressing the left mouse button, substitute the vbPopupMenuRightButton variable with the vbPopupMenuLeftButton variable instead.
* Defining the location of pop-up menus
Normally, the pop-up menu appears wherever the mouse pointer happens to be. However, if you want pop-up menus to appear in a specific location on the screen, you can specify the exact coordinates.
For example, the following PopupMenu BASIC command displays a pop-up menu named mnuTools at X-coordinate 500 and Y-coordinate 650:
PopupMenu mnuTools, 500, 650
For those people who are finicky about where the pop-up menu appears relative to the mouse pointer, you can specify whether the menu appears to the left, to the right, or dead center of the mouse. Use one of the following commands:
PopupMenu mnuEdit, 0 ' Left aligned
PopupMenu mnuEdit, 4 ' Center aligned
PopupMenu mnuEdit, 8 ' Right aligned
Using numbers to specify right or left alignment can be easy to type but ultimately confusing to understand what the code actually does. To simplify matters, Visual Basic lets you use honest-to-goodness English phrases that represent the above numbers, such as the following:
PopupMenu mnuEdit, vbPopupMenuLeftAlign ' Left aligned
PopupMenu mnuEdit, vbPopupMenuCenterAlign ' Center aligned
PopupMenu mnuEdit, vbPopupMenuRightAlign ' Right aligned
Essentially, these statements tell Visual Basic, "Hey stupid, whenever you see the words vbPopupMenuRightAlign, substitute the number 8 instead."
Using the old method in a Visual Basic procedure looks like the following:
Private Sub Form_MouseDown(Button As Integer, Shift As _ Integer, X As Single, Y As Single)
If Button = 2 Then ' Right mouse button pressed
PopupMenu mnuEdit, 4 ' Old, confusing way
End If
End Sub
Now here's what the modern, easy-to-read method using English looks like:
Private Sub Form_MouseDown(Button As Integer, Shift As _ Integer, X As Single, Y As Single)
If Button = vbPopupMenuRightButton Then
PopupMenu mnuEdit, vbPopupMenuCenterAlign
End If
End Sub
In case your mind is racing ahead already to new possibilities, you can left-, center-, or right-align a menu around a specific X-coordinate. For example, if you want to center-align the pop-up menu around a specific X-coordinate, do the following:
PopupMenu mnuTools, 4, 500, 650
or
PopupMenu mnuTools, vbPopupMenuCenterAlign, 500, 650
This example displays the pop-up menu at the X- and Y-coordinates of 500 twips and 650 twips, respectively.
If you change the units of measurement on a form from twips to inches or centimeters, you may have to change the X- and Y-coordinate values as well to correspond to your new scale. (See Chapter 6 in case you want to define different units of measurement for your form.)
* Defining the right mouse button to work with pop-up menus
Normally, pop-up menus work like ordinary pull-down menus. To choose a command, you just click on the command with the left mouse button. However, because the right mouse button is used about as often as you use your wisdom teeth, you may want to give users the capability to use either the left or right mouse button.
The following code example activates the right mouse button:
PopupMenu mnuEdit, 2
To replace numbers with English, you can substitute the following:
PopupMenu mnuEdit, vbPopupManuRightButton
By default, Visual Basic assumes that you always want to use the left mouse button with any pop-up menu.
Trick question: How do you define both center-alignment and right-button activation? The answer is to use the Or operator, as shown in the following:
PopupMenu mnuEdit, 2 Or 4
To use English instead of numbers, you can use the following:
PopupMenu mnuEdit, vbPopupMenuRightButton Or vbPopupMenuCenterAlign
Most popular programs (such as WordPerfect, Excel, and Paradox) use submenus, dynamically growing menus, and pop-up menus. But for less complicated programs, such as games, you probably aren't going to need all these different menu features.
Fortunately, most users are already familiar with all these menu features, so when a pop-up menu or submenu appears on the screen, the user is not going to be shocked. The secret is to use these features only if you have to. Remember, the more fancy features you add to your program, the more programming concerns you have to worry about. (Now aren't you glad you decided to find out how to program your computer?)
* Test your newfound knowledge
1. Explain why you may want your menu captions to change while your program is running.
a. To confuse your users so that they think they're doing something wrong.
b. To move important commands every five minutes to keep users on their toes.
c. To toggle menu captions like Undo and Redo.
d. Because you aren't considered a real programmer unless you use every possible feature of a programming language to make your programs more complicated to use.
2. Explain what the following BASIC code does:
Private Sub
Form_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
If Button = vbPopupMenuRightButton Then PopupMenu mnuFont
End If
End Sub
a. When the user presses and releases the right mouse button, the program displays the mnuFont menu as a popup menu.
b. When the user presses the right mouse button, World War Ill is declared.
c. When the user presses any button, the mouse self-destructs with a loud popI
++ CHAPTER 13
DIALOG BOXES
In This Chapter:
Creating dialog boxes
Adding icons and buttons to dialog boxes
Using common dialog boxes
Pull-down menus certainly make life easier for users (provided, of course, that the users know how to use the menus). In addition to pulldown menus, nearly every program also uses dialog boxes.
Dialog boxes are those tiny windows that pop up on the screen. Most of the time, the computer uses dialog boxes to let the user know what the computer is doing, such as "Now printing page 4 of 67"' or "Windows 98 just crashed again and here's an application error number that you won't understand anyway."
However, dialog boxes also let the computer ask questions of users, such as "Cancel printing?" or "Do you really want to exit out of Windows?" A fancy dialog box may be crammed full of options so that the user can make multiple choices all at one time. Just as most Windows-based programs use similar pull-down menus (File, Edit, Help), these programs also use similar dialog boxes.
** Creating a Simple Dialog Box
A dialog box displays a brief message on the screen along with one or more command buttons. Dialog boxes typically contain the following four parts.
A title bar
A message
An eye-catching icon
One or more command buttons
The title bar identifies the purpose of the dialog box, such as About This Program. The message contains text that appears in the dialog box, such as "Are you sure that you want to start World War Ill?" The icon provides visual information about the dialog boxs importance. The number and type of command buttons can vary from one to three.
The simplest dialog box is one that displays a message on the screen and provides an OK command button so that the user can make the dialog box go away.
The following BASIC code creates the simple dialog box.
Private Sub Form_Load ( )
MsgBox "Get a Macintosh", ,"Another Windows Crash"
End Sub
This simple dialog box does nothing more than appear on the screen and then disappear when the user clicks on OK.
* Adding icons to a dialog box
Icons can help grab a user's attention to your dialog box. Visual Basic uses four icons.
Critical Message: Alerts the user to an extremely important question, such as "If you continue, you are going to erase all the files on your hard disk. Are you sure that you want to do this?"
Warning Query. (A question mark.) Highlights less-threatening questions, such as "Do you really want to exit from Microsoft Word?"
Warning Message: (An exclamation mark.) Emphasizes warnings that the user needs to know about, such as "You are about to replace all 79 pages of your document with a period!"
Information Message: Makes otherwise drab and boring messages look interesting, such as "Printing all 3,049 pages of your document may take a long time. Click on OK if you want to go through with this."
To add an icon to a dialog box, just add the numeric value of the icon between the dialog box message and the title bar text, as follows:
Private Sub Form_Load( )
MsgBox "It crashed again!", 16, "Windows Error Message'
End Sub
Normally, Visual Basic lets you display only one of four possible icons in a dialog box from scratch. To do this, create a separate form, set the form's BorderStyle property to Fixed Dialog, and draw command buttons and an image box directly on this form. You can then draw an image box on the form and load any type of icon you want.
Just remember that creating a dialog box by using a separate form requires you to draw the command buttons, label, and image box, and to write BASIC code to make the whole dialog box work. If you just want to create a dialog box quickly and easily, use the MsgBox command instead.
* Defining the number and type fo command buttons in a dialog box
Dialog boxes can contain from one to three command buttons. A numerical value represents each command button. Table 13-1 lists the six command button combinations.
||
TABLE 13-1
Command Button Combinations Available in Visual Basic
Displays - OK button
Value - 0
Visual Basic Constant - vbOKOnly
Displays - OK and Cancel buttons
Value - 1
Visual Basic Constant - vbOKCancel
Displays - Abort, Retry, and Ignore Buttons
Value - 2
Visual Basic Constant - vbAbortRetryIgnore
Displays - Yes, No, and Cancel buttons
Value - 3
Visual Basic Constant - vbYesNoCancel
Displays - Yes and No button
Value - 4
Visual Basic Constant - vbYesNo
Display - Retry and Cancel buttons
Value - 5
Visual Basic Constant - vbRetryCancel
To define a command button combination, choose the combination you want and type the numerical value of the combination or the Visual Basic constant between the dialog box's message text and title bar text, such as
Private Sub Form_Load( )
MsgBox "File not found", 2, "Error Message"
End Sub
Private Sub Form_Load( )
MsgBox "File not found", vbAbortRetryIgnore, "Error Message"
End Sub
* Which command button did the user select in a dialog box?
If a dialog box just displays an OK command button, clicking on that OK command button usually makes that dialog box go away. However, dialog boxes with two or more command buttons give users a choice. When dealing with multiple command buttons on a dialog box, you have to write BASIC code to figure out:
Which button the user chose
What your program should do depending on which button the user chose
The seven possible buttons a user can choose are represented by the numerical balues in TABLE 13-2.
To make your program determien which command button a user chose, you have to set a variable equal to the MsgBox BASIC code, as shown in the following line:
Reply = MsgBox("File not foune", 2, "Error Message")
This code displays a dialog box with the Abort, Retry, Ignore command buttons. If the user clicks on Abort, the value of Reply is 3. If the user clicks on Retry, the value of Reply is 4. If the user clicks on Ignore, the value of Reply is 5.
Note.. Whenever you assign a variable to represent the value chosen from a dialog box, you must use parentheses to enclose the dialog box parameters.
||
TABLE 13-2
Command Buttons a User Can Choose
Button Selected - OK
Numerical Value - 1
Visual Basic Constant - vbOK
Button Selected - Cancel
Numerical Value - 2
Visual Basic Constant - vbCancel
Button Selected - Abort
Numerical Value - 3
Visual Basic Constant - vbAbort
Button Selected - Retry
Numerical Value - 4
Visual Basic Constant - vbRetry
Button Selected - Ignore
Numerical Value - 5
Visual Basic Constant - vbIgnore
Button Selected - Yes
Numerical Value - 6
Visual Basic Constant - vbYes
Button Selected - No
Numerical Value - 7
Visual Basic Constant - vbNo
** Commonly Used Dialog Boxes
While a simple dialog box may be sufficient occasionally, you may want to use a more complicated, yet more common, dialog box such as:
Open
Save As
Color
Font
Before you can use one of these common dialog boxes, you may have to load the Common Dialog Box icon into the Visual Basic Toolbox. To do this, follow these steps:
1. Choose Project->Components or press Ctrl+T. A Components dialog box appears.
2. Click on the Controls tab.
3. Make sure that a check mark appears in the Microsoft Common Dialog Control 6.0 check box. If a check mark does not appear, click on the check box.
4. Click on OK. The Visual Basic Toolbox displays the Common Dialog Box icon.
To display one of the five commonly used dialog boxes in your program, follow these steps:
1. Click on the Common Dialog Box Icon in the Visual Basic Toolbox.
2. Move the mouse anywhere on the form. Hold down the mouse button and move the mouse down and to the right. Let go of the mouse button.
This draws the Common Dialog Box icon on the form.
If you double-click on the Common Dialog Box icon in the Visual Basic Toolbox, Visual Basic draws the Common Dialog Box icon on the form for you automatically.
Where you place the Common Dialog Box icon on a form is irrelevant because the icon is always invisible when your program runs. Putting the Common Dialog Box icon on a form essentially tells Visual Basic, "Okay, this icon gives you the magical power to display the Open, Save As, Print, Color, or Font dialog box whenever I tell you to."
Because you need only one Common Dialog Box icon per form to display the different types of common dialog boxes, you don't have to change the icon's name. Just use the default name that Visual Basic gives the icon, which is CommonDialog1.
* Test Your Newfound Knowledge
1. Why do you want do display an icon in a dialog box?
a. In case an illiterate computer user wants to use your program.
b. To catch the eye of the user and provide a visual cue. For example, a Critical Message icon can warn users that something terrible is about to happen if they don't do something immediatley.
c. To see if the user is smart enough to realize that the dialog box has nothing important to say.
2. To use the predefined Open, Save As, Color, Font, or Print dialog box, what must you do first?
a. Buy the Visual Basic manuals that used to come with the program for free
b. Create a new form, draw three command buttons, two check boxes, one list box, and a partridge in a pear tree.
c. Save your file and exit Visual Basic
d. Make sure that you have drawn the Common Dialog Box icon on your form.
* Displaying the Open dialog box
The Open dialog box lets users choose a drive, directory, and file to open. The user also has the choice of displaying only specific file types, such as those matching the *.TXT or *.EXE criteria.
To display the Open dialog box, you need only one magic BASIC command that looks like this:
CommonDialog1.ShowOpen
If you want to define the list of files that the Open dialog box displays, you have to use something technical called a filter. A filter tells Visual Basic what types of files to display, such as all those with the TXT or BAT file extension.
A filter consists of two parts: the label that appears in the list box and the filter itself. Table 13-3 lists some examples of labels and filters. For added clarity, labels usually include the filter they use.
Text files, for example, usually have the file extension TXT, but sometimes they have the file extension ASC. So the label "Text Files (*.TXT)" lets you know that the dialog box shows only text files with the TXT file extension (and not text flies with the ASC file extension).
To define your labels and filters, use BASIC code, as in the following example:
CommonDialog1.Filter = "All Files (*.*) |*.*|Text Files - (*.TXT) |*.TXT|Batch Files (*.BAT) - |*.BAT| Executable Files
(*.EXE) |*.EXE"
This code also establishes the filter order, which you can utilize with the corresponding filter index number. In the above example, the All Files filter is first, and so has a filter index number of 1. After you define the filter, you have to tell Visual Basic which filter to display by default. Again, you use BASIC code:
CommonDialog1.FilterIndex = 1
This code line displays "All Files (*.*)" in the List Files of Type list box. Or you can choose the following:
CommonDialog1.FilterIndex = 4
This line displays "Executable Files (*.EXE)" in the List Files of Type list box.
The order in which you define your filter (using the Common Dialog1.Filter command) determines the FilterIndex number. For example, if you change the filter to
CommonDialog1.Filter = "Text Files (*.TXT) - |*.TXT|All Files(*.*)|*.*"
the following code displays "Text Files (*.TXT)" in the List Files of Type list box:
CommonDialog1.FilterIndex = 1
||
TABLE 13-3
Labels and Filters
Label - All Files (*.*)
Filter - *.*
Label - Text Files (*.TXT)
Filter - *.TXT
Label - Batch Files (*.BAT)
Filter - *.BAT
Label - Executable Files (*.EXE)
Filter - *.EXE
* How to create an Open dialog box
Obviously, if your program is going to store data in a file, the program is going to have to open a file at some point. Because nearly every program needs to open files, make your life easy and use the Open dialog box.
To actually display an Open dialog box, you need to write BASIC code, such as:
Private Sub mnuFileOpen_Click( )
CommonDialog1.Filter "Text Files (*.TXT) |*.TXT |AII Files (*.*)l*.*"
CommonDialog1.Filterlndex = 1
CommonDialog1.ShowOpen
End Sub
This event procedure tells Visual Basic:
1. When the user clicks on the Open command underneath the File pulldown menu, follow the instructions sandwiched in between the first and last lines of the Private Sub mnuFileOpen_Click( ) event procedure.
2. The second line tells Visual Basic what types of files to display in the Open dialog box.
3. The third line tells Visual Basic to display files matching the first list of files def ined by the Filter property. In this case, the third line tells the Open dialog box to display *. TXT text files.
4. The fourth line tells Visual Basic to display the Open dialog box on the screen, displaying only files with the TXT file extension.
At this point, the Open dialog box looks nice and seems to work, but because you haven't written any BASIC code to tell the dialog box what to do, the box doesn't do a thing but look pretty.
* Which file did the user choose from an Open dialog box?
After you display an Open dialog box, the next big question is to find out which file the user chose. When the user clicks on a file displayed by the Open dialog box, Visual Basic stores the filename in the Filename property of the Common dialog box. So if you want to retrieve the filename that the user clicked on, you need to set a variable to store the filename property, such as:
WhatFile = CommonDialog1.filename
So the complete event procedure may look like this:
Private Sub mnuFileOpen_Click( )
Dim WhatFile As String
CommonDialog1.Filter = "Text Files (*.TXT) |*.TXT|All_Files (*.*)|*.*"
CommonDialog1.FilterIndex = 1
CommonDialog1.ShowOpen
WhatFile = commonDialog1.filename
End Sub
The CommonDialog1.file name property contains both the filename and the directory that the file is stored in, such as C:\MyDocuments\Secrets\Resume.txt. If the user clicks the Cancel command button in the Open dialog box, the filename property is set to " " (no text).
* Displaying a Save As dialog box
A Save As dialog box is nearly identical to the Open dialog box. However, the text of the title bar is not the same. (The Open dialog box's title bar says "Open" and the Save As dialog box's title bar says "Save As".)
The only BASIC command you need to use to display a Save As dialog box is:
CommonDialog1.ShowSave
However, you may also want to use filters to display certain types of files, such as:
Private Sub mnuFileSaveAs_Click( )
CommonDialog1.Filter = "Text Files (*.TXT) | .TXT | All_Files (*.*) |*.*"
CommonDialog1.FilterIndex = 1
CommonDialog1.ShowSave
End Sub
* Which file did the user choose from a Save As dialog box?
Like the Open dialog box, the Save As dialog box stores the filename in the Filename property of the Common dialog box. So if you want to retrieve the filename on which the user clicked, you need to set a variable to store the filename property, such as:
WhatFile = CommonDialog1.filename
So the complete event procedure may look like this:
Private Sub mnuFileSaveAs_Click( )
Dim WhatFile As String
CommonDialog1.Filter - "Text Files (*.TXT) |*.TXT |All_Files (*.*) |*.*"
CommonDialog1.FilterIndex = 1
CommonDialog1.ShowSave
WhatFile = CommonDialog1.fiIename
End Sub
The Save As dialog box doesn't actually save files on its own. To save a file under a different name, you have to write additional BASIC code that tells your computer to save a file onto a disk.
Rather than use Visual Basic's Open and Save As dialog boxes, try experimenting with a free File dialog box control that offers more flexibility and features while being smaller to use. You can download this FileDialog control from the CCRP Web site at www.mvps.org/ccrp. (Note that an independent group of programmers created this FileDialog box control, so if it doesn't work, don't blame Microsoft.)
* Displaying a Color dialog box
The Color dialog box lets users choose colors or mix their own.
To display a Color dialog box, you have to use two magic BASIC commands:
CommonDialog1.Flags = cdlCCRGBInit
CommonDialog1.ShowColor
The first line tells Visual Basic that the Common dialog box, defined by CommonDialog1, can be used to retrieve a color from the Color dialog box.
The second line tells Visual Basic to display the Color dialog box.
For example:
Private Sub Command1_Click( )
CommonDialog1.Flags = cdlCCRGBInit
CommonDialog1.ShowColor
End Sub
* Which color did the user choose from the Color dialog box?
The Color dialog box stores the color the user chose in the Color property of the Common dialog box. So if you want to retrieve the color (which Visual Basic stores as a number) that the user clicked on, you need to set a variable to store the Color property, such as:
WhatColor = CommonDialog1.Color
So the complete event procedure may look like this:
Private Sub Command1_Click( )
Dim WhatColor as Long
CommonDialog1.Flags = cdICCRGBInit
CommonDialog1.ShowColor
WhatColor = CommonDialog1.Color
End Sub
* Displaying a Font dialog box
The Font dialog box lets users choose different fonts, font styles, and point sizes. Each time the user chooses an option, this dialog box displays a sample so that the user can see whether the font, font style, or point size looks okay.
To display a Font dialog box, you have to use two magic BASIC commands:
CommonDialog1.Flags = cdlCFEffects Or cdlCFBoth
CommonDialog1.ShowFont
The first line tells Visual Basic that the Common dialog box, defined by CommonDialog1, can be used to retrieve a font value from the Font dialog box.
The second line tells Visual Basic to display the Font dialog box. For example:
Private Sub Command1_Click ( )
CommonDialog1.Flags = cdlCFEffects Or cdlCFBoth
CommonDialog1.ShowFont
End Sub
* Which captions did the user choose from the Font dialog box?
The Font dialog box lets the user choose a variety of options, as Table 13-4 shows:
TABLE 13-4
Properties That Store Values from the Font Dialog Box
Property - Color
What Information It Contains - The selected color. To use this property, you must first set the Flags property to cdlCFEffects.
Property - FontBold
What Information It Contains - Whether bold was selected.
Property - Property
What Information It Contains - What information it contains.
Property - FontItalic
What Information It Contains - Whether italic was selected.
Property - FontStrikethru
What Information It Contains - Whether strikethru was selected. To use this property, you must first set the Flags property to cdlEFEffects.
Property - FontUnderline
What Information It Contains - Whether underline was selected. To use this property, you must first set the Flags property to cdlCFEffects.
Property - FontName
What Information It Contains - The selected font name
Property - FontSize
What Information It Contains - The selected font size.
So the complete event procedure may look like this:
Private Sub mnuFormatFont_Click( )
Dim TextColor As Long
Dim Bold As Boolean
Dim Italic As Boolean
Dim Underline As Boolean
Dim Strikethru As Boolean
Dim Font As String
Dim Size As Integer
CommonDialog1.Flags = cdlCFEffects Or cdlCFBoth
CommonDialog1.ShowFont
TextColor = CommonDialog1.Color
Bold = CommonDialog1.FontBold
Italic = CommonDialog1.FontItalic
Underline = CommonDialog1.FontUnderline
Strikethru = CommonDialog1.FontStrikethru
Font = CommonDialog1.FontName
Size = CommonDialog1.FontSize
End Sub
* Displaying a Print dialog box
A Print dialog box lets users choose the printer, the print range, and the print quantity.
To display a Print dialog box, use the following BASIC command:
CommonDialog1.ShowPrinter
For example:
Private Sub mnuFilePrint_Click( )
CommonDialog1.ShowPrinter
End Sub
To set the default value for the number of copies to print, you have to use the following BASIC commands:
CommonDialog1.Copies = 1
CommonDialog1.ShowPrinter
For example:
Private Sub Command1_Click( )
CommonDialog1.Copies = 1
CommonDialog1.ShowPrinter
End Sub
Like the Open, Save As, Color, and Font dialog boxes, the Print dialog box looks like it works but really doesn't do anything until you write BASIC code to tell the box how to work after the user clicks on OK.
Using the common dialog boxes that I discuss in this chapter can give your programs that all-important professional look and feel that people have come to expect from software. As any professional programmer can tell you, the more your program looks to be in working order, the more likely that people are going to believe that the program does work.
++ PART IV
THE BASICS OF WRITING CODE
In this part.....
Hurray! Here's the first chapter where you actually find out how to write your own BASIC code to make your computer do something worthwhile. Until now, you may have only drawn the parts that make up a user interface (with an occasional BASIC command thrown in). But everyone knows that looks aren't everything (unless you're a centerfold, looking to marry a multimillionaire who will die within the year). What matters is not only that your user interface looks good, but that your user interface also responds to the user.
Although the thought of writing BASIC code may seem intimidating, it's not. BASIC code is nothing more than a set of step-by-step instructions that tell the computer exactly what to do. So get ready to start coding (a programmers' term for writing computer commands). You'll find that programming can really be fun, easy, and almost as addictive as drawing your user interface or playing a computer game.
++ CHAPTER 14
EVENT PROCEDURES
In This Chapter:
Creating event procedures
Viewing different event procedures
Editing in the code window
Whenever the user takes any action, such as clicking the mouse, pressing a key, passing out on the keyboard, or putting a bullet through the monitor, the action is called an event. The moment an event occurs, Visual Basic looks for BASIC code to tell the program what to do. The BASIC code that responds to a specific event is called an event procedure.
A single Visual Basic program can consist of several thousand event procedures. If you have that many, however, you either have a tremendously complicated program or you're an incredibly incompetent programmer.
With so many possible events and so many possible event procedures in a single program, how does Visual Basic know which event procedure to use?
The answer is easy. When an event occurs, this event is usually directed at some part of your program's user interface. For example, most users click the mouse button only when the mouse is pointing at an object, such as a command button, check box, or menu command on the screen.
Every object can have one or more event procedures, and each event procedure responds to one specific event, such as clicking the mouse or pressing a key.
** Types of Events
Events can be classified into three categories:
Keyboard events occur when the user presses a certain key, such as Tab, or a certain keystroke combination, such as Ctrl+P.
Mouse events occur when the user moves the mouse, clicks or doubleclicks the mouse button, or drags the mouse across the screen.
Program events occur when a Visual Basic program does something on its own, such as loading, opening, or closing a form. Whereas keyboard and mouse events occur when the user does something, program events occur when BASIC code does something.
Although Visual Basic can respond to a multitude of events, you generally want your user interface to respond only to a few events, such as clicking the mouse or pressing a certain key. As soon as Visual Basic detects an event, your program immediately looks to see what part of the user interface needs to respond.
When the user clicks the mouse, for example, Visual Basic first identifies the event. ("Okay, that was a mouse click.") Next, Visual Basic looks to see where the user clicked the mouse. ("The user clicked the mouse on the OK command button.")
Visual Basic then finds that particular command button's event procedure, which contains BASIC code that tells your program what to do when the user clicks the mouse button.
** Creating event procedures
One object can respond to one or more events. For example, a command button can respond to the user's clicking the mouse button or pressing Enter.
Two or more objects can respond to the same event. For example, both a command button and a check box can respond to a mouse click, but they may have different instructions that tell Visual Basic what to do next.
To write an event procedure, you have to perform the following tasks:
Identify the part of your user interface that is going to respond
Open the Code window.
Identify the event to which Visual Basic is to respond.
Write BASIC code to process the event.
Make sure that all the objects of your user interface have names before creating any event procedures. If you create an event procedure for an object and later change that object's name, you will have to rewrite your event procedures.
The following three parts of a user interface can have events associated with them:
Forms
Objects (command buttons, check boxes, and so on)
Pull-down menus
* To create an event procedure for a form, follow these steps:
1. Click anywhere on the form, but not on any objects on the form.
2. Open the Code window by pressing F7, choosing View->Code, or double-clicking anywhere on the form (but not on any objects on the form).
Visual Basic displays the Code window on the screen along with an empty event procedure.
* To create an event procedure for an object, such as a command button or check box, follow these steps:
1. Click on the object so that little black rectangles (handles) appear around it.
2. Open the Code window by pressing F7, choosing View->Code, or by double-clicking on the object.
Visual Basic displays the Code window on the screen along with an empty event procedure. You may still have to click on the Procedure list box to choose a specific event to respond to, such as Click or KeyPress.
If you double-click on an object (such as a command button), Visual Basic displays the Code window right away.
* To create an event procedure for a pull-down menu, follow these steps:
1. Click on the pull-down menu title containing the menu command you want.
2. Click on the menu command you want to write BASIC code for.
Visual Basic displays the Code window on the screen along with an empty event procedure.
** Getting to know the parts of event procedures
When you create an event procedure for the first time, Visual Basic displays an empty event procedure in the Code window. All empty event procedures consist of two lines, such as:
Private Sub cmdExit_Click( )
End Sub
The first line of any event procedure contains five parts:
Private Sub: Identifies the procedure as a subroutine.
The object's name: In this example, the object is a command button named cmdExit.
An underscore
The event name: In this example, the event is a mouse click.
A pair of parentheses, containing any data that the subroutine may need to work: In this example, the parentheses are empty, indicating that no additional data is necessary.
The preceding event procedure says to the computer, "Here are the instructions to follow whenever the user clicks the mouse on the command button named cmdExit. Now leave me alone."
Because this example contains no instructions to follow, this event procedure does absolutely nothing, much like many co-workers you may know.
Any time you change the name of an object, make sure that you change the name of all event procedures connected to the newly named object as well. Otherwise, Visual Basic doesn't know which event procedures belong to which objects on your user interface.
** Splitting the Code window in half
After you start to write lots of event procedures, the Code window may not be able to display all your event procedures at the same time. If you want to view two or more event procedures on the screen at the same time, you can split the Code window in half horizontally. You can divide the Code window only in half (not in thirds, quarters, and so on).
* To split the Code window in half, follow these steps:
1. Move the mouse pointer to the Split bar.
The Split bar appears at the top of the vertical scroll bar. As soon as the mouse pointer appears over the Split bar, the mouse pointer turns into two horizontal parallel lines with arrows pointing up and down.
2. Hold down the left mouse button and drag the mouse down.
When the Split bar divides the Code window the way you want, let go of the mouse button.
* To display the Code window as a single window again, follow these steps:
1. Move the mouse pointer over the Split bar that divides the Code window in half.
The mouse pointer turns into two horizontal parallel lines with arrows pointing up and down.
2. Hold down the left mouse button and drag the mouse all the way up to the top (or bottom) of the Code window, then let go of the mouse button.
** Editing in the Code Window
The Code window works like a simple word processor. Table 14-1 lists the different keystroke commands you can use to edit your event procedures.
TABLE 14-1
Common Editing Keys
Keystroke - Delete
What Happens - Deletes the character to the right of the cursor
Keystroke - Backspace
What Happens - Deletes the character to the left of the cursor
Keystroke - Ctrl+Y
What Happens - Deletes the line that the cursor is on
Keystroke - Home
What Happens - Moves the cursor to the front of the line that the cursor is on
Keystroke - End
What Happens - Moves the cursor to the end of the line that the cursor is on
Keystroke - Ctrl+Home
What Happens - Moves the cursor to the first event procedure
Keystroke - Ctrl+End
What Happens - Moves the cursor to the last event procedure
Keystroke - Ctrl+Down arrow
What Happens - Displays the next procedure
Keystroke - Ctrl+Up Arrow
What Happens - Displays the previous procedure
Keystroke - Ctrl+Page Down
What Happens - Goes to the first line of the next procedure
Keystroke - Ctrl+Page Up
What Happens - Goes to the first line of the current or previous procedure
Keystroke - Shift+F2
What Happens - If the cursor appears in a procedure name, this command displays the BASIC code of that procedure
Keystroke - Ctrl+Right arrow
What Happens - Goes one word to the right
Keystroke - Ctrl+Left arrow
What Happens - Goes one word to the left
Keystroke - Page Down
What Happens - Displays the next page down in the Code window
Keystroke - Page Up
What Happens - Displays the next page up in the Code window
Keystroke - Insert
What Happens - Toggles the Insert mode on or off
Keystroke - Ctrl+X
What Happens - Cuts a selected block of text
Keystroke - Ctrl+C
What Happens - Copies a selected block of text
Keystroke - Ctrl+V
What Happens - Pastes a previously Cut or Copied block of text
Keystroke - Ctrl+Z
What Happens - Undoes the last thing you did (typed a letter, erased a sentence, and so on)
Keystroke - Ctrl+F
What Happens - Finds a word that you specify
Keystroke - F1
What Happens - Displays the Visual Basic help system
Keystroke - F3
What Happens - Finds the next word that you specified previously using the Ctrl+F command
Keystroke - Shift+F3
What Happens - Finds the last occurrence of the word that you specified previously using the Ctrl+F command
Keystroke - F6
What Happens - Switches between Code window panes (if the Code window is split)
Keystroke - Ctrl+H
What Happens - Searches for a word and replaces it with something else
Keystroke - Ctrl+P
What Happens - Displays the Print Dialog box
To help you write BASIC code, the Code window also automatically high-lights BASIC reserved keywords in color. This way you can see which commands are BASIC reserved keywords and which are commands you've created on your own.
To delete an entire event procedure, highlight the procedure by using the mouse or the cursor keys and press Delete.
** Viewing Different Event Procedures
A typical Visual Basic program consists of event procedures stored in FRM form files. To help you find a particular event procedure to examine or edit, you have two choices:
Choose an object name from the Object list box and then choose an event from the Procedure list box in the Code window.
Use the Object Browser window.
** Choosing an event procedure with the Object and Procedure list boxes
To use the Object and Procedure list boxes to find an event procedure, you must know the FRM form file where Visual Basic has stored the event procedure. The Object list box contains all the objects stored on a form. If you click on the Object list box, you can find the object containing the event procedure that you want to see.
The Procedure list box contains all the events that an object can respond to. Each time you select a different event, Visual Basic displays a different event procedure in the Code window.
* To display an event procedure by using the Object and Procedure list boxes, follow these steps:
1. Choose one of the following to switch to the Project Explorer window:
* Click on the Project Explorer window with the mouse
* Press CtrL+R
* Choose View->Project Explorer
2. Click on the form file that contains the event procedure that you want.
3. Click on the View Code icon in the Project Explorer window (or choose View->Code).
4. Click on the downward-pointing arrow of the Object list box and click on an object name.
5. Click on the downward-pointing arrow of the Procedure list box and click on an event name.
Visual Basic displays your chosen event procedure.
If an event procedure contains BASIC code, the Procedure list box displays the event name in bold-face type. If an event name appears in normal type, then that event procedure is empty.
** Choosing an event procedure with the Object Browser window
The Object Browser is most useful when you want to view event procedures stored in different files.
* To display an event procedure with the Object Browser, follow these steps:
1. Choose View->Object Browser, or press F2 to display the Object Browser Window.
2. Click on the downward-pointing arrow in the Project/Library list box and click on the Project that contains the event procedure you want to see.
3. Click on a Form file name that appears in the Classes left pane.
All event procedures stored in that Form file appear in bold-face in the right pane.
4. Double-click on the event procedure that appears in the right pane.
Table 14-2 lists the most common events to which objects can respond.
** TABLE 14-2
Common Events
Event - Activate
Occurs When - A form becomes the active window
Event - Change
Occurs When - The contents of a combo box, directory list box, drive list box, scroll bar, label, picture box, or text box change.
Event - Click
Occurs When - The user clicks the mouse button once on the object
Event - DblClick
Occurs When - The user clicks the mouse button twice in rapid succession on an object
Event - Deactivate
Occurs When - A form changes from being an active window to an inactive window
Event - DragDrop
Occurs When - The user holds down the mouse button on an object, moves the mouse, and releases the mouse button
Event - DragOver
Occurs When - The user holds down the mouse button on an object and moves the mouse.
Event - DropDown
Occurs When - The list portion of a combo box drops down to display a list of choices
Event - GotFocus
Occurs When - An object becomes highlighted when the user presses Tab or clicks on an object, or if a form loads.
Event - KeyDown
Occurs When - The user presses a key
Event - KeyPress
Occurs When - The user presses and releases an ANSI key, such as a keyboard character, Ctrl key combination, Enter, or backspace key. (Basically, an ANSI key can be any letter, number, or oddball keystroke combination that you press).
Event - KeyUp
Occurs When - The user releases a key
Event - LostFocus
Occurs When - An object is no longer highlighted because the user pressed Tab or clicked on another object, or if a form has unloaded.
Event - MouseDown
Occurs When - The user presses a mouse button
Event - MouseMove
Occurs When - The user moves the mouse
Event - MouseUp
Occurs When - The user releases a mouse button
The combination of the object name and the event name defines the name for an event procedure. Because object names must always be unique, no two event procedures on the same form can have the same name.
Although no two event procedures on the same form can share the same name, event procedures on different forms can have the same name. For example, you might have a command button named cmdExit that appears on two different forms. If this happens, you can have the following event procedure stored on both forms:
Private Sub cmdExit_Click( )
End Sub
Two or more objects can share the same name if you make them into a control array. No need to memorize this term right now. Just make a note of this term and move on. If you really want to know about control arrays, pick up a copy of the latest edition of More Visual Basic For Dummies.
** The Event Procedure That Every Program Needs
The simplest and most important event procedure that every program needs is one that stops your program. The following event procedure tells Visual Basic to stop running your program the moment the user clicks on a command button named cmdExit:
Private Sub cmdExit_Click( )
Unload Me
End Sub
If you don't include an event procedure to stop your program, the only way a user can stop your program is by rebooting the computer or turning the whole system off. Because this isn't the best way to exit a program, always make sure that your program contains at least one (or more) ways for the user to exit your program at any given moment.
The old way to stop a Visual Basic program was to use the End keyword, such as:
Private Sub cmdExit_Click( )
End
End Sub
However, Microsoft recommends that instead of using the End keyword, you should use the Unload Me command instead. However, the Unload Me command stops a Visual Basic program only if all forms of your program have been unloaded using the Unload command. If even one form of your program is not unloaded, the Unload Me command will not stop a Visual Basic program from running.
** Test your newfound knowledge
1. What is an event, and what are the three types of events?
a. An event is something that you must get tickets for, such as a concert, a sports event, or the circus.
b. Events are things that happen to your computer, such as having a drink spilled on the keyboard, having all your files
erased by mistake, and having the dog eat a floppy disk.
c. Events occur when the user presses a key or mouse button or when the program changes appearance. The three types are keyboard, mouse, and program events.
d. An event is a holiday or celebration that lets you take the day off from work. The three types of events are legal holidays,
reunions, and funerals for non-existent relatives.
2. What do the Object list box and Procedure list box do in the Code window?
a. They list all the possible reasons why you need to write your program in C++ or Java rather than in Visual Basic.
b. The Object list box lets you choose an object for which you can write an event procedure. The Procedure list box lets you choose all the possible events to which an object can respond.
c. The Object list box contains a list of all the blunt objects you can use to hit your computer. The Procedure list box lists all the events that you can attend instead of staring at your computer screen.
d. Neither list box does anything worth remembering, so don't bother to ask me this question again.
++ CHAPTER 15
USING VARIABLES
In This Chapter....
Using Variables
Assigning numbers and strings to variables
Declaring data types
After you know what you want your program to do, you can start writing BASIC code. The first code you need to write is inside your event procedures.
At the simplest level, an event procedure tells the computer what to do. An event procedure for exiting a program, for example, gives the computer a single Unload Me instruction:
Private Sub cmdExit_Click( )
Unload Me
End Sub
This event procedure requires absolutely no information from the user beyond the simple event of the user's clicking on the cmdExit command button.
What happens, however, when a user types a name, an address, or a telephone number into a program? Obviously, the program must read this information from the user interface and then do something with the information.
The CD-ROM contains a simple program to show you how to declare, use, and display a string variable in a text box on two different forms.
** Reading Data
Any information that a program gets from outside the computer is data. Nearly all but the simplest programs receive data, do something to the data, and spit the data out again.
A word processor receives data as words, which the word processor formats to look pretty and then prints neatly on paper. A database receives data as names, addresses, and phone numbers. The database stores this information someplace and then displays the data in a way that you think is useful. A nuclear-missile guidance system receives data as target coordinates. The missile system uses this data to guide a warhead to a target and wipe entire cities off the planet in the name of peace.
Every useful program in the world follows these three basic steps:
1. Get data.
2. Do something to the data.
3. Spit the data back out again.
Every useless program in the world has these four characteristics:
Is too hard to understand and use
Costs a great deal of money
Claims to be user-friendly
Doesn't work
The whole purpose of a program is to turn computers into electronic sausage grinders. Stick information in one end and out comes the information on the other end. No matter what kind of program you examine - word processor, spreadsheet, database, or game - all programs manipulate the following:
Numbers
Strings
Numbers can be positive or negative, whole numbers or fractions, or just about any other type of number you can think of (including telephone numbers to hot dates, numbers that form a combination to a safe containing wads of money, and imaginary numbers that no one except mathematicians truly understand).
Strings are characters strung together. A character is anything you can type from the keyboard, including letters, punctuation marks, and (don't get confused now) numbers.
Depending on how the program decides to treat them, numbers can be considered as numbers or as a string. For example, most programs treat your telephone number or street address as a string but treat your age or weight as a number.
A single letter is considered a string. An entire sentence is also a string. You can even consider the first chapter of War and Peace a string. Strings can be any collection of letters, spaces, and numbers grouped together.
** Values and Variables
When you type a number or a string into a program, how does the computer keep track of that number or string? After all, you may know that 555-1234 represents a phone number, but to the computer, the phone number is just another number or string.
To store data, programs use variables, a time-tested concept from algebra. When you write a program, you have to tell the program, "Okay, when someone types 555-1234, give this number a name, such as PhoneNumber, and store the number someplace where you can find it again."
When your program needs to retrieve or manipulate this data, the program says, "Okay, where did I put this information? Oh, that's right, I stored the information in a place (variable) called PhoneNumber." The computer obediently rushes to the PhoneNumber variable and yanks out whatever number or word the computer stored there.
Variables can hold a wide variety of data (which is why they're called variables, a more scientific-sounding name than flaky, wishy-washy, or schizophrenic). The information stored in a variable is called a value because a value represents either a string or a number.
** Using variabtes
Two types of variables exist:
Those you make up
Those already defined as the properties of every object on a form
Every time you draw an object to make your user interface, Visual Basic automatically creates a whole bunch of variables (called properties) set with default values. To look at the values of an object's properties, you have to use the Properties window. (Press F4 or choose View->Properties Window.)
Property values can represent numbers (such as defining the width and height of an object), True or False (such as defining whether an object is visible), or strings (such as captions on a command button). Properties simply define the appearance of an object on the screen.
Variables are names that can represent any type of value. Properties are special names for variables that affect the appearance of an object.
To create a variable on your own, just give the variable a name. After you type a name for a variable, the variable magically springs into existence. You have two ways to create a variable:
Use the Dim statement to declare the variable.
Just name the variable and assign a value to it.
The only place where you can type (and create) a variable name is in the Code window. The only place in the Code window where you can type a variable name is sandwiched between the first and last lines of a procedure.
** Declaring variables:
Creating a variable is as simple as typing a name and assigning a value to the variable, as shown in the following example:
Private Sub Form1-Load( )
PetName "Bo the cat"
Age = 6
End Sub
The preceding event procedure says the following:
1. When a form named Form loads, follow these instructions.
2. First, create a variable called PetName and set the value to the string "Bo the cat".
3. Second, create a variable called Age and set the value to the number 6.
Although creating a variable out of thin air in the middle of a procedure is perfectly acceptable, this is not considered good programming practice. Unless you examine an event procedure line by line, you have no idea how many variables the procedure may be using.
A better programming practice is to declare your variables at the beginning of each event procedure. To declare a variable, use the Dim command, as shown in the following example:
Dim VariableName1, VariableName2
Type as many variable names as you want in the preceding command.
Rewriting the preceding event procedure causes the procedure to look like the following:
Private Sub Form1_Load( )
Dim PetName, Age
PetName = "Bo the cat"
Age = 6
End Sub
You can also declare variables explicitly, as in the following:
Dim PetName as String, Age as Integer
I give you the details on this technique in this chapter's section called "Data types."
Although declaring a variable can add an extra line or two to an event procedure, do you see how easy you can find the names of all the variables used? Instead of examining an event procedure line by line, you can just glance at the first few lines and see a list of all the variable names used.
Using the Dim statement simply helps you, the programmer, understand what the procedure does. As far as the computer is concerned, the computer doesn't care whether you use the Dim statement or not.
Naming variables:
You can name your variables anything you want, and you can store anything you want in them.
However, naming a variable PhoneNumber and then stuffing somebody's address in the variable is pretty foolish.
To make your life easier, give your variables names that represent the data you're going to store in them. For example, naming a variable PhoneNumber makes sense if you're going to store phone numbers in the variable. Likewise, you want a variable named BusinessName to hold only the words that make up business names.
When naming your variables, you must adhere to some unbreakable rules, which follow; otherwise, Visual Basic throws a tantrum. All variables must:
Begin with a letter
Be a maximum of 255 characters in length (with an obvious minimum of one character in length).
Contain only letters, numbers, and the underscore character (_); spaces and punctuation marks are not allowed.
Be any word except a Visual Basic reserved word, such as End or Sub.
If your variable names meet these criteria, all is going to be well. (Of course, that doesn't mean your program is going to work, but at least Visual Basic is going to be happy.) The following are examples of Visual Basic-approved variable names:
Phone
Here_is_Your_Name
Route66
The following are some no-nos for variable names, which Visual Basic refuses to use:
123Surprise (This name begins with a number)
Just Work (This name contains a space)
Sub (This name is a Visual Basic reserved keyword).
* Assigning numbers to variables
Now that you know how to create variables by naming them, how do you assign a value to a variable and get the value back out again? Easy - you use something mysterious called an equal sign (=).
To assign a value to a variable, you have to write a BASIC command, as in the following example:
VariableName = Value
Rather than telling the computer, "Hey, stupid. Assign the number 36 to a variable named Age," you can just write:
Age = 36
Variables can hold only one value at a time. If a variable already holds a value and you assign another one to the variable, the variable cheerfully tosses out the old value and accepts the new one. You can give two commands, as in the following example:
Age = 36
Age = 49
Visual Basic first says, "Okay, let my variable named Age hold the number 36." Then Visual Basic looks at the second line and says, "Okay, let my variable named Age hold the number 49, and forget that the number 36 ever existed."
Because the properties of an object are variables, you can assign values to an object's property in the same way. For example, suppose that you wanted to change the Height property of a text box named txtPassword to 1200. Here's how you do this:
txtPassword.Height = 1200
This tells Visual Basic, "Find the object named txtPassword and change the Height property to 1200"
If you want to be more specific, you can even do this:
frmSecret!txtPassword.Height = 1200
This tells Visual Basic, "On the form named frmSecret, find the object named txt Password and change the Height property to 1200"
If you don't include the form's name, Visual Basic assumes that the object you want is located on the form containing your BASIC code. To retrieve the value from an object's property (such as finding out the height of a text box), assign a variable to that object's property such as:
DimButtonHeight As Integer
ButtonHeight = txtEatThis.Height
1. Create a variable called ButtonHeight as an integer variable.
2. Assign the variable called ButtonHeight to the value stored in the Height property of a text box named txtEatThis.
** Assigning strings to variables
Assigning strings to variables is similar to assigning numbers to variables. The only difference is that you have to surround a string with quotation marks so that Visual Basic knows where the string begins and ends.
For example, you can assign a variable with a single-word string:
Name = "John"
Or you can assign a variable with a string consisting of two or more words:
Name = "John Doe".
Or
Name = "John Smith Doe the Third and proud of it"
Not all strings consist of letters. Sometimes you may want to assign a variable with a phone number or social security number, as follows:
PhoneNumber = "555-1234"
SocialSecurity = "123-45-6789"
What happens if you don't include the quotation marks and just type the following?
PhoneNumber = 155-1234
SocialSecurity = 123-45-6789
Without the quotation marks, Visual Basic thinks the hyphen is a subtraction symbol and that you want the program to calculate a new result. Instead of storing 555-1234 in the PhoneNumber variable, Visual Basic stores the number -679. Instead of storing 123-45-6789 in the Social Security variable, Visual Basic stores -6711.
The golden rule of assigning variables is this: When you assign a variable with letters or numbers that you want treated as a string, put quotation marks around the letters or numbers.
** Modifying properties
Assigning a variable with numbers or strings isn't some dry, academic exercise that has little relation to anything in real life. For example, if you want to display a message on the screen, you need to modify the properties of a label or text box. If you want to create animation, you need to constantly change the Left and Top properties that define an object's position on the screen. Because the properties of an object are variables, you can modify an object by assigning new values to the object's properties.
Suppose that the two command buttons and the text boxes, shown in Figures 15-2 and 15-3, have the following properties:
Object - Text Box
Property - Name
Setting - txtMessage
Property - Text
Setting - (Empty)
Object - Top Command button
Property - Name
Setting - cmdHello
Property - Caption
Setting - &Hello
Object - Bottom command button
Property - Name
Setting - cmdBye
Property - Caption
Setting - &Good-bye
Now, suppose that the first command button has the following event procedure:
Private Sub cmdHello_Click( )
txtMessage.Text = "Hello, world!"
End Sub
And the second command button has the following event procedure:
Private Sub cmdBye_Click( )
txtMessage.Text - "Good-bye, cruel world!"
End Sub
When you click on the Hello command button, this is what happens:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdHello.
2. Visual Basic quickly finds the event procedure named cmdHello-Click( ) and looks for further instructions.
3. The cmdHello-Click( ) event procedure tells Visual Basic, "Find a text box named txt Message and replace the Text property with the string "Hello, world!' "
4. The string "Hello, world!" pops up inside the text box named txtMessage.
When you click on the Good-bye command button, the following process happens:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdBye.
2. Visual Basic quickly finds the event procedure named cmdBye-Click and looks for further instructions.
3. The cmdBye_Click( ) event procedure tells Visual Basic, "Find a text box named txtMessage and replace the Text property with the string "Good-bye, cruel world!" "
4. The string "Good - bye, cruel world! " pops up inside the text box named txtMessage .
You can use BASIC code to modify the properties of any object that appears on a form. By modifying the properties of other objects, you can display messages and information to the user.
The only property that BASIC code cannot change is the Name property of any object. The only way to change the Name property of an object is through the Properties window.
** Assigning variables to other variables
Besides assigning numbers or strings to a variable, you can also assign the value of one variable to another variable. To do this, you have to write a BASIC command like the following:
FirstVariableName = SecondVariableName
For example, consider adding a second text box with the following properties.
Object - Second text box
Property - Name
Setting - txtCopyCat
Property - Text
Setting - (Empty)
Consider the following modifications to the cmdHello_Click event procedure:
Private Sub cmdHello_Click( )
txtMessage.Text = "Hello, world!"
txtCopyCat.Text = txtMessage.Text
End Sub
And to the cmdBye-Click event procedure:
Private Sub cmdBye_Click( )
txtMessage.Text = "Good-bye, cruel world!"
txtCopyCat.Text = txtMessage.Text
End Sub
Now when you click on the Hello command button, this is what happens:
1 . Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdHello.
2. Visual Basic quickly finds the event procedure named cmdHello_Click( ) and looks for further instructions.
3. The cmdHello_Click( ) event procedure tells Visual Basic, "Find a text box named txtMessage and replace the Text property with the string "Hello, world!" "
4. Visual Basic sees the second instruction that says, 'find a text box named txtCopyCat and replace its Text property with whatever is stored in the txtMessage. Text property."
5. The string "Hello, world! " pops up inside the text box named txtMessage and the text box named txtCopyCat.
And when you click on the Good-bye command button, the following happens:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdBye.
2. Visual Basic quickly finds the event procedure named cmdBye_Click( ) and looks for further instructions.
3. The cmdBye_Click( ) event procedure tells Visual Basic, "Find a text box named txtMessage and replace its Text property with the string "Good-bye. cruel world!" "
4. Visual Basic sees the second instruction that says, "Find a text box named txtCopyCat and replace its Text property with whatever is stored in the txtMessage. Text property."
5. The string "Good-bye, cruel world!" pops up inside the text box named txtMessage and the text box named txtCopyCat.
** Assigning Values to Objects Stored in Other Forms
To assign a value into an object's property, use this simple command:
ObjectName.PropertyName = Value
ObjectName is the name of the object. PropertyName is the property you want to change. Value is the number or string you want to assign to the property that is to affect the object named ObjectName.
If you want to change the properties on a form that hasn't been loaded yet, you have to specify the form name such as:
FormName!Ob,jectN,ame.PropertyName = Value
For example, to change the Text property of the txtMessage text box, you have to type the following:
txtMessage.Text = "Hello, world!"
So how can you change the property of an object stored on another form? The solution is easy. You just have to specify the name of the form on which the object is stored.
Figure 15-5, for example, shows two forms. Form 1 contains two text boxes, named txtMessage and txtCopyCat, and two command buttons, named cmdHello and cmdBye. Form 2 contains one text box with the following properties:
Object - Form
Property - Name
Setting - A Second Form
Object - Text Box
Property - Name
Setting - txtNewBox
Property - Text
Setting - (Empty)
So how can the event procedure stored in Form 1 modify the Text property of an object stored on another form? You can simply use the following command:
FormName!ObjectName.PropertyName = Value
FormName specifies the name of the form that contains the object you want to modify. ObjectName is the name of the object. PropertyName is the property you want to change. Value is the number or string you want to assign to the property.
If you want the text box named txtNewBox to display the same message that the text boxes txtMessage and txtCopyCat display, you add the following command to the cmdHello_Click and the cmd Bye_Click event procedures:
Form2!txtNewBox.Text = txtMessage.Text
The cmdHello-Click event procedure now looks like this:
Private Sub cmdHello_Click( )
txtMessage.Text = "Hello, world!"
txtCopyCat.Text = txtMessage.Text
Form2.Show
Form2!txtNewBox.Text = txtMessage.Text
End Sub
And the cmdBye-Click event procedure now looks like this:
Private Sub cmdBye_Click( )
txtMessage.Text = "Good-bye, cruel world!"
txtCopyCat.Text = txtMessage.Text
Form2.Show
Form2!txtNewBox.Text = txtMessage.Text
End Sub
The following explains what happens when you click on the Hello command button:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdHello.
2. Visual Basic quickly finds the event procedure named cmdHello_Click and looks for further instructions.
3. The cmdHello_Click( ) event procedure tells Visual Basic, "Find a text box named txtMessage and replace the Text property with the string "Hello, world!""
4. Visual Basic sees the second instruction that says, "Find a text box named txtCopyCat and replace its Text property with whatever is stored in the txtMessage.Text property."
5. The Form2.Show command tells Visual Basic, "Find a form named Form2 and display the form on the screen."
6. The Form2!txtNewBox.Text = txtMessage.Text command tells Visual Basic, "On the form named Form2, look for a text box named txtNewBox and stuff its Text property with the value stored in the text box named txtMessage."
7. The string "Hello, world! " pops up inside all three text boxes, named txtMessage, txtCopyCat, and txtNewBox.
When you click the mouse on the Good-bye command button, the following happens:
1. Visual Basic detects the Click event and notices that the mouse is pointing to a command button named cmdBye.
2. Visual Basic quickly finds the event procedure named cmdBye_Click and looks for further instructions.
3. The cmdBye_Click event procedure tells Visual Basic, "Find a text box named txtMessage and replace its Text property with the string "Good-bye, cruel world!""
4. Visual Basic sees the second instruction that says, "Find a text box named txtCopyCat and replace its Text property with whatever is stored in the txtMessage.Text property."
5. The Form2.Show command tells Visual Basic, "Find a form named Form2 and display this form on the screen."
6. The Form2! txt NewBox.Text = txtMessage.Text command tells Visual Basic, "On the form named Form2, look for a text box named txtNewBox and stuff the Text property with the value stored in the text boxnamed txtMessage."
7. The string "Good-bye, cruel world! " pops up inside all three text boxes, named txtMessage, txtCopyCat, and txtNewBox.
** Data Types
Variables can hold numbers and strings. However, you may want a variable called FirstName to contain nothing but strings. If a variable called FirstName winds up holding the number 56, that variable can cause an error if the computer expects a string but gets a number instead. To restrict the type of information a variable can hold, you can declare a variable to hold a specific data type. Data types tell Visual Basic, "See this variable? This variable can hold only strings or certain types of numbers, so there!"
Here are three primary reasons to use data types:
So that you can easily see the type of data each variable can hold.
To prevent variables from accidentally storing the wrong type of data and causing an error.
To use memory more efficiently because some data types (such as double) require more memory to use than other data types (such as byte).
If you write a BASIC command that attempts to assign a string into a data type that accepts only numbers, Visual Basic squawks and displays an error message. This squawking helps you catch possible errors in your program long before you finish - and distribute - your program to others.
Visual Basic provides the ten data types shown in Table 15-1.
** TABLE 15-1
The Visual Basic Data Types
Data Type - Byte
Accepts Numbers That Range From - 0 to 255
Data Type - Boolean
Accepts Numbers That Range From - True or False
Data Type - Currency
Accepts Numbers That Range From - -922337203685477.5808 to 922337203685477.5807
Data Type - Date
Accepts Numbers That Range From - Dates between January 1,100 and December 31, 9999
Data Type - Double
Accepts Numbers That Range From - -1.79769313486232E308 to -4.94065645841247E-324 and 4.94065645841247E-324 to 1.79769313486232E308
Data Type - Integar
Accepts Numbers That Range From - -32,768 to 32,767
Data Type - Long
Accepts Numbers That Range From - -2,147,483,648 to 2,147,483,647
Data Type - Numbers
Accepts Numbers That Range From - Same range as Double
Data Type - Single
Accepts Numbers That Range From - -3.402823E38 to -1.401298E-45 and 1.401298E-45 to 3.402823E38
Data Type - String
Accepts Numbers That Range From - 0 to 65,5000 characters
Data Type - Variant
Accepts Numbers That Range From - (When storing numbers, same range as Double. When storing strings, same range as String).
Visual Basic actually gives you two ways to declare a variable:
Dim MyString As String
or
DimMyString$
The first method can be verbose but clear. The second method uses something called type declaration characters. This makes declaring variables easier but makes your commands harder to read and understand at first glance.
If you want to be clear and don't mind typing a lot of extra words like "As String" or "As Integer," use the first method. If you want to save time and don't care to make your code readable, use the second method. Here's a short table listing all the type declaration characters you can use to declare variables as different data types:
Data Type - Currency
Character - @
Example - Dim Loot@
Equivalent To - Dim Loot As Currency
Data Type - Double
Character - #
Example - Dim Average#
Equivalent To - Dim Average As Double
Data Type - Integar
Character - %
Example - Dim Age%
Equivalent To - Dim Age As Integer
Data Type - Long
Character - &
Example - Dim Huge&
Equivalent To - Dim Huge As Long
Data Type - Single
Character - !
Example - Dim Tiny!
Equivalent To - Dim Tiny As Single
Data Type - String
Character - $
Example - Dim Name$
Equivalent To - Dim Name As String
** Declaring objects as data types:
To store whole numbers, use the Integer data type. If you need to store really small or really large numbers, use the Long data type. If you just need to store numbers no smaller than 0 or larger than 255, use the Byte data type.
To store numbers with decimal points, use the Single data type. If you need to store really small or really large numbers with decimal points, use the Double or Numbers data type.
To store numbers representing currency (that's money, in non-technical terms), use the Currency data type.
To store words and letters, use the String data type.
To store dates, use the Date data type.
To store True or False values, use the Boolean data type.
You can also use the Variant data type to store numbers or strings.
By default, Visual Basic assigns all variables as a Variant data type unless you specifically tell Visual Basic otherwise. The only reason to specifically declare a variable as a Variant data type is for clarity in reading your code. As far as Visual Basic is concerned, declaring a variable as a Variant data type is redundant, such as telling people that you drive a Ford Mustang automobile.
To declare a variable as a particular data type, use the following command:
Dim VariableName As DataType
For example, to declare a variable named MyName as a string data type, you want to use the following command:
Dim MyName As String
When Visual Basic sees this statement, Visual Basic thinks, "Okay, this is a variable named MyName, and the programmer defined the variable as a String data type so that this variable can hold only strings."
When Visual Basic sees the statement:
Dim MyName As Variant
Visual Basic thinks, "Okay, this is a variable named MyName, and the programmer defined this variable as a Variant data type so that it can hold numbers or strings."
And when Visual Basic sees the statement:
Dim MyName
Visual Basic thinks, "Okay, this is a variable named MyName; because the programmer is too lame to define the data type, I can automatically assume that this variable is a Variant data type."
If you want to declare multiple variables on a single line, you must explicitly declare each variable, such as:
Dim MyName As String, PetName As String
Suppose you declare your variables like this:
Dim MyName, PetName As String
Visual Basic assumes that the MyName variable is a Variant data type and PetName is a String data type.
An everyday event procedure:
To study an actual, honest-to-goodness event procedure that declares variables as specific data types, consider the following:
Private Sub Form1_Load( )
Dim PetName As String, Age As Integer
PetName = "Bo the cat"
Age = 6
End Sub
The variable declaration statement Dim PetName As String, Age As Integer tells Visual Basic, "Okay, create a variable named PetName and make sure that you use this variable to hold any strings."
Visual Basic continues, "Then create a variable named Age and make sure that the variable holds only numbers greater than or equal to -32,768 but less than or equal to 32,767."
The next command tells Visual Basic, "Assign the string "Bo the cat" to the variable PetName." Faster than a speeding bullet, Visual Basic checks to make sure that the PetName variable really can hold string values. Thankfully, the statement Dim PetName As String defined PetName to hold only string values, so everything is a-okay.
Finally, Visual Basic says, "Assign the number 6 to the variable Age." Quickly, Visual Basic checks that the variable Age really can hold a number as massive as 6. Because 6 falls within the declared range of an integer (between -32,768 and 32,767), Visual Basic cheerfully allows this statement to pass as valid.
Always use the smallest data type possible. For example, if you know the variable Age is never going to hold a number larger than 32,767, declare Age as a Byte data type. If you need larger or smaller numbers, choose the Long data type. By choosing the appropriate data type, you can optimize the use of your computer's memory.
* String data types
In case you want to restrict the length of the strings a variable can hold, you can define the maximum length by using the following command:
Dim VariableName As String * Size
The value of Size can vary from 1 to 65,500. So if you want to keep a variable from storing more than ten characters, use the following command to specify Size as 10:
Dim VariableName As String * 10
For example, suppose that you declare the following:
Dim FirstName As String * 5
The following are valid strings that you can assign to the FirstName variable:
" 12345 "
"Bo"
"Jacob"
"Will"
If you try to assign to the FirstName variable a string that is longer than five characters, however, this is how Visual Basic actually stores a string:
String Assignment What Really Happens
FirstName = Marilyn FirstName = Maril
FirstName = Bobcat FirstName = Bobca
FirstName = King Edwards FirstName = King
If a string is too long for the declared string length of a variable, Visual Basic ruthlessly chops off the string. If you don't define a maximum string length, or if you define the variable as a Variant data type, the variable can hold up to 65,500 characters.
* Scope of variables
The scope of a variable determines the accessibility of a variable within a Visual Basic program. Visual Basic lets you declare the scope of variables in three ways:
Local
Module
Public
A local variable exists only within the procedure in which the variable is created, and a local variable can be used only within the procedure in which the variable is declared. The purpose of a local variable is to isolate a specific variable in the single procedure in which the variable is being used. That way, if a variable is screwing up and storing the wrong value, you can easily isolate the problem and fix it.
To declare a local variable, declare the variable within an event procedure, as follows:
Private Sub Command1_Click( )
Dim FulIName As String
End Sub
Remember, a local variable can be used only in the one event procedure in which the variable is declared. But what if you want to create a variable that two or more event procedures can share? In that case, you have to create a module variable.
Module variables are variables that can be used only by other procedures stored in the same file.
To declare a module variable, follow these steps:
1. Open the Code window by pressing F7, choosing View->Code, or double-clicking anywhere on the form (but not on any objects on the form).
2. Click on the Object list box in the Code window and choose (General).
Visual Basic displays (Declarations) in the Procedure list box.
3. Type your variable declaration using the Dim command.
Module variables are useful for sharing a variable amoung procedures stored in the same file, such as the same Form (FRM) file. If you want to use a variable that can be used by any event procedure, no matter what file the variable may be stored in, you need to create a public variable.
Public variables are variables that can be the most convenient to use, because every event procedure in your Visual Basic program can access them. But be careful! Most programmers avoid using public variables because if your program stores the wrong value in a public variable, you have to search your entire program to find the part that is messing up.
By comparison, if your program messes up a module variable, you can isolate the problem in the file in which you declared the module variable. Likewise, if your program messes up a local variable, the only possible place the problem can occur is in the procedure in which you declared the local variable.
To declare a public variable, follow these steps:
1. Choose Project->Add Module to create a BAS module file. An Add Module dialog box appears.
2. Click on Module and click on Open. Visual Basic displays the Code window with (General) displayed in the Object list box and (Declarations) in the Procedure list box.
3. Declare your public variable by using the Public command. For example:
Public FullName As String
To make sure that programs are easy to understand and modify later, create local, module, or public variables. If you want to throw caution aside, you can create variables as you need them in the middle of your programs, but doing so will make your programs harder to read and understand later.
If you plan to write large programs, always declare your variables. Like eating your vegetables, this is something that may seem distasteful at first but, later on, can actually be useful.
To force yourself to declare your variables, just add the following command in the General area (like a module variable) in every file (Form or Module file) of your program:
Option Explicit
When you use the Option Explicit command, Visual Basic screams every time you try to use a variable without declaring it first.
++ CHAPTER 16
RESPONDING TO THE USER
In This Chapter .....
Getting data from text boxes
Determining which radio button and check box the user chose
Retrieving data from list boxes and scroll bars
A user interface makes your program look nice and pretty. Unfortunately, a nice and pretty user interface can be as useless as an attractive person without any brains (think of some of your co-workers). If you want your program to have more substance than an empty-headed fashion model, you have to make sure that your program can respond intelligently to the user.
To make your user interface responsive, your program must:
Get information from the user interface
Calculate a result
Display that result back on the user interface
For example, when the user chooses an item from a list box, the program has no idea which item the user chose. If you look at the screen, you may be tempted to say, "Hey, stupid computer. If I can see which item the user has chosen, why can't you?"
But what you see on the user interface isn't what the computer sees. From the computer's point of view, the computer still has no idea which item the user selected from the list box.
To tell the computer what action a user took, you have to write BASIC code. This BASIC code grabs information off the user interface so that your program can then do something with the information.
** Getting Data from the User Interface
A user interface is a simple way for users to give information to your program. For example, a user interface can offer a list box that displays several filenames. That way the user can just click on the filename they want to use.
When a user gives information to a program (by pressing a particular key or clicking on a command button, files list box, and so on), Visual Basic stores this information in the object's properties. From this point on, the program can use the information.
The nine basic types of objects that can get data from the user interface are as follows:
Check boxes
Radio buttons
Combo boxes
List boxes
Text boxes
Horizontal and vertical scroll bars
Drive list boxes
Directory list boxes
File list boxes
** Finding Information In an Object
To find the information stored in an object, you need to know:
The name of the object (such as txtWriteHere)
The object's property that contains the information (such as the Text property)
The name of the form on which the object is located (such as frmMain)
When the user puts information in an object (such as typing text in a text box), Visual Basic stores the data in one of the object's properties. Different objects store information from the user in different properties. For example, a text box stores information in the Text property, but a check box stores information in the Value property.
Knowing which property you want isn't enough. You also need the name of the object that holds the property you want. And because objects on different forms can have the same names, you may also need to know the name of the form where the object appears.
The combination of form name, object name, and property defines the specific location of the information from the user. Think of objects as mail boxes. To retrieve a letter, you have to know the state (form name), the city (object name), and the address (property).
So if you want to retrieve information from an object, you have to use the following combination:
FormName!ObjectName.Property
For example, you may want to retrieve information from a form named frmAttack, containing a text box named txtWarning, which stores data in the Text property, such as
FrmAttack!txtWarning.Text
.
Whenever Visual Basic sees this code, Visual Basic automatically says to itself, "Okay, let me find the form named frmAttack, look for the object named txtWarning, and find the value stored in the Text setting. Ah, here is the value I want."
If you omit the form's name, Visual Basic looks only for objects that appear on the current form. If that's where you want to look, you can shorten this combination to the following:
ObjectName.Property
* Getting Data from Text Boxes
When a user types something in a text box, Visual Basic stores the information in the Text property. Typing in a text box is equivalent to assigning a value in the text box's Text property.
If the user types "Greetings from Mars!" in a text box named txtSecret, this is the equivalent to the following BASIC code:
TxtSecret.Text - "Greetings from Mars!"
* Determining which radio button the user chose
A radio button can have one of two possible values stored in its Value property:
True (selected)
False (unselected)
An unselected radio button has the Value property set to False. Clicking on an unselected radio button changes the Value property to True.
A selected radio button has the Value property set to True. Clicking on a selected radio button changes the Value property to False.
To determine which radio button the user chose, you have to check the Value properties of all your radio buttons. For example, if the user chooses an unselected radio button named optStation, choosing the button is equivalent to the following BASIC code:
optStation.Value = True
If the user selects a different radio button (other than the radio button named optStation), this is equivalent to the following BASIC code:
optStation.Value = False
* Determining which check box the user chose
A check box can have one of three possible values stored in the Value property:
0 - unchecked
1 - checked
2 - grayed
An unselected check box has the Value property set to 0. Clicking on an unselected check box changes the Value property to 1.
A selected check box has the Value property set to 1. Clicking on a selected check box changes the Value property to 0.
A grayed check box has the Value property set to 2. Clicking on a grayed check box changes the Value property to 0.
Note that a user can still click on a grayed check box, unlike a disabled check box which also looks gray.
A selected check box obviously means that the user chose the command represented by the check box. Likewise, an unselected check box means the user did not choose the command represented by the check box. A grayed check box can draw the user's attention that the grayed check box's command represents a special exception or feature.
You have only two ways to set a check box's Value property to 2 and have the check box appear grayed. The first approach is to change the Value property in the Property window when you design your program. The second way is to use BASIC code. The following BASIC code sets the Value property to 2 for a check box named chkBold:
chkBold.Value = 2
To determine which check box the user chooses, you have to review the Value properties of all your check boxes. For example, if a check box named chkBold is blank, this is equivalent to the following BASIC code:
chkBold.Value = 0
If the check box is selected, this is equivalent to:
chkBold.Value = 1
If the check box is grayed, this is equivalent to:
chkBold.Value = 2
* Retrieving data from Drive, Directory, and File list boxes.
When a user makes a selection from the Drive list box, Visual Basic stores the selection as a string in the Drive list box's Drive property. If a user clicks on c: in a Drive list box named drvWhichDrive, for example, this is equivalent to the following BASIC code:
drvWhichDrive.Drive = "c:"
When a user makes a selection from the Directory list box, Visual Basic stores the selection as a string in the Directory list box's Path property.
If a user clicks on c:\dos in a Directory list box named dirFolder, for example, this is equivalent to the following BASIC code:
dirFolder.Path = "c:\dos"
When a user makes a selection from a File list box, Visual Basic stores the file name as a string in the File list box's FileName property.
If a user clicks on autoexec.bat in a File list box named filGetFiles, for example, this is equivalent to the following BASIC code:
filGetFiles.FileName = "c:\dos\autoexec.bat"
* Obtaining choices from a combo box
When a user chooses or types an item in a combo box, Visual Basic stores this information in the combo box's Text property.
The text that a combo box displays is stored in the combo box's List property. When the user actually types an item or clicks on an item in the combo box, the user's choice is stored in the combo box's Text property.
If the user types or selects an item named " ASAP " in a combo box named cboPriorities, for example, this is equivalent to the following BASIC code:
cboPriorities.Text = " ASAP "
* Getting values from horizontal and vertical scroll bars
Scroll bars that are not part of a text box, list box, or combo box represent a number. This number is stored in the scroll bar's Value property.
The value that a scroll bar can represent is determined by the scroll bar's Min and Max properties. The lowest possible value is -32,768. The highest possible value is 32,767.
Scroll bars let users visually represent a value, rather than typing a number from the keyboard. Ideally, you want to use a scroll bar along with a label that shows the actual value of the scroll bar. That way, as users move the scroll box in the scroll bar, they can also see the value of the scroll bar changing.
Although scroll bars can represent a range of values, your program eventually needs to read a single value from the scroll bar. If you want to retrieve the value of a horizontal scroll bar named hsbSensitivity, for example, you have to set a variable to equal the Value property of the scroll bar. So if you have a variable named MouseSensitivity and a scroll bar named hsbSensitivity, you could use the following command:
MouseSensitivity = hsbSensitivity.Value
* Test your newfound knowledge
1. What does the following BASIC command do?
WhatIsIt = chkBold.Value
a. The command tried to identify UFOs named chkBold.Value
b. The command questions the need for anything named chkBold.Value
c. The command makes the computer ask, "What do you want me to do? Tell me and then leave me alone. I'm feeling bold today."
d. The command yanks a number that's stored in the Value property of a check box named chkBold and stuffs the number in a variable named WhatIsIt. If the check box named chkBold is selected, the value of WhatIsIt is 1.
2. Examine the following command and explain what this code does.
FrmDataSheet!txtMessage.Text = "Warning!"
a. The command tells Visual Basic, "Okay, look for a form named frmDataSheet, and on this form find a text box named txtMessage. When you find that, stuff the Test property with the string "Warning!""
b. The command tells everyone that the person who wrote this command probably had a good reason for not being present to help you interpret this command.
c. The command warns you that your computer is about to explode and you had better take cover immediately.
* Retrieving data from a list box
A user can select one or more items from a list box depending on the list box's MultiSelect property. If the MultiSelect property is set to 0 (the default value) and a user selects an item, the list box stores the item in the Text property.
If the user selects an item named " Cat food " in a list box named 1st Groceries, this is equivalent to the following BASIC code:
1stGroceries.Text = "Cat food"
If a list box's MultiSelect property is 1 or 2, users can hold down the Shift key and click on an item to select two or more items in the list box. Because a list box's Text property can hold only one string at a time, you can't store all the selections made by a user in the list box's Text property.
To retrieve data from a list box that allows multiple selections, you have to do the following:
1. Create a second list box to temporarily store any selected items from the first list box.
2. Set this second list box's Visible property to False so that the list box isn't visible on the user interface.
3. Every time a user selects an Item from the first list box, copy the item into the second (invisible) list box.
The invisible list box contains only those items selected from another list box, and stores all items in a List property. Items in a list are assigned an index number. The first item in the list is given an index number of 0, the second item in a list is given an index number of 1, and so on. To better understand this concept, create on a blank form two list boxes and a command button with the following properties:
If you don't feel like typing in the following program, just load the LISTBOXES.VBP program off the CD-ROM.
Object - First List Box
Property - Name
Value - 1stChoose
Property - MultiSelect
Value - 1 - Simple
Object - Second List box
Property - Name
Value - 1stTemp
Property - Visible
Value - True
Object - Command button
Property - Name
Value - cmdStore
Property - Caption
Value - Store
If the Visible property of the second list box is False, the box is (obviously) invisible and you can't see what is happening. So, when you test the procedure, set the Visible property for the second list box to True so that you can see what happens. Then, after you've become educated, change the Visible property back to the original False setting.
The following procedure stores items in the 1stChoose list box:
Private Sub Form_Load( )
1stChoose.AddItem "Butter"
1stChoose.AddItem "Lard"
1stChoose.AddItem "Cat food"
1stChoose.AddItem "Milk"
1stChoose.AddItem "Ammunition"
1stChoose.AddItem "Ice Cream"
1stChoose.AddItem "Cookies"
1stChoose.AddItem "Sugar"
1stChoose.AddItem "Camouflage"
1stChoose.AddItem "Survival rations"
End Sub
This loads the 1stChoose list box with items for the user to select.
An event procedure to retireve multiple selected items from the 1stChoose list box may look like the following:
Private Sub cmdStore_Click( )
Dim I
1stTemp.Clear
For I = 0 to 1stChoose.ListCount - 1
If 1stChoose.Selected(I) Then
1stTemp.AddItem 1stChoose.List(I)
End If
Next I
End Sub
This is how Visual Basic responds to the code:
1. The first line tells Visual Basic, "Follow these instructions whenever the user clicks on the command button named cmdStore."
2. The second line says, "Declare a variable called I and assume the data type is Variant."
3. The third line says, "Find a list box named 1stTemp and clear out anything the box may be storing."
4. The fourth line says, "Set the value of I to 0. Continue to count by one until the value of I equals the number of items displayed in the 1stChoose list box."
5. The fifth line says, "If the user selected an item displayed in the 1stChoose list box, follow the instructions in the sixth line."
6. The sixth line says, "Copy the item selected in the list box named 1stChoose and put the copied item in the list box named 1stTemp."
7. The seventh line says, "This is the end of all the instructions to follow for now."
8. The eighth line says, "Add one to the value of I and start back at line five."
9. The ninth line says, "This is the end of the instructions to follow when the user clicks on the command button named cmdStore."
These instructions do nothing more than copy all selected items from one list box into a second list box, which is invisible.
Each item in a list box (and a combo box, for that matter) is identified by an index number. The top item of the list is assigned an index number of 0, the second item from the top is assigned an index number of 1, and so on.
To get at the items stored in the list box named 1stTemp, you have to use index numbers. The following command retrieves the top item stored in the 1stTemp list box:
1stTemp.List(0)
The following command retrieves the second item from the top in the 1stTemp list box:
1stTemp.List(1)
Each increase in the index number retrieves the next item further from the top.
++ CHAPTER 17
MATH 101: ARITHMETIC, LOGICAL AND COMPARISON OPERATORS
In This Chapter:
Adding, subtracting, multiplying, and dividing numbers
Using the not, And, Or, and Xor operators
Comparing numbers and strings
Comparing strings and operators
After a program gets data from the user (either as a number or as a string), the next step is to do something with that data. If your program mimics an overworked clerical worker, the program may just lose the data and blame the loss on something else. But most likely, you want your program to calculate some sort of result with the data the program receives from the user.
To calculate a result, your program needs to get data from the user interface and then somehow change, modify, mutilate, or spindle that data. Changing anything involves an operation, so the special commands to work with data are called operators.
Visual Basic provides the following three types of operators:
Arithmetic
Logical
Comparison
** Arithmetic Operators
Arithmetic operators essentially turn your $2,000 computer into a $4.95 pocket calculator. These operators let you add, subtract, multiply, and divide numbers or variables that represent numbers. Table 17-1 shows the most common arithmetic operators.
||
TABLE 17-1
Arithmetic Operators
Operator - +
What the Operator Does - Adds two numbers
Operator - -
What the Operator Does - Subtracts two numbers
Operator - *
What the Operator Does - Multiplies two numbers
Operator - /
What the Operator Does - Divides two numbers and returns a floating-point (decimal) number, such as 3.14, 16.2, or 392.2398
Operator - \
What the Operator Does - Divides two numbers and returns an integer, such as 8, 16, 302, or 25
Operator - Mod (or modulo)
What the Operator Does - Divides two numbers and returns only the remainder
Operator - ^
What the Operator Does - Raises a number to an exponential power
Operator - &
What the Operator Does - Adds (concatenates) two strings
* Adding two numbers with the + operator
To add two numbers, use the addition operator (+), as shown in the following example:
X = 10
Y = 15.4
Sum = X + Y
In case these three BASIC commands mystify you, here's how they work:
1. The first command says, "Create a variable called X and set the value of X to 10."
2. The second command says, "Create a variable called Y and set the value of Y to 15.4."
3. The third command says, "Create a variable called Sum and set the value of Sum equal to the value of X plus the value of Y." In this case, the value of Sum equals 10 + 15.4, or 25.4.
* Subtracting two numbers with the - operator
To subtract two numbers, use the subtraction operator (-), as shown in the following example:
Income = 2000
Taxes = 2500
Real_Income = Income - Taxes
Here's how Visual Basic interprets these three BASIC commands:
1. The first command says, "Create a variable called Income and set the value of Income to 2000".
2. The second command says, "Create a variable called Taxes and set the value of Taxes to 2500."
3. The third command says, "Create a variable called Real_Income and set the value of Real_Income equal to the value of Income minus the value of Taxes." In this case, the value of Rea1_Income equals 2000 - 2500, or -500.
* Negating numbers with the - operator
The subtraction operator ( - ), used by itself, can turn a positive number into a negative number and vice versa. To negate a number, place the -operator in front of any number or variable, as shown in the following example:
Amount = 250
Balance = - Amount
This is how the tiny little brain of Visual Basic interprets these BASIC commands:
1. The first command says, "Create a variable called Amount and set the value of Amount to 250."
2. The second command says, "Create a variable called Balance and set the value of Balance to the negative value of Amount." In this case, the value of Balance is -250.
* Multiplying two numbers with the * operator
To multiply two numbers, use the multiplication operator (*), as shown in the following example:
Hours = 25
Wages = 5.75
Salary = Hours * Wages
To see how Visual Basic understands these three BASIC commands, this is how they work:
1. The first command says, "Create a variable called Hours and set the value of Hours to 25."
2. The second command says, "Create a variable called Wages and set the value of Wages to 5.75."
3. The third command says, "Create a variable called Salary and set the value of Salary equal to the value of Hours multiplied by the value of Wages." In this case, the value of Salary equals 25 * 5.75, or 143.75.
* Dividing two numbers with the /operator
To divide two numbers and calculate a floating-point (decimal) number, use the forward-slash division operator ( / ), as shown in the following example:
GamesWon = 104
TotalGames = 162
WinningPercentage = GamesWon / Total Games
Visual Basic interprets these three BASIC commands as follows:
1. The first command says, "Create a variable called GamesWon and set the value of GamesWon to 104."
2. The second command says, "Create a variable called TotalGames and set the value of Total Games to 162."
3. The third command says, "Create a variable called WinningPercentage and set the value of WinningPercentage equal to the value of GamesWon divided by the value of TotalGames." In this case, the value of WinningPercentage equals 104 / 162, or 0.6419753.
* Dividing two numbers with the \ operator
To divide two numbers and calculate an integer, use the backslash division operator ( \ ), as shown in the following example:
CrateCapacity = 72
Bottles_in_Crate = 1900
FullCrates = Bottles_in_Crate \ CrateCapacity
So how does Visual Basic interpret these three BASIC commands? Glad you asked. Here's how:
1. The first command says "Create a variable called CrateCapacity and set the value of CrateCapacity to 72."
2. The second command says, "Create a variable called Bottles_in_Crate and set the value of Bottles_in_Crate to 1900."
3. The third command says, "Create a variable called FullCrates and set the value of FullCrates equal to the value of Bottles_in_Crate divided by the value of CrateCapacity." In this case, the value of FullCrates equals 1900 \ 72, or 26.
Dividing two numbers often calculates a floating-point (decimal) number, so how does Visual Basic handle rounding? Consider the following example:
Operand1 = 2.5
Operand2 = 1.5
Result = Operand1 \ Operand2
Before Visual Basic performs a calculation using the \ operator, the operands are rounded to the nearest whole number. (If an operand is halfway between two whole numbers, such as 2.5 or 1.5, then the operand is rounded up.) In this example, 0perand1 is rounded up to 3 and 0perand2 is rounded up to 2; therefore, Result = 3 \ 2, or 1.5. Because the \ operator must return an integer, the value of Result is rounded down to 1.
* Dividing with the modulo (Mod) operator
To divide two numbers and calculate the remainder, use the modulo operator ( Mod ), as shown in the following example:
CrateCapacity = 72
Bottles_in_Crate = 1900
LooseBottles = Bottles-in-Crate Mod CrateCapacity
For those curiosity seekers, this is how Visual Basic interprets these commands:
1. The first command says, "Create a variable called CrateCapacity and set the value of CrateCapacity to 72."
2. The second command says, "Create a variable called Bottles_in_Crate and set the value of Bottles_in_Crate to 1900."
3. The third command says, "Create a variable called LooseBottles and set the value of LooseBottles equal to the remainder of the value of Bottles_in_Crate divided by the value of CrateCapacity." In this case, the value of LooseBottles equals 1900 Mod 72, or 28.
* Calculating an exponential with the ^ operator
An exponential is a fancy mathematical term that means to multiply the same number by itself a certain number of times. For example, multiplying the number 2 four times is represented by 2 and small 4, or 2 * 2 * 2 * 2.
Because you can't type 2 and small 4, and typing 2 * 2 * 2 * 2 is a bit cumbersome, Visual Basic provides the caret operator (^), as shown in the following example:
2 ^ 4
* Adding (concatenating) two strings with the & operator
Adding or concatenating two strings means smashing them together. For this operation, use the ampersand operator ( & ), as shown in the following example:
FirstName = "John "
LastName = "Doe"
FullName = FirstName & LastName
When concatenating strings, always make room for a space between the two strings. Otherwise Visual Basic just slams the two strings together as one word like "JohnDoe."
This is how Visual Basic follows these three BASIC commands:
1. The first command says, "Create a variable called FirstName and set the value of FirstName to " John " (note the space at the end)."
2. The second command says, "Create a variable called LastName and set the value of LastName to " Do "."
3. The third command says, "Create a variable called FullName and set the value of FullName equal to the value of FirstName and the value of LastName smashed together. " In this case, the value of FullName equals "John" & "Doe", or "John Doe".
Besides concatenating strings using the ampersand character (&), you can also use the plus sign (+). However, you should use the ampersand character instead because the plus sign is also used with numerical addition. Using the ampersand simply makes your code easier to read.
** Logical Operators
Logical operators manipulate True and False values. Visual Basic represents a value of True as -1 and a value of False as 0. Table 17-2 shows the most common logical operators.
||
Table 17-2
Logical Operators
Operator - And
How to Use - Variable1 and Variable2
Operator - Or
How to Use - Variable1 Or Variable2
Operator - Xor
How to Use - Variable1 Xor Variable2
Operator - Not
How to Use - Not Variable
* Using the Not operator
It's a sad commentary on the negative impact that television has on children when their vocabulary degenerates to the monosyllabic utterance, "Not!"
Of course, the computer world laid claim to Not long before MTV materialized. The Not operator simply changes a True value to False and a False value to True, as in the following example:
Variable Name - Another_Computer_Book
Value - True
Variable Name - Not Another_Computer_Book
Value - False
For clarity, cool programmers like to use parentheses. If you use parentheses in the preceeding example, it would look like this:
Not(Another_Computer_Book)
* Using the And operator
The And operator compares the True or False values of two variables and calculates a new True or False value. This allows your program to mae decisions, as the following example illustrates:
KicktheCat = CatPresent And CatMisbehaving
So when is the variable KicktheCat True or False? This depends on the True or False value of CatPresent and CatMisbehaving.
KicktheCat CatPresent CatMisbehaving
True True True
False False False
False True False
False False True
The And operator returns a True value only if both CatPresent and CatMisbehaving are True.
* Using the Or operator
Like the And operator, the Or operator compares the True or False values of two variables and calculates a new True or False value. This allows your program to make decisions, as the following example illustrates:
LoafInside = GameOnTV Or WeatherBad
So when is the variable LoafInside True or False? This depends on the True or False values of GameOnTV and WeatherBad.
LoafInside GameOnTV WeatherBad
True True True
True False True
False True True
False False False
The Or operator returns a False value only if both GameOnTV and WeatherBad are False.
* Using the Xor operator
As the the And or Or operators, the Xor operator compares the True of False values to two variables and calculates a new True or False value. This allows your program to make decisions, as the following example illustrates:
TellOffBoss = BossPresent Xor AtWork
So when is the variable TellOffBoss True or False? This depends on the True or False value of BossPresent and AtWork.
TellOffBoss BossPresent AtWork
True True False
True False True
False True True
False False False
The Xor operator returns a False value if both BossPresent and AtWork are True or if both are False.
** Comparison Operators
Comparison operators compare two numbers or strings to see whether the numbers or strings are equal to, not equal to, greater than, or less than one another. Table 17-3 shows the most common arithmetic operators.
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Table 17-3
Comparison Operators
Operator - <
Meaning - Less than
Operator - <=
Meaning - Less than or equal to
Operator - >
Meaning - Greater than
Operator - >=
Meaning - Greater than or equal to
Operator - =
Meaning - Equal to
Operator - < >
Meaning - Not equal to
* Comparing numbers and strings
As the following example illustrates, comparison operators compare the values of numbers and strings in order to return a value of True or False:
Age = 18
MinimumAge 21
Pass = (Age >= MinimumAge)
This is how Visual Basic interprets these three BASIC commands:
1. The first command says, "Create a variable called Age and set the value of Age to 18."
2. The second command says, "Create a variable called MinimumAge and set the value of MinimumAge to 2l."
3. The third command says, "Compare the value of Age and see whether the value is greater than or equal to the value of MinimumAge. If the value of Age is greater than or equal to the value of MinimumAge, create a variable called Pass and set the value of Pass to True. If the value of Age is not greater than or equal to the value of MinimumAge, create a variable called Pass and set the value of Pass to False."
Comparing numbers is fairly easy, but comparing strings is a bit trickier. When comparing strings, Visual Basic calculates the ANSI character code value of each letter.
* ANSI character codes
At the simplest level, computers understand only two numbers: zero and one. You can represent all numbers with zeros and ones, such numbers are called binary numbers.
Since computers only understand numbers, not letters, humans created a simple system where certain numbers represent certain letters, punctuation marks, and characters. So the number 97 represents the letter a, the number 65 represents the letter A, and the number 33 represents an exclamation mark (!).
To make sure that all computers use the same numbers to represent the same letters and punctuation marks, the American National Standards Institute (ANSI) defined an ANSI Character Set that specifies which number represents which letter or punctuation mark.
* Comparing strings with the = and <> operators
Two strings are equal only if they are absolutely identical. As you can see in the following example, the equal to operator ( = ) always calculates a False value unless the operator compares two identical strings, such as " a " = " a ":
Operation - " a" = " a "
Value of Operation - True
Operation - " a " = " A "
Value of Operation - False
Operation - " a " = " aa "
Value of Operation - False
In the next example, however, you see that the not equal to operator ( <>) always calculates a True value unless this operator compares two identical strings, such as " Abott " <> " Abott ":
Operation - " A " <> " a "
Value of Operation - True
Operation - "Abott" <> "Abott"
Value of Operation - False
Visual Basic always treats uppercase and lowercase letters as completely different entitles when comparing strings.
* Comparing strings with the >, >=, <, and <= operators
When comparing strings, Visual Basic calculates the ANSI character code for each letter in each string, beginning with the first letter. The string whose character has the higher ANSI character code is considered greater.
For example, the letter A has an ANSI character code of 65 and the letter a has an ANSI character code of 97. So consider the following line:
Flag = ("Air" < "aardvark")
Because the first letter in " Air " has a lower character code number than the first letter in "aardvark", Visual Basic considers the value of "Air" to be less than "aardvark", so the value of Flag is going to be True.
Now consider the following example:
Flag = ("air" < "aardvark")
Here the value of Flag is False. How does Visual Basic decide whether "air" is less than "aardvark"? First, Visual Basic calculates the ANSI character code for the first letter of each string. Because both begin with a, Visual Basic looks at the second letter. Because i has a higher ANSI character code than a, "air " is considered greater than "aardvark" and Flag is therefore False.
Consider one final example:
Flag = ("air" < "airplane")
In this example, the value of Flag is True. The first three letters of each string are identical, but the fourth letter is not. Because "air" doesn't have a fourth letter and "airplane" does, "airplane" is considered greater and Flag is therefore True.
* Test your newfound knowledge
1. What is the difference between the / operator and the \ operator?
a. One is called a forward slash and one is called a backslash. Other than that, they both look like typos.
b. The / operator divides two numbers and the \ operator puts them back together again.
c. The / operator calculates a floating-point (decimal) number, such as 3.54, and the \ operator calculates an integer, such as 5 or 34.
d. The / operator doesn't work at all, so you have to use the \ operator instead.
2. Is the following statement True or False?
"aeroplane" < "airplane"
a. False, because I don't know what to think; besides, the answer hasn't been the letter (a) for a long time.
b. True, because the second letter in aeroplane is less than the second letter in airplane.
c.. True and False, because I'm hedging my bets.
d. False, because an aeroplane is an oldfashioned way of saying airplane, so both strings are exactly the same.
**Precedence
With all these operators crowding your BASIC commands, what happens if you lump them all together on one line, like this:
Mess = 4 / 7 + 9 * 2
If you guessed that the value of Mess is 18.57143, congratulations! But how does Visual Basic handle this? First, Visual Basic calculates those operators that have higher priority, or precedence.
Not all operators are equal. Some have a higher precedence than others, which means that they demand attention first, just like crying babies. Table 17-4 lists the order in which Visual Basic pays attention to the various operators. The higher an operator appears in Table 17-4, the higher that operator's precedence, so the exponential operator (^) has higher precedence than the less than operator ( < ).
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Table 17-4
Precedence of Operators
Operator - Exponential ( ^ )
Type of Operator - Arithmetic
Operator - Negation ( - )
Type of Operator - Arithmetic
Operator - Multiplication and Division ( * and / )
Type of Operator - Arithmetic
Operator - Integer division ( \ )
Type of Operator - Arithmetic
Operator - Modulo ( mod )
Type of Operator - Arithmetic
Operator - Addition and Subtraction ( + and - )
Type of Operator - Arithmetic
Operator - String concatenation ( & )
Type of Operator - Arithmetic
Operator - Equality ( = )
Type of Operator - Comparison
Operator - Inequality ( <> )
Type of Operator - Comparison
Operator - Less than ( < )
Type of Operator - Comparison
Operator - Greater than ( > )
Type of Operator - Comparison
Operator - Less than or equal to ( <= )
Type of Operator - Comparison
Operator - Greater than or equal to ( >= )
Type of Operator - Comparison
Operator - Like
Type of Operator - Comparison
Operator - Is
Type of Operator - Comparison
Operator - Not
Type of Operator - Logical
Operator - And
Type of Operator - Logical
Operator - Or
Type of Operator - Logical
Operator - Xor
Type of Operator - Logical
Operator - Eqv
Type of Operator - Logical
Operator - Imp
Type of Operator - Logical
How does Visual Basic calculate the value of Mess in the following equation?
Mess = 4 / 7 + 9 * 2
To help you understand how Visual Basic calculates a result, these are the steps Visual Basic follows:
1. Multiplication and division have a higher precedence than addition, so Visual Basic looks at the multiplication and division operators first.
2. Because multiplication and division have the same precedence, Visual Basic starts with the one furthest to the left. So Visual Basic calculates the value of 4 / 7 and comes up with 0.57143. Now the equation has been simplified to:
Mess = 0.57143 + 9 * 2
3. Visual Basic sees that the multiplication operator has a higher precedence than the addition operator, so Visual Basic calculates the value of 9 * 2 and comes up with 18. The equation is now:
Mess = 0.57143 + 18
The final value of Mess is 18.57143.
What if you really wanted Visual Basic to add the two numbers first before doing any division or multiplication? For clarity, and to make sure that calculations come out the way you intend, enclose particular operations in your equations in parentheses,
as shown in the following example:
Mess = 4 / (7 + 9) * 2
This is how Visual Basic calculates the result:
1. The parentheses tell Visual Basic to add 7 + 9 first, which creates the following equation:
Mess = 4 / 16 * 2
2. Because the division and multiplication operators have the same precedence, Visual Basic begins with the leftmost operator. Visual Basic calculates 4 / 16, and comes up with 0.25. The equation is now:
Mess = 0.25 * 2
3. Finally, Visual Basic multiplies these numbers and assigns the value of 0.5 to the variable Mess.
Whenever you use two or more operators, use parentheses to provide clarity and to ensure that Visual Basic calculates everything in the exact order you want.
To help you better understand precedence, the enclosed CD-ROM provides a program that shows you how precedence works when using parentheses and when omitting parentheses while calculating a numeric result.
++ CHAPTER 18
STRINGS AND THINGS
In This Chapter:
Converting the case of strings
Using parts of strings to search for and replace other strings
Converting strings into numbers and numbers into strings
Converting strings into ASCII values
In addition to manipulating numbers, your program can manipulate strings as well. Strings are any combination of letters, numbers, or symbols that you want the program to treat literally.
For example, computers blindly interpret phone numbers and social security numbers as mathematical expressions. A typical computer interprets the phone number 123-4567 as the expression, "Subtract 4567 from the number 123."
To tell your program to treat strings literally, always surround your strings with quotation marks, "like this". So if you want to assign the string 123-4567 to a variable, you use quotation marks, as in the following example:
Private Sub Count( )
Phone = "123-4567"
End Sub
If you forget to add the quotation marks, Visual Basic stupidly tries to interpret the string of numbers as an actual command. In this case, Visual Basic would try to subtract 4567 from 123.
After you designate particular data as a string, Visual Basic provides all sorts of weird ways to examine, manipulate, and mutilate the string.
** Manipulating Strings
You are not limited to using strings exactly as they originally appear. You can modify them in many ways. You can convert the case of a string, use parts of a string to look for and replace other strings, and shorten a string by removing extra spaces.
* Counting the length of a string
A string is only as long as the number of characters (including spaces) that the string contains. To count the length of a string, use the following BASIC command:
VariableName = Len("String")
For example:
Private Sub Command1_Click( )
Dim Name As String
Dim NameLength As Integer
Name = "Bo the cat"
NameLength = Len(Name)
End Sub
In this case, the length of the string "Bo the cat" is 10 (eight letters and two spaces), so this value is assigned to the variable called Name Length.
* Converting from uppercase to lowercase
To convert a string to all lowercase letters, use the following BASIC command:
Lcase ("String")
For example:
Private Sub Form_Click( )
Dim Name As String, LowerCase As String
Name = "DOESN'T THIS LOOK OBNOXIOUS?"
LowerCase = LCase(Name)
End Sub
In this case, the value of Lower Case is the following string:
doesn't this look obnoxious?
Notice that the LCase command affects only letters. (How do you present a lowercase question mark, anyway?)
To convert a string to all uppercase letters, use the following BASIC command:
Ucase("String")
For example:
Private Sub Form_Click( )
Dim Name As String, UpperCase As String
Name = "whisper when you speak"
UpperCase = UCase(Name)
End Sub
In this case, the value of UpperCase is the following string:
WHISPER WHEN YOU SPEAK
* Reversing strings
The newest string manipulation command in Visual Basic reverses strings, which can be useful for playing with palindromes (phrases that appear the same written forward as they do backward). To reverse a string, use the following BASIC command:
StrReverse("String")
For example:
Private Sub Form_Click( )
Dim Phrase As String
Phrase = "Madam, I'm Adam"
Phrase = StrReverse(Phrase)
End Sub
In this case, the value of Phrase is the following string:
madA m'I ,madaM
* Extracting characters from a string
Sometimes a string contains more information than you want. For example, you may have stored a person's full name in a variable called FullName, as in the following:
FullName = "John Doe"
To extract characters starting from the left of the string, use the following BASIC command:
Left(FullName, N)
The preceding command says, "See that string over there called FullName? Yank out N number of characters, starting from the left." For example:
Private Sub Form_CIick( )
Dim FulIName As String, First As String
FulIName = "John Doe"
First = Left(FullName, 4)
End Sub
In the preceding example, the value of First is John.
To extract characters starting from the right of the string, use the following BASIC command:
Right(FullName, N)
This command says, "See that string over there called FullName? Yank out N number of characters, starting from the right."
For example:
Private Sub Form_Click( )
Dim FulIName As String, Last As String
FulIName = "John Doe"
Last = Right(FulIName, 3)
End Sub
In this example, the value of Last is Doe.
Another command for extracting characters from a string is the Mid command, such as:
Mid(FulIName, X, Y)
This command says, "See that string over there called FullName? Count X number of characters from the left, and rip out the next Y number of characters. For example,
Private Sub Form_Click( )
Dim FulIName As String, Middle As String
FulIName = "John Q. Doe"
Middle = Mid(FulIName, 6, 2)
End Sub
In this example, the value of Middle is Q. (including the period).
* Finding part of a string with another string
If one string is buried in the middle of another string, you can find the location of the burial string by using the following BASIC command:
InStr("TargetString", "WantedString")
This command returns a number defining the exact location from the left where the "WantedString" begins inside the "Target String". For example:
Private Sub Form_Click( )
Dim FulIName As String
Dim Location As Integer
FulIName = "John Plain Doe"
Location = InStr(FulIName, "Plain")
End Sub
In this case, the value of Location is 6.
If the string you want isn't located inside the string you're searching for, the InStr command returns 0.
When you search for a string within another string, you have to search for the exact uppercase or lowercase string. For example, the following command returns a value of 0:
InStr("John Plain Doe", "PLAIN")
In this case, "Plain" is not the same string as "PLAIN", so InStr returns 0. Essentially, a zero is the Visual Basic way of saying, "Sorry, I can't find your exact string anywhere."
* Replacing part of a string with another string
In case you get the creative urge to write your own word processor in Visual Basic (complete with search and replace features), you can do so with the following BASIC command:
Mid("TargetString", Position) = "NewString"
This command says, "See that string called Target String? Find the value defined by Position, count that number of characters from the left, and insert the string called NewString."
Of course, you have to be careful when inserting a new string into an existing one. For example, consider the following code:
FullName = "John Plain Doe"
Mid(FulIName, 6) = "Vanilla"
Here's how Visual Basic interprets this code:
First, Visual Basic assigns the string "John Plain Doe" to the variable called FullName.
Next, Visual Basic looks at the string "John Plain Doe", finds the sixth character from the left, and inserts the string "Vanilla" replacing the original string beginning with the sixth character. So the following is what happens:
John Plain Doe (Original string)
^ (Sixth character from the left)
John Vanillaoe (New string)
After you tell Visual Basic to replace part of a string with another one, Visual Basic gets overzealous and wipes out anything that gets in the way of the new string.
* Trimming spaces from strings
Strings aren't always nice and neat. Sometimes spaces lie in front of or behind the string, as the following examples illustrate:
" This is an example of leading spaces"
"This is an example of trailing spaces "
To strip away leading spaces, use the following BASIC command:
LTrim("TargetString")
For example:
Private Sub Form_Click( )
Dim FullName As String
FullName = " John Doe"
FullName = LTrim(FullName)
End Sub
The value of FullName is "John Doe" with the leading spaces removed.
To strip away trailing spaces, use the following BASIC command:
RTrim("TargetString")
For example:
Private Sub Form_Click( )
Dim FullName As String
FullName = "John "
FullName = RTrim(FullName)
FullName = FullName & " " & "Doe"
End Sub
In the preceding example, the RTrim command removes the trailing spaces so the value of FullName is just plain "John". Then the last command adds the value of FullName("John") to a blank space and the string "Doe" to create the string "John Doe".
In case you have both leading and trailing spaces, you can combine the two commands like this:
LTrim(RTrim("TargetString"))
This command says, "First, remove all trailing spaces and then remove all leading spaces." For an even simpler method, use the following BASIC command instead:
Trim("TargetString")
For example:
Private Sub Form_Click( )
Dim FulIName As String
FulIName = " John Dull "
FulIName = Trim(Ful]Name)
FulIName = FulIName & " " & "Doe"
End Sub
The Trim command removes both the leading and trailing spaces in one quick stroke, then the last command adds the string "John Dull" to a blank string (" ") and the string "Doe" to create "John Dull Doe".
** Converting Strings and Values
Visual Basic handles numbers and strings differently. There may come a time, however, when you need to convert a string into a number so that you can use the string for calculations. Or you may need to convert a number into a string so that you can manipulate the string. You also may need to convert a string into the equivalent ASCII or ANSI value.
* Converting a string into a number
What if you have a text box in which users can type their hourly wages? Unfortunately, the Text property of any text box stores data as a string, not as a number. To convert this string into a number, you have to use one of the following BASIC commands:
CDbl("TargetString")
CSng("TargetString")
The first command says, "Take the string called Target String and convert TargetString to a Double data type."
The second command says, "Take the string called TargetString and convert TargetString to a Single data type.
For example:
Private Sub Form_Click( )
Dim GetNumber As Double
GetNumber = CDbl(txtHourlyWage.Text)
End Sub
The following is how Visual Basic interprets this code:
1. The first statement says, "Declare a variable called GetNumber as a Double data type."
2. The second statement says, "Get the string stored in the Text property of a text box called txtHourlyWage and convert the string to a number that's a Double data type."
3. Finally, the value stored in the txtHourlyWage. Text property is assigned to the GetNumber variable.
If the user types 6.25 in the txtHourlyWage text box, the value of GetNumber is 6.25.
If the user types 6.25 Hourly wage or My hourly wage is 6.25 in the txtHourlyWage text box, Visual Basic chokes and screams about a type mismatch error because neither CDbl nor CSng knows how to handle characters.
* Converting a number into a string
What if you have a number and need to convert it into a string so that you can do fancy string manipulations to the number? Then you have to use the following BASIC command:
CStr(Number)
This command says, "Take the number represented by Number and turn this number into a string."
For example, Visual Basic considers these to be two completely different creatures:
10 ' This is a number
"10" ' This is a string
The following converts a number into a string:
CStr(10) ' The string " 10"
CStr(10.5) ' The string " 10.5"
CStr( - 10) ' The string "-10"
When Visual Basic converts a number into a string, the string has an extra leading space if the string is a positive number or a minus sign (-) if the string is a negative number.
* Converting a string into an equivalent ASCII value
As a programmer, you have to practically memorize the ASCII table at some point, so you may as well find a copy of one and hang it near your computer somewhere so that you can find this table easily.
An ASCII table shows the codes that computers use to represent most of the characters you need. For example, the letter A has an ASCII value of 65, and the letter a has an ASCII value of 97.
Whenever you need the ASCII value of a one-character string, you can use this BASIC command:
Asc("Character")
The following shows how to convert a character into its ASCII value:
X = Asc ("A") ' X = 65
X = Asc ("a") ' X = 97
* Converting an ANSI value into a string
Microsoft Windows doesn't use the ASCII table. Instead, Windows uses the ANSI table, which is practically the same as the ASCII table anyway. (You can read about the ANSI table in Chapter 17)
To use an ANSI value, use the following BASIC command:
Chr("Character")
The only time you need to use the ANSI value of anything is for special control codes, such as for line feeds, carriage returns, and new lines.
The following commands shows common ANSI values:
LineFeed = Chr(I0)
FormFeed = Chr(12)
Carriage = Chr (13)
By using all these fancy string-manipulation commands, you can make sure that your strings look exactly the way you want them to before displaying them in a text box or label. Either that, or you can just have fun playing with words and numbers and pretend you're doing serious research on your job.
To show you how Visual Basic can manipulate strings, examine the ELIZA.VBP program on the enclosed CD-ROM. This program is similar to the famous ELIZA artificial intelligence program that mimics a psychiatrist who simply echoes back portions of what the user types in.
++ CHAPTER 19
DEFINING CONSTANTS AND USING COMMENTS
In This Chapter:
Naming and calculating constants
Declaring the scope of constants
Creating and using the three types of comments
A constant is a fixed value that never changes, no matter what happens to your program. Numbers, strings, and dates can be constant values.
But why bother using constants? Several good reasons exist, none of which make any sense until you start writing your own programs.
For example, suppose that you want to write a program that pays employees according to the current minimum wage. If the minimum wage is $5.95, you have to type the number 5.95 everywhere in your program.
Unfortunately, the number 5.95 means nothing in itself. Even worse, if the minimum wage changes from $5.95 to $6.25, you have to change 5.95 to 6.25 everywhere in your program.
To overcome these problems, you can use constants. A constant is simply a word that represents a specific value. A constant not only uses plain English to describe what the value means, but a constant also lets you change values quickly and easily throughout an entire program.
** Naming Constants
Constant names must meet the following criteria. They must:
Begin with a letter
Be 40 characters or fewer
Contain only letters, numbers, and the underscore character (_); punctuation marks and spaces are not allowed
Be any word except a Visual Basic reserved keyword
To make constant names stand out, use all uppercase letters. For example, the following are acceptable constant names:
AGE
MY_BIRTHDAY
MINIMUM_WAGE
LIFEBOAT CAPACITY
To provide additional information about the type of data a constant represents, use a three-letter prefix as part of the constant name such as:
intAGE (int represents an Integer data value)
curMINIMUM_WAGE (cur represents a Currency data value)
sngGPA (sng represents a Single data value)
Table 19-1 provides Microsoft's list of suggested three-letter prefixes for naming constants (or variables, for that matter).
||
Table 19-1
Three-Letter Prefixes for Naming Constants and Variables
Data Type - Boolean
Prefix - bln
Example - blnIsItDeadYet
Data Type - Byte
Prefix - byt
Example - bytThisNumber
Data Type - Currency
Prefix - cur
Example - curCEOBonus
Data Type - Data (Time)
Prefix - dtm
Example - dtmAnniversary
Data Type - Double
Prefix - dbl
Example - dblHeight
Data Type - Integer
Prefix - int
Example - intTotalNumber
Data Type - Long
Prefix - lng
Example - lngWidth
Data Type - Single
Prefix - sng
Example - sngAverage
Data Type - String
Prefix - str
Example - strMyName
Data Type - Variant
Prefix - vnt
Example - vntWhatEver
* Declaring Constants
Before you can use a constant, you have to declare the constant. To declare a constant, you just give the constant a name and assign it a specific value, such as any of the following:
Numbers
Strings
Dates
The following code declares number, string, and date constants:
Private Sub Command1_Click( )
Const intAGE = 21
Const strCOMPANY = "Acme Manufacturing"
Const dtmCHRISTMAS = #25 December 1995#
End Sub
Place all constant declarations at the top of your event procedures. Instead of typing one constant declaration on each line, you can smash them all together and separate them with commas, as illustrated in the following code:
Private Sub Command1_Click( )
Const intAGE = 21, strCOMPANY = "Acme Manufacturing"
End Sub
Note that number constants are only numbers, string constants are anything enclosed in quotation marks and date constants are dates surrounded by the pound sign (#).
Here are some of the ways in which dates can display:
#12-25-95#
#December 25, 1995#
#Dec-25-95#
#25 December 1995#
* Calculating constants
Constants normally represent a fixed value. However, constants can also be mathematic values based on other constants. For example:
Const intRETIREMENT_AGE = 65
Const sngHALFWAY-THERE = intRETIREMENT_AGE / 2
In this case, the value of the constant intRETIREMENT_AGE is 65 and the value of the constant sngHALFWAY_THERE is 65/2, or 32.5.
* Using constants
After you declare a constant, you can use the constant just like any other value. Consider the following:
Const curMINIMUM_WAGE = 5.75
Salary = curMINIMUM_WAGE * 20
Here's how Visual Basic interprets this code:
1. The first command says, "Create a constant named curMINIMUM_WAGE and set the value of curMINIMUM_WAGE to 5.75."
2. The second command says, "Multiply the value of curMINIMUM_WAGE by 20 and store this value in the variable called Salary." In this case, the value of curMINIMUM_WAGE is 5.75, so you multiply 5.75 by 20, which equals 115. Then Visual Basic stores this value in Salary.
** Scope of Constants
Visual Basic lets you declare the scope of constants in the following three ways:
Local
Module
Public
* Local constants
You can use a local constant only within the procedure in which you declare the constant. The purpose of local constants is to isolate specific constants in a single procedure where they are used.
Declare a local constant within an event procedure, as in the following:
Private Sub Command1_Click( )
Const intSPEED_LIMIT = 55
End Sub
You can use a local constant only in the one event procedure in which you declare the constant. However, what if you want to create a constant that two or more event procedures can share? In that case, you have to create a module constant.
* Module constants
A module constant can be used only in an event procedure stored in the same file.
To declare a module constant, follow these steps:
1. Open the Code window by pressing F7, choosing View->Code, or double-clicking anywhere on the form.
2. Click the Object list box in the Code window and choose (General).
3. Click the Procedure list box and choose (Declarations).
4. Now type your constant declaration using the Const statement, as in the following:
Const intDRINKING_AGE = 21
Module constants are useful for sharing a constant value among one or more event procedures, but isolating the constant to only those event procedures stored in the same file. If you want a constant that any procedure in your program can use, you need to create a public constant.
To help you identify module constants buried in your code, Microsoft recommends that you put the letter m in front of your constant name such as:
Const mintDRINKING_AGE = 21
* Public constants
A public constant (also called a global constant) can be the most convenient to use because every procedure in your Visual Basic program can access such a constant. However, cool programmers use public constants only when absolutely necessary; cluttering up your program with public constants that only a few procedures ever use is bad programming practice.
Using public constants is poor programming etiquette because changing a public constant can affect your entire program. Experienced programmers may blush in embarrassment for you if they catch you using public constants needlessly, and you may never get invited to any of the really great programmer parties as a result.
You need to declare public constants in a BAS (module) file. To declare a public constant, follow these steps:
1. Open the Project window.
To do this, press Ctrl+R and click on the BAS (module) file where you want to put the public constant. (If you need to create a BAS file, click the Module icon or choose Module from the Insert menu.)
2. Open the Code window by pressing F7, choosing View->Code, or double-clicking anywhere on the form.
3. Click on the Object list box in the Code window and choose (General).
4. Click on the Procedure list box and choose (Declarations).
5. Type your public constant using the Public command, as in the following:
Public Const AGE_LIMIT = 18
To help you identify public (or global) constants buried in your code, Microsoft recommends that you put the letter g in front of your constant name such as:
Const gsngHEIGHT_LIMIT = 21.67
If you want to see a list of all the global constants that Visual Basic has already defined for you, follow these steps:
1. Choose Help->lndex. A dialog box appears.
2. Click the Index tab.
3. Type Constants in the Type in the keyword to find text box.
4. Under the Constants category, click on Visual Basic.
5. Click Display. Visual Basic displays the Visual Basic Constants window.
6. Click on the constant category that you want to view, such as Button Constants.
** Using Comments
When you're coding your program (note the proper use of the word coding), the way your program works may be clear to you. Unfortunately, if you put your program aside and try to modify it five years later, you may have forgotten why you wrote certain commands and even how some of those commands work.
For just this reason, add plenty of comments to your programs. Comments are short descriptions that programmers add to their program to explain what certain commands mean or to explain what is supposed to happen in a particular part of the program.
As far as the computer is concerned, comments do absolutely nothing to help or hinder the way your program works. However, from a programmer's point of view, comments help explain how and why a program functions.
* Creating comments
Visual Basic lets you create comments by using the apostrophe ( ' ) symbol, followed by anything you care to type. The following, for example, is a valid comment:
Private Sub Command1_Click( )
' This event procedure does absolutely nothing
End Sub
As far as Visual Basic is concerned, the computer ignores anything that appears to the right of the apostrophe symbol.
Comments can appear on separate lines or they can appear as part of another line, as in the following example:
Private Sub Command1_Click( )
X = Y * 2 ' Y represents the number of employees
End Sub
You can also cram several comments together on multiple lines:
Private Sub Command1_Click( )
Y = 200 ' Y represents the number of employees
X = Y * 2
' X represents the number of employees who would like
' to slash the tires on the boss's car.
End Sub
Just remember that the computer ignores anything that appears to the right of the apostrophe symbol and considers it a comment.
* Commenting for readability
The main reason for using comments is to make your programs easy to understand. For this reason, most cool programmers put comments at the beginning of every procedure.
These comments explain what data the procedure gets (if any) and what calculations the procedure performs. By just looking at the comments, anyone can quickly see what the procedure does without needing to decipher several lines of cryptic BASIC code. For example, can you figure out what the following event procedure does?
Private Sub Command1_Click( )
A = SQR(B ^ 2 + C ^ 2)
End Sub
To make the procedure's function easier to understand, add a bunch of comments at the top of this procedure:
Private Sub Command1_Click( )
' The following equation uses the Pythagorean theorem
' for calculating the length of a side of a right
' triangle if the lengths of two sides are known. In
' this case, the length of one side of the triangle is
' represented by B and the length of the second side of
' the triangle is represented by C.
A = SQR(B ^ 2 + C " 2)
End Sub
If several people share the responsibilities for writing procedures, you can use comments to note the name of the programmer and the date each procedure was last modified. (That way, you know who to blame when the procedure doesn't work.) For example:
Private Sub Command1_Click( )
' Programmer: JOHN DOE
' Last modified: 1/1/80 (our computer clock doesn't work)
A = SQR((B ^ 2 + C ^ 2))
End Sub
Of course, if you get too wordy, your comments can be more intrusive than helpful - like billboards along the highway. Just remember: Provide enough information to be helpful, but not so much that people nod off to sleep while reading your comments. You're not writing a classic novel here, just a brief description that other people can understand.
* Test your newfound knowledge
1. Why do you want to add comments to your program?
a. To summarize and explain how your BASIC code works.
b. To exercise your literary skills and prove that programmers can write, too.
c. To prove that you have something to say besides BASIC commands.
d. So that you can leave cryptic messages for other programmers to decipher
2. Comment on the simplicity and brevity of this lesson.
a. All right! Now I can quit and go home early.
b. Why can't all the lessons in this book be this simple and short?
c. I still can't write a program, but I know how to use comments. Maybe I need to get a job as a commentator.
d. Comments are cool. If we can write comments in our programs, does that mean we can write programs with our word
processors?
* Comments for legibility
If your program contains lots of BASIC code, you can use comments and blank lines to make your code easy to read. For example, to make each chunk more easily seen, separate chunks of code:
Private Sub Command1_Click( )
Const dblINTEREST_RATE = .055 ' 5.5% interest rate
Dim Msg As String ' Declares Msg as a string ' variable
BankBalance = 500
BankBalance = BankBalance * dblINTEREST_RATE
' Subtract bank fees
BankFees = BankBalance * 2
BankBalance = BankBalance - BankFees
' Display a message box saying that the user owes the
' bank a certain amount of money.
Msg = "Please pay this amount: " & - BankBalance
MsgBox Msg, vbCritical, "Amount You Owe"
End Sub
As the preceeding example shows, you can insert hard returns to add blank lines between chunks of code, thereby enabling yourself to see more easily what each chunk actually does.
Stripping out all comments and blank lines gives you the following equivalent, but uglier, version:
Private Sub Command1_Click( )
Const INTEREST_RATE = .055
Dim Msg As String
BankBalance = 500
BankBalance = BankBalance * INTEREST_RATE
BankFees = BankBalance * 2
BankBalance = BankBalance - BankFees
Msg = "Please pay this amount: " & -BankBalance
MsgBox Msg, vbCritical, "Amount You Owe"
End Sub
Notice how this new version seems cramped and cluttered, much like your bathroom counter or your garage.
* Comments for disability
With comments, you can not only add explanations about your program and visually break up your code but also temporarily disable one or more BASIC commands.
For example, as you're writing a program, you may find that a command isn't working as you want it to. To test how your program works without this command, you have two choices:
Erase the command.
Comment the command out.
If you erase a command and then decide you need it, you have to type the command all over again. If you comment the command out, however, you only have to erase the apostrophe symbol in order to put the command back in.
The following example contains a fairly long line of numbers:
Private Sub Command1_Click( )
X = 3.14 * 650 - (909 / 34.56) + 89.323
End Sub
If you erase the second line, typing the line again can be a real pain in the neck. However, you can just comment the line out, as follows:
Private Sub Command1_Click( )
' X = 3.14 * 650 - (909 / 34.56) + 89.323
End Sub
Remember, the computer ignores anything that appears to the right of the apostrophe symbol. So, to the computer, this procedure now looks like the following:
Private Sub Command1_Click( )
End Sub
Placing the apostrophe in front of this statement turns the statement into a comment and disables this new comment as a BASIC command. By removing the apostrophe symbol, you can quickly turn the comment back into a real-life BASIC command.
By using comments wisely, you can ensure that you or another programmer can easily understand any programs you write. Then again, if you really want to sabotage a programming project, add comments that don't make any sense or leave them out altogether and see what happens.
If you want to comment multiple lines of code quickly, follow these steps:
1. Highlight the lines of code you want to turn into a comment.
2. Choose View->Toolbars->Edit.
3. Click on the Comment Block icon. Visual Basic adds an apostrophe to the front of each line that you highlighted in Step 1.
To uncomment blocks of code, just repeat the above three steps except click on the Uncomment Block icon on the Editor toolbar in Step 3.
++ CHAPTER 20
KILLING BUGS
In This Chapter:
Examining the various types of bugs
Embarking on the big bug hunt
Killing the bugs you find
Even if you've written millions of different programs before, you are still going to make a mistake at one time or another. You may misspell a word or forget to type a command. So no matter how carefully you write your program, the program may not work exactly as you design it to. The problems hindering your program from working are bugs.
Every program in the world has bugs, including Netscape Navigator, WordPerfect, Lotus 1-2-3, Quicken, and Microsoft Windows 98. The only difference between the bugs in your program and the bugs in a commercial program is that nobody is paying you to eliminate bugs in your program. However, with a fair amount of planning, application design, and just plain common sense, you can avoid quite a few bugs.
Don't worry. Many bugs are relatively harmless. These minor bugs normally aren't going to prevent you from using a program correctly, but they may slow down your computer or display odd colors or objects on the screen at random times.
Major bugs are more devastating. For example, a major bug allegedly blew up one of NASA's multimillion-dollar weather satellites because someone mistyped a single command in the program.
Nobody is perfect, so no program can be guaranteed to be completely bug-free. Even an experienced professional with a doctorate in computer science regularly writes bug-ridden programs.
Bugs are a fact of life, like cockroaches in a kitchen. You may never get rid of them all, but you can kill as many as possible along the way.
* Why computer problems are called bugs
The first computer in the world used mechanical relays instead of modern electronics. One day the computer stopped working for no apparent reason. The scientists checked their programming (the program was supposed to have worked), the electric cord (it was plugged in), and the wires inside the computer (they were still connected).
Eventually, someone noticed that a moth had gotten smashed in one of the relays, preventing the relay from closing all the way. Because the moth's dead body prevented the computer from working, problems with computers were henceforth known as bugs (which is a lot easier to say than Chihuahuas, so we should be glad that a dog didn't get smashed in the first computer).
** Types of Bugs
The art of killing bugs is known as debugging. Before you can kill a bug, you first have to find the bug. With small programs, such as ones that display Hello, world! on the screen, you have only so many places a bug can hide. With large programs, a bug can be hiding anywhere, and this can be as frustrating as trying to find a single tsetse fly in a high-rise apartment building.
To make hunting for bugs easier, computer scientists classify bugs in three categories:
Syntax errors
Run-time errors
Logic errors
* Syntax errors
A syntax error is a bug that occurs when you misspell a command. If you type INTTEGER instead of INTEGER, for example, Visual Basic is going to have no idea what INTTEGER means and isn't going to even bother trying to run the rest of your program.
When Visual Basic runs across a syntax error, Visual Basic politely high-lights the misspelled word on the screen to show you exactly what the problem is. Just type the correct spelling and run your program again.
Even one syntax error is going to keep your program from running. When you finally get your program to run for the first time, you know that your program is completely free of syntax errors. Then you have to worry only about run-time and logic errors.
* Run-time errors
A run-time error occurs when your program gets data that the program doesn't quite know how to handle. A run-time error is more subtle than a syntax error. Your program may be riddled with run-time errors, but you may never know this until you actually run your program.
To simulate a run-time error in your own life, pull into the drive-through window at your nearest Burger King. When the cashier asks, "May I help you?" order a Big Mac. Because the cashier expects you to order something from Burger King's menu, this person has no idea how to respond to your question and is likely to suffer a run-time error.
For an example of a run-time error in a program, consider this formula for calculating a result:
TaxRate = TaxesOwed / YearlyIncome
This equation normally works - unless the YearIy Income equals 0. Because dividing any number by 0 is impossible, the program stops running if the value in YearlyIncome is 0.
To discover a run-time error, you must test your program for every possible occurrence: from someone pressing the wrong key to some idiot typing a negative number for his or her age.
Because the number of things that can ever go wrong is nearly infinite (Murphy's Law), you can understand why every large program in the world has bugs. (Now isn't this a comforting thought to remember the next time you fly in a computer-controlled airplane?)
* Logic errors
The trickiest type of bug is a logic error. A logic error occurs when the program doesn't work correctly because you gave the program the wrong commands or the commands you issued are out of sequence with other commands. Huh? How can you give a program the wrong commands when you're the one writing the program? Believe it or not, entering the wrong commands is easy.
Anyone raising teenagers knows that when you tell them to mow the lawn or clean up their rooms, they may do the task - but not quite the way you wanted the task accomplished. Instead of mowing the lawn in neat rows, a teenager may move around in circles and give up. Or instead of cleaning a room by picking up dirty clothes and tossing out papers, a teenager may shove the whole mess under the bed or out into the hallway.
In both cases, the teenager followed your instructions, but your instructions weren't specific enough. If a teenager can find a loophole in your instructions, he or she will, and a computer is no different.
Because you thought you gave the computer the right commands to follow, you have no idea why your program isn't working. Now you have to find the one spot where your instructions aren't clear enough. If you have a large program, this may mean searching the entire program, line-by-line. (Isn't programming fun?)
** Bug Hunting
Basically, you need to go through four steps to hunt down and kill bugs in your program:
1. Realize that your program has a bug.
2. Find the bug.
3. Find what's causing the bug.
4. Squash the bug.
* Realizing that your program has a bug
The best way to discover bugs in your program is to let unsuspecting individuals use your program. (In the world of commercial software, these unsuspecting individuals are often called paying customers.)
The more people you find to test your program, the more likely that these guinea pigs are going to uncover bugs you never knew existed. Bugs can be as glaring as ones that cause the computer to crash, or they can be as sneaky as ones that round off numbers to the wrong decimal place.
After you conclude that your program has a bug, you have to track the bug down. (For the optimists in the group, you can call your program's bugs undocumented features.)
* Finding the bug
Finding where a bug is hiding is the toughest part. The simplest (and most tedious) way to find a bug's hiding place is to run your program and examine it line-by-line. The moment the bug appears, you know exactly which line caused the bug.
For small programs, this approach is acceptable. For large programs, this is crazy.
As a faster alternative, just examine the parts of your program in which you think the bug may be hiding. If your program doesn't print correctly, for example, the bug may be in your BASIC code that tells the computer how to print.
* Finding what's causing the bug
After you isolate where you think the bug is hiding, you have to figure out what is causing the bug in the first place.
Suppose that your program is to print your name on the screen but is printing your social security number instead. The program may seem to be printing everything correctly but is simply getting the wrong type of information to print.
By using your incredible powers of deductive reasoning, you realize that the bug is (probably) wherever your program first tries to print your name.
* Squashing the bug
After you find the cause of your bug, you've reached the time to correct your program. But be careful! Sometimes correcting one bug introduces two or three more by mistake. Huh? How can that be?
Compare bug squashing to repairing a problem with the plumbing in your house. The easiest solution may be to tear out a wall and put in new pipes. This may solve the plumbing problem, but tearing out a wall can also tear out electrical wires inside the wall. So now you've fixed your plumbing problems but also created a new electrical problem. If you put up a new wall with electrical wiring, you may inadvertently block a vent for the central air-conditioning. Move the wall back 3 feet, and now the roof may be too weak in the middle to hold up the wall. See - your small plumbing "bug" has just multiplied.
So when fixing a bug, be careful. Sometimes rewriting a huge chunk of code is easier than trying to fix a bug within the code.
The best way to avoid bugs is to not have any in the first place. Of course, that's like saying to avoid money problems, just make sure you always have enough money.
Because bugs appear in even the best of programs, the most you can hope for is to reduce the number of bugs that can pop up in your programs. Here are some tips that may help:
To avoid bugs, write (and test) lots of tiny programs and paste them together to make one huge program. The smaller your programs, the easier isolating any bugs is going to be. In military terms, this is known as the divide and conquer method.
Test your program each time you modify it. If your program worked fine until you changed three lines, the problem probably can be isolated to those three lines.
Have someone you can pin the blame on. If your program refuses to work, blame your spouse, your dog, or your favorite deity. This isn't going to help fix your program, but blaming someone or something else can make you feel better for a moment or two.
** How Visual Basic Tracks and Kills Bugs
Visual Basic provides two primary ways to help you track and kill bugs:
stepping and watching.
Stepping means that you go through your program line-by-line and examine each instruction. After each line runs, look to see what the program did. If your program works the way you wanted it to, the line is okay. If not, you just located a bug.
Watching lets you see what data your program is using at any given time. If you watch for specific data, such as a name or phone number, you can see whether your program is storing, printing, or modifying the specific data correctly.
By stepping through a program line-by-line and watching to see what data your program is using, you can find any bugs in your program.
* Stepping through a program line-by-line
If you have absolutely no idea where your bug may be, you need to examine your entire program line-by-line. To step through a program, Visual Basic provides three commands:
Debug->Step Into (or press F8)
Debug->Step Over (or press Shift+F8)
Debug->Step Out (or press Ctrl+Shift+F8)
The Step Into command runs through your entire program one line at a time, including every line stored in every procedure in your program.
The Step Over command runs through your entire program but whenever Visual Basic runs into a procedure, Visual Basic skips over the instructions that make up that procedure.
The Step Out command is used with the Step Into command. The Step Into command shows you, line-by-line, how a procedure is working. However, if you suddenly decide that you don't want to examine how a procedure is working, line-by-line, use the Step Out command.
You can combine the three commands at any time. First, use the Step Into command to examine your program line-by-line. Next, use the Step Out command to get out of any procedures that the Step Into command starts displaying. Finally, use the Step Over command to skip over any procedures that you're positive already work.
To use the Step Into or Step Over commands, follow these steps:
1. Press F8 or Shift+F8, or choose Debug->Step Into or Step Over.
Visual Basic displays and outlines a line in your program.
2. Choose Run->End when you want to stop.
* Watching your variables
Stepping through your program line-by-line can be even more useful if you watch how your program handles data at the same time. To help you see what values your variables contain at any given time, Visual Basic provides a Watch window.
The Watch window tells Visual Basic, "These are the variables I want to examine. Show me the contents of these variables as I step through my program line-by-line."
To use the Watch window to watch your variables, follow these steps:
1. Open the Code window by pressing F7, choosing View->Code, or clicking on the View Code icon in the Project Explorer window.
2. Highlight the variable that you want to watch and choose Debug->Add Watch. An Add Watch dialog box appears.
3. Click on OK.
4. Press F8 or Shift+F8 to choose the Step Into (if you want to examine the guts of a procedure) or Step Over (if you want to skip over the code trapped inside a procedure) commands. The Watch window displays the value of your watched variable each time you choose the Step Into or Step Over command.
5. Choose Run->End or click on the End icon In the toolbar when you want to stop.
* Setting breakpoints
Both the Step Into and Step Over commands start from the beginning of your program and continue until they reach the end. This is acceptable for small programs, but the process can get tedious for large programs.
To skip over large sections of your program that you know (or hope) already work, you can set a breakpoint. A breakpoint tells Visual Basic, "Run the program up until you reach me. Then wait until I give you the Step Into, Step Over, or Run command."
To set a breakpoint, follow these steps:
1. Open the Code window by pressing F7, choosing View->Code, or clicking on the View Code icon in the Project Explorer window.
2. Click on the line where you want to set your breakpoint
3. Press F9 (or choose Debug->Toggle Breakpoint).
After you set a breakpoint, press F5 to run your program until it reaches your breakpoint. At this point, you can use the Step Into or Step Over commands along with the Add Watch command.
To remove a breakpoint, just follow the above three steps again, clicking on the line that contains the breakpoint you want to remove.
To quickly remove all breakpoints in your program, press Ctrl+Shift+F9 or choose Debug->Clear All Breakpoints.
++ PART V
MAKING DECISIONS (SOMETHING YOU STOP DOING WHEN YOU GET MARRIED)
In this part....
Programs contain nothing more complicated than instructions that tell the computer what to do next. The simplest programs just contain one massive list of instructions that the computer follows, one instruction after another, like a shopping list.
But blindly following instructions doesn't make for a very useful program. Most programs need to receive data and then decide how to use the data.
This decision-making capability can make your computer seem alive, responsive, and intelligent. (Well, alive, anyway.) When a program can tell your computer how to make its own decisions, your computer can begin doing something useful.
++ CHAPTER 21
THE IF-THEN AND IF-THEN-ELSE STATEMENTS
In This Chapter.....
Specifying a condition
Using If-Then and If-Then-End If statements
Using If-Then-Else and If-Then-ElseIf statements
Everyone has made decisions based on certain conditions, such as, "Don't worry, honey. If the ball game is over early, then I am going to mow the lawn." Or "If you stop feeding the cat, then maybe it won't keep getting fatter." Visual Basic is no different when making decisions is at hand. When you write a program, Visual Basic can check for certain conditions and then respond.
** Conditions
A condition must represent a value that is either True or False. Conditions can be:
A single variable
An expression
If a condition is a single variable, that variable must have a value that is either True or False. You can check the value of a variable in two ways. The first way is to specifically check if a variable is equal to True, as in the following example:
If TooHot = True Then
The second way (which is shorter to write) lets you eliminate = True and just write the following:
If TooHot Then
You don't have to specify whether this variable is equal to True because Visual Basic checks whether the value is True or False anyway.
If you want to specifically test whether a variable is False, you can do the following:
If TooCold = False Then
As ashortcut, eliminate = False and just write
If Not TooCold Then
If a condition is an expression, that entire expression must represent a value that is either True or False, such as:
If Age >+ 21 Then
In the above example, the condition is Age >= 21.
For this condition, if the value of Age is greater than or equal to 21, the expression is True. Otherwise, the expression is False.
In the following example, if the string stored in the Text property of a text box called txtName contains the string "Captain Mike", the expression is True. Otherwise, the expression is False.
If (txtName.Text = "Captain Mike") Then
The parentheses in the preceding line of code don't affect your precious code one bit; they just help make the condition easier to identify.
In the following example, the expression Cat Present And CatMisbehaving evaluates to False:
CatPresent = False
CatMisbehaving = True
If CatPresent And CatMisbehaving Then
To make the condition in an If-Then statement easier to see, you can also surround the condition with optional parentheses, as in the following line of code:
If (CatPresent And CatMisbehaving) Then
Now that you understand what and how conditions work, you can find out how to plug them into an If-Then statement in the following section, "The If-Then Statement."
** The If-Then Statement
To make decisions, Visual Basic uses something called an If-Then statement. An If-Then statement is the way Visual Basic checks whether a condition is True or False.
If the condition is True, Visual Basic follows a certain instruction. If the condition is False, Visual Basic ignores this instruction.
All of this may look rather wordy, so here's the condensed version of the statement:
If Condition Then Instruction
Essentially, this code tells Visual Basic that if a certain condition is True, Visual Basic needs to obey the instruction that follows.
Whatever the condition may be, the condition must always return a True or False value.
Here are a few real-life examples:
If Number > 25 Then txtNote.Text = "Full"
Here's how Visual Basic interprets this code:
1. This command says, "Check a variable called Number and see whether the value is greater than 25. If the value is greater than 25, then stuff the string "Full" into the Text property of a text box called txtNote."
2. "If the value of the Number variable is equal to or less than 25, then skip to the next instruction in your Visual Basic program."
If Hungry Or Bored Then Message = "Let's eat."
Here's how Visual Basic interprets this code:
1. This command says, "Check the value of a variable called Hungry and check the value of a variable called Bored. If either one has a value of True, then create a variable called Message and set the value equal to the string " Let ' s eat."
2. "If both the variable Hungry and the variable Bored are False, skip to the next instruction in your Visual Basic program."
The typical If-Then statement tests whether a certain condition is True or False and then follows a single instruction. But what happens if you want to test for a condition and then make the computer follow two or more instructions? In that case, you have to use a different form of the If-Then statement that is called the If-Then-End If statement.
** The If-Then-End If Statement
The If-Then-End If statement lets Visual Basic check a single condition. If the condition is True, the program follows a bunch of instructions. Here's the proper If-Then-End If syntax:
If Condition Then
Instruction1
Instruction2
End If
Essentially, this code tells Visual Basic, "Check a condition; if the condition is True, then obey all the following instructions until you reach End If."
Here is an honest-to-goodness example:
If Electricity_is_Out = True Then
Light-candles = True
TxtWarning.Text = "You just lost all your work."
End If
And here's how Visual Basic interprets this code:
1. The first line says, "Check the value for a variable called Electricity_is_Out. If the variable's value is True, then follow the
next two instructions. If the value is False, then do nothing."
2. "Assign a value of True to the variable Light_candles."
3. "Assign the string "You just lost all your work." to the Text property of the txtWarning text box."
** If-Then-Else Statement
The If-Then statement gives your program the capability to make decisions based on certain conditions. If this isn't the pinnacle of your computer programming career, hold on to your hats for the If-Then-Else statement.
The problem with using an If-Then statement or an If-Then-End If statement is that you may need too many of them to check for both True and False conditions. Is there an easier way? The answer, of course, is yes! Visual Basic offers something called an If-Then-Else statement.
The simplest If-Then-Else statement looks like this:
If Condition Then
Instructions1
Else
Instructions2
End If
This statement tells Visual Basic, "If the condition is True, then follow the first batch of instructions. If the condition is False, then follow the second batch of instructions."
So how can you modify the following?
If Day > 15 Then txtReadMe.Text = "Bills are past due!"
If Day <= 15 Then txtReadMe.Text = "Pay your bills!"
Depending on the condition you use, you can rewrite these statements in two ways. If you use the condition Day > 15, you get the following:
If Day > 15 Then
txtReadMe.Text = "Bills are past due!"
Else
txtReadMe.Text = "Pay your bills!"
End If
But if you use the condition Day <= 15, you get the following:
If Day <= 15 Then
txtReadMe.Text = "Pay your bills!"
Else
txtReadMe.Text = "Bills are past due!"
End If
Both types of If-Then-Else statements are perfectly acceptable. Which statement you choose to use is just a matter of personal preference.
You can shove as many instructions as you want between the If-Then and Else lines and the Else and End If lines.
One possible drawback with an If-Then-Else statement is that if the first condition is False, Visual Basic blindly follows the second group of instructions. If you don't want this to happen, you have to specify a condition for the second set of instructions. To do that, you have to use an If-Then-ElseIf statement.
** The If-Then-ElseIf Statement
An If-Then-ElseIf statement looks like the following:
If Condition1 Then
Instructions1
ElseIf Condition2 Then
Instructions2
End If
This code tells Visual Basic, "If Condition1 is True, then follow the first set of instructions. But if Condition1 is False, then check to see if Condition2 is True. If Condition2 is True, then follow the second set of instructions. If Condition2 is False, then don't do anything at all."
With an If-Then-Else statement, the computer always follows at least one set of instructions. But with an If-Then-ElseIf statement, the computer could possible ignore all instructions - much like a rebellious teenager.
For example:
If Day > 15 Then
txtReadMe.Text - "Bills are past due!"
ElseIf Day > 10 Then
txtReadMe.Text = "Pay your bills!"
End If
So what happens if the value of Day is 12?
1. Visual Basic checks the first condition and concludes that the expression 12 > 15 is False (because the value of Day is 12).
2. Then Visual Basic checks the second condition and concludes that the expression 12 > 10 is True, so Visual Basic assigns the string, "Pay your bills!" to the Text property of a text box called txtReadMe.
Here's the tricky part. What happens if the value of Day is 6?
1. Visual Basic checks the first condition and concludes that the statement 6 > 15 is False, so Visual Basic ignores the first set of instructions.
2. Next, Visual Basic checks the second condition and concludes that the statement 6 > 10 is False, so Visual Basic ignores the second set of instructions.
3. Finally, Visual Basic reaches the end of the If-Then-ElseIf statement. Because none of the statements were True, none of the instructions were followed.
To handle multiple possibilities, you need to add more ElseIf conditions.
** Making multiple choices with If-Then-ElseIf
For checking multiple conditions, use multiple ElseIfs, as follows:
If Condition1 Then
Instructions1
ElseIf Condition2 Then
Instructions2
ElseIf Condition3 Then
Instructions3
End If
This code tells Visual Basic, "If Condition1 is True, then follow Instructions1. But if Condition1 is False, check whether Condition2 is True. If Condition2 is True, then follow Instructions2. If Condition1 is False and Condition2 is False, then check to see whether Condition3 is True. If Condition3 is True, then follow Instructions3."
Once again, the possibility exists that all condi tions are going to be False, so the computer may never follow any of the instructions.
You can use as many ElseIf lines as you need. Of course, the more you use, the more confusing your entire If-Then-ElseIf statement gets. ("Now if Condition3 is False but Condition4 is True, wait a minute, what's supposed to happen?")
** Making sure that the computer follows at least one set of instructions
You can have a huge If-Then-ElseIf statement and still not have a single instruction that the computer follows. To make sure that the computer follows at least one set of instructions, add an Else statement at the end, as shown in the following:
If Condition1 Then
Instructions1
ElseIf Condition2 Then
Instructions2
Else
InstructionsDefault
End If
This code tells Visual Basic, "If Condition1 is True, then follow the first set of instructions. But if Condition1 is False, check the value of Condition2. If Condition2 is True, then follow the second set of instructions. If all conditions are False, then go ahead and follow the last set of instructions."
* * Nesting If-Then statements
If you want, you can nest your statements (cram multiple If-Then statements inside one another), such as:
If Age > 21 Then
If Rating = 10 Then
txtAction.Text "Ask for a date,"
End If
Else
txtAction.Text = "Sorry, you're too young."
End If
If the value of Age were 23 and the value of Rating were 10, Visual Basic interprets this code as follows:
1. Visual Basic checks the first condition and concludes that the expression Age > 21 is True (because the value of Age is 23).
2. Then Visual Basic checks the second condition and concludes that the expression Rating = 10 is True, so Visual Basic assigns the string, "Ask for a date." to the Text property of a text box called txtAction.
If the value of Age were 23 but the value of Rating were only 9, Visual Basic interprets this code as follows:
1. Visual Basic checks the first condition and concludes that the expression Age > 2 1 is True ( because the value of Age is 23).
2. Then Visual Basic checks the second condition and concludes that the expression Rating = 10 is False, so nothing happens.
Finally, if the value of Age were 13 and the value of Rating were 10, Visual Basic interprets this code as follows:
1. Visual Basic checks the first condition and concludes that the expression Age > 21 is False (because the value of Age is 13).
2. Visual Basic skips to the Else part of the If-Then-Else statement and stuffs the string " Sorry, you' re too young. " in the Text property of the text box called txtAction. Notice that in this case, the value of Rating is irrelevant.
Be careful when nesting If-Then statements inside one another because nested If-Then statements may act in ways that you didn't expect. For example, in the preceding code, if the value of Age were 23 but the value of Rating were only 9, you may be surprised to find that this code isn't going to put any string in the Text property of the text box called txtAction.
++ CHAPTER 22
THE SELECT CASE STATEMENT
In This Chapter....
Examining the Select Case statement
Making sure that the computer follows at least one set of instructions.
Using nested control structures
The main problem with using massive If-Then-ElseIf statements, which I discuss in Chapter 21, is that these statements are ugly, hard to read and understand, and cumbersome to write. Consider the following:
If Caller = "Frank" Then
txtReply.Text = "Yes!"
ElseIf Caller = "Matt" Then
txtReply.Text = "Okay, but only if you buy."
ElseIf Caller = "Jeff" Then
txtReply.Text = "I'm washing my hair tonight."
ElseIf Caller = "Steve" Then
txtReply.Text = "This is a recording."
End If
So what's the alternative to an endless proliferation of ElseIfs that can be confusing to look at?
One alternative is to toss your copy of Visual Basic out the window and find someone to write your programs for you. But the more practical alternative is to use something called the Select Case statement.
** The Select Case Statement
The Select Case statement looks like the following:
Select Case VariableName
Case X
Instructions1
Case Y
Instructions2
Case Z
Instructions3
End Select
This statement tells Visual Basic, "Look at the value of the variable called VariableName. If this value is equal to X, then follow Instructions1. If this value is equal to Y, then follow Instructions2. If this value is equal to Z, then follow Instructions3."
Replacing the multiple If-Then-ElseIf statement at the beginning of this chapter with the Select Case statement changes the code to look like the following:
Select Case Caller
Case "Frank*
txtReply.Text "Yes!"
Case "Matt"
txtReply.Text - "Okay, but only if you buy."
Case "Jeff"
txtReply.Text "I'm washing my hair tonight.
Case "Steve"
txtReply.Text "This is a recording."
End Select
Notice the cleaner look and the elimination of repetitive words such as ElseIf and Then.
Depending on how many values you need to check, you can sandwich as many Case lines in a Select Case statement as you want.
** Using the Select Case Statement with Comparison Operators
Normally, the Select Case statement requires an exact value to examine. However, by using comparison operators, such as <, <=, or <>, you can make the Select Case statement examine whether a variable falls within a range of values.
To make a Select Case statement use comparison operators, you have to use the magic reserved word is. Therefore, the following Select Case statement:
Select Case Day
Case is > 15
txtReadMe.Text = "Bills are past duel"
Case is > 10
txtReadMe.Text = "Pay your bills!"
End Select
is equivalent to the following If-Then statement:
If Day > 15 Then
txtReadMe.Text - "Bills are past due!"
ElseIf Day > 10 Then
txtReadMe.Text = "Pay your bills!"
End If
** Making Sure the Computer Follows at Least One Set of Instructions
Like the If-Then-ElseIf statement, the possibility that none of the instructions within the Select Case statement are going to be followed exists. To make sure that the computer follows at least one set of instructions, you have to use the magical Else command again. Take a look at the following example:
Select Case Day
Case 1
Instructions1
Case 2
Instructions2
Case3
Instructions3
Case Else
InstructionsDefault
End Select
The preceding code tells Visual Basic, "If the value of Day equals 1, then follow the first set of instructions. If the value of Day equals 2, then follow the second set of instructions. If the value of Day equals 3, then follow the third set of instructions. If the value of Day doesn't equal 1, 2, or 3, then follow the last set of instructions."
** Nesting Case Statements
Some of the simplest toys that amuse children to no end are those Chinese boxes stacked one inside the other. Each time you open a box, you find a smaller one inside. Eventually, you reach a point where no more boxes remain and you have to stop.
Normally an ordinary Select Case statement contains one or more groups of instructions, such as the following:
Select Case ID
Case 123
chkFrank.Value = True
Case 124
chkBob.Value
Case 125
chkMartha.Value = True
End Select
Rather than shoving boring old instructions inside a Select Case statement, however, you can shove If-Then and Select Case statements within a Select Case statement, as the following example illustrates:
Select Case IQ
Case 120
Select Case Age
Case is <= 9
txtAnalysis.Text = "You must be a smart kid."
End Select
End Select
Here's how Visual Basic interprets this code:
1. The first line says, "Check the value stored in a variable called IQ. Then continue to the second line."
2. The second line says, "If the value of IQ is exactly equal to 120, continue to the third line. If the value of IQ is anything else (such as 119, 121, or 3), skip to the seventh line.
3. The third line says, "Check the value stored in a variable called Age. Then continue to the fourth line."
4. The fourth line says, "If the value stored in the variable called Age is equal to or less than 9, then continue to the fifth line. If the value of Age is anything greater than 9 (such as 13, 86, or 10), then skip to the sixth line."
5. The fifth line says, "Assign the string, " You must be a smart kid." to the Text property of a text box called txtAnalysis."
6. The sixth line says, "This is the end of one Select Case statement."
7. The seventh line says, "This is the end of another Select Case statement."
For kicks and grins, you can put an If-Then statement inside a Select Case statement or a Select Case statement inside an If-Then statement. (Some fun, huh?)
Although no theoretical limit exists as to how many If-Then or Select Case statements you can place inside one another, the fewer you use, the easier your code is going to be to figure out. As a general rule, if you have nested more than three If-Then or Case Select statements inside one another, you probably don't know what you're doing.
When nesting multiple statements, you should indent statements so that seeing where they begin and end is easier. For example, notice how confusing the following program appears without indentation:
Select Case Salary
Case 1200
If Name = "Bob" Then
txtReview.Text = "No raise this year, ha, ha, ha!"
ElseIf Name = "Karen" Then
txt Review.Text = "Okay, how about a 5% raise?"
End If
End Select
Here's what the same program looks like with indentation:
Select Case Salary
Case 1200
If Name = "Bob" Then
txtReview.Text = "No raise this year, ha, ha, ha!"
ElseIf Name = "Karen" Then
txt Review.Text = "Okay, how about a 5% raise?"
End If
End Select
From the computer's point of view, both programs are the same. But from a programmer's point of view, the program using indentation is much easier to read and understand.
* Test your newfound knowledge
1. What is the limit to the number of control structures (If-Then or Select Case statements) you can nest?
a. The limit is determined by the restrictions your government may place upon you.
b. The limit is determined by the theoretical applications pursuant to the implications of Einstein's theory of relativity, as reworded by a lawyer.
c. The limit is 65. If you go over that, you risk getting pulled over by a state trooper.
d. No limit exists. But if you have too many nested control structures, your program is going to be harder to read and understand.
2. To make nested control structures easier to read and understand, what do you need to do?
a. Avoid using nested control structures
b. Avoid programming altogether.
c. Limit the number of nested control structures you use, and use indentation to make each If-Then or Select Case statement easy to find.
d. Print in big, bold, block letters and use short statements like "See Dick run. Dick runs fast."
++ PART VI
GETTING LOOPY
In this part....
Every program contains instructions that tell the computer what to do. Sometimes these instructions are used only once, such as instructions that tell the computer what to do when the program loads.
But other times, the computer may need certain instructions over and over again. Rather than retyping these instructions each time, programmers invented something magical called loops.
Essentially a loop tells the computer, "See those instructions over there? Keep repeating them again and again a certain number of times, and then stop." By using loops, programmers spare themselves the tedium of repeatedly writing the same instructions - and saving time is what programmers know how to do best.
++ CHAPTER 23
THE DO WHILE AND DO-LOOP WHILE LOOPS
In This Chapter ....
Using the Do While loop
Finding out how often the Do While loop runs
Examining how the Do-Loop While loop works and when to use it
Do While loops don't do anything without checking if a certain condition is True first. If a certain condition is True, then the Do While loop goes ahead and does something. Otherwise, it jumps to the next instruction in the Visual Basic program.
You can find Do While loops in such everyday experiences as when office workers tell themselves, "Keep stuffing office supplies in my briefcase while no one is looking. The moment someone looks in my direction, stop and do something else."
A Do While loop looks like this:
Do While Condition
Instructions
Loop
The condition must be a variable or an expression that represents a True or False value. A Do While loop can hold one or more instructions.
** How the Do While Loop Works
The first time Visual Basic sees a Do While loop, it says, "Okay, is the value of the condition True or False? If it's False, ignore all the instructions inside the Do While loop. If it's True, follow all the instructions inside the Do While loop."
For example, the following code has a Do While loop:
Counter = 0
Do While Counter <> 5
Counter = Counter + 1
txtCounter.Text = CStr(Counter)
Loop
Visual Basic interprets the code like this:
1. The first line says, "Stuff the value of 0 (zero) inside a variable called Counter."
2. The second line says, "As long as Counter is not equal to 5, keep repeating all the instructions sandwiched between the Do While line and the Loop line."
3. The third line says, "Add 1 (one) to the value of the Counter variable."
4. The fourth line says, "Take the value of Counter, convert it into a String, and stuff it in the Text property of a text box called txtCounter."
5. The fifth line says, "This is the end of the Do While loop. Go back to the second line where the Do While loop begins as long as the condition that Counter <> is True."
Each time this loop runs, it increases the value of Counter by one. As soon as the value of Counter equals 5, the condition Counter < > 5 suddenly becomes False and the Do While loop stops.
* How many times does the Do While loop run?
If the condition of a Do While loop is False, none of the instructions inside the Do While loop runs. In that case, the Do While loop runs zero times.
If the condition of a Do While loop is True, the Do While loop runs at least once.
If the condition of a Do While loop is always True, the Do While loop repeats itself an endless number of times until you turn off the computer or the universe explodes in another Big Bang.
When a loop repeats itself endlessly, it is called an endless loop.
Endless loops send your program into infinity and keep it from working properly (because the loop never ends). To avoid an endless loop, make sure that at least one instruction inside your Do While loop changes the True or False value of the condition that the loop checks.
* When to use a Do While loop
Use a Do While loop whenever you need to do the following:
Loop zero or more times
Loop as long as a certain condition remains True
** How the Do-Loop While Loop Works
Do-Loop While loops essentially tell Visual Basic, "Go ahead and do something until a certain condition tells you to stop."
You can find Do-Loop While loops in such everyday experiences as when parents tell their kids, "Go ahead and do what you want, just as long as you don't bother me."
A Do-Loop While loop looks like the following:
Do
Instructions
Loop While Condition
The condition must be a variable or an expression that represents a True or False value. A Do-Loop While loop can hold one or more instructions.
The first time Visual Basic sees a Do-Loop While loop, it says, "Let me follow all the instructions inside the loop first. After this, check whether the value of the condition is True or False. If it's False, stop. If it's True, repeat all the instructions inside the Do-Loop While loop again."
For example, the following code has a Do-Loop While loop.
Counter = 0
Do
Counter = Counter + 1
Loop While Counter < 5
Visual Basic interprets the code like this:
1. The first line says, "Create a variable called Counter and set its value to 0."
2. The second line says, "This is the beginning of the Do-Loop While loop."
3. The third line says, "Take the value of Counter and add 1 (one) to it. Now store this new value in the Counter variable."
4. The fourth line says, "This is the end of the Do-Loop While loop. As long as the value of Counter is less than 5, keep repeating all the instructions sandwiched between the Do line and the Loop While line. Otherwise, exit the loop."
Each time this loop runs, it increases the value of Counter by one. As soon as the value of Counter equals 5, the condition Counter < 5 suddenly becomes False and the Do While loop stops.
* How many times does the Do-Loop While Loop repeat itself?
No matter what the condition of a Do-Loop While loop may be, it always runs at least once. As long as the condition remains True, the Do-Loop While loop can keep running until infinity (or until you neglect to pay your electricity bill).
To avoid an endless Do-Loop While loop, you have to make sure that at least one instruction inside your Do-Loop While loop changes the True or False value of the condition that the loop checks.
* When to use a Do-Loop While loop
Use a Do-Loop While loop whenever you need to do the following:
Loop at least once
Loop as long as a certain condition remains True
Choose your loops carefully because loops may look similar but act differently. To avoid confusion, try to stick with one type of loop throughout your program so that it is easier for you to figure out how all your program's loops may work.
* Test your newfound knowledge
1. What are two main differences between a Do While loop and a Do-Loop While loop?
a. The words are in a different order, and they use a different number of consonants.
b. The Do While loop runs zero or more times and repeats only if its condition is True. Regardless of its condition, the Do-Loop While Loop runs at least once, and if its condition is True, it can run an infinite number of times.
c. Both loops run endlessly, faster and faster, until your computer is flung up against a wall because they're running so fast.
d. Four out of five dentists recommend the Do While loop, sugar-free chewing gum, and Crest toothpaste.
2. Why is it possible that a Do While loop runs zero or more times?
a. Because it's the number one loop used by two out of three programmers employed at Microsoft, IBM, and Symantec.
b. Nobody knows, but I remember seeing a segment on Unsolved Mysteries, asking viewers to call in if they had any information that might help resolve this question.
c. Because it checks its condition before it runs even once.
d. Because it used steroids; So not only does it run once, it runs faster than any other loop that isn't doped up.
++ CHAPTER 24
TO DO UNTIL AND DO-LOOP UNTIL LOOPS
In This Chapter...
Using the Do Until loop
Finding out how often the Do Until loop repeats itself
Examining how the Do-Loop Until loop works and when to use it
Do Until loops keep repeating until some condition becomes True, such as, "Keep stealing from the cash register until someone catches you."
A Do Until loop looks like this:
Do Until Condition
Instructions
Loop
The condition must be a variable or an expression that represents a True or False value. A Do Until loop can hold one or more instructions.
** How the Do Until Loop Works
The first time Visual Basic sees a Do Until loop, it says, "Let me check whether the value of the condition is True or False. If it's False, follow all the instructions inside the Do Until loop. If it's True, exit the Do Until loop."
For example, the following is a typical Do Until loop that counts:
Counter = 0
Do Until Counter > 4
Counter = Counter + 1
Loop
Here's an explanation of the code:
1. The first line says, "Stuff the value of 0 inside a variable called Counter."
2. The second line says, "This is the beginning of the Do Until loop. As long as the value of Counter > 4 is False, keep repeating all the instructions sandwiched between the Do line and the Loop line. Otherwise, if the value of Counter > 4 is True (when Counter is 5), exit the loop."
3. The third line says, "Take the value of Counter and add 1 (one) to it. Now store this new value in the Counter variable."
4. The fourth line says, "This is the end of the Do Until loop."
Each time this loop runs, it increases the value of Counter by one. As soon as the value of Counter equals 5, the condition Counter > 4 suddenly becomes True and the Do Until loop stops.
* How many times does the Do Until loop repeat itself?
The Do Until loop can run zero or more times. This loop keeps repeating itself until its condition becomes True.
Notice that this is the opposite of the Do While and Do-Loop While loops. These two loops keep running as long as their condition is True. The Do Until loop keeps running until its condition becomes True. (This is the same thing as saying that the Do Until loop keeps running as long as its condition is False.)
To avoid an endless Do Until loop, make sure that at least one instruction inside your Do Until loop changes the True or False value of the condition that the loop checks.
* When to use a Do Until loop
Use a Do Until loop whenever you need to do the following:
Loop zero or more times
Loop until a certain condition becomes True
The Do Until loop works like the following two Do While loops:
Do While Not Condition
Instructions
Loop
Do While Condition = False
Instructions
Loop
* How the Do-Loop Until Loop Works
Do-Loop Until loops not only sound like you're stuttering, but they keep repeating until a condition becomes True.
A Do-Loop Until loop looks like this:
Do
Instructions
Loop Until Condition
The condition must be a variable or an expression that represents a True or False value. A Do-Loop Until loop can hold one or more instructions.
The first time Visual Basic sees a Do-Loop Until loop, it says, "Follow all the instructions inside the loop once. Then check whether the value of the condition is True or False. If it's True, stop. If it's False, follow all the instructions inside the Do-Loop Until loop again."
For example, the following is a typical Do-Loop Until loop that counts:
Counter = 0
Do
Counter = Counter + 1
Loop Until Counter > 4
Here's what this code means:
1. The first line says, "Stuff the value of 0 inside a variable called Counter."
2. The second line says, "This is the beginning of the Do-Loop Until loop."
3. The third line says, "Take the value of Counter and add 1 (one) to it. Now store this new value in the Counter variable."
4. The fourth line says, "This is the end of the Do-Loop Until loop. As long as the value of Counter is 4 or less, keep repeating all the instructions sandwiched between the Do line and this Loop Until line. Otherwise, exit the loop."
Each time this loop runs, it increases the value of Counter by one. As soon as the value of Counter equals 5, the condition Counter > 4 suddenly becomes True and the Do-Loop Until loop stops.
* How many times does the Do-Loop Until loop repeat itself?
No matter what the condition of a Do-Loop Until loop may be, it always runs at least once. As long as the condition remains False, it keeps running until its condition becomes True.
Notice that this is the opposite of the Do While loop and the Do-Loop While loop. These two loops keep running as long as their conditions are True. The Do-Loop Until loop, on the other hand, keeps running as long as its condition is False.
To avoid an endless Do-Loop Until loop, make sure that at least one instruction inside your Do-Loop Until loop changes the True or False value of the condition that the loop checks.
* Test your newfound knowledge
1. If you need to loop until a certain condition becomes True, which type of loop would you use?
a. Either a Do Until loop or a Do While Not loop.
b. A loop twisted in the shape of a pretzel.
c. A loop-the-loop.
d. Have you noticed that if you stare at the word loop long enough, it starts to look funny?
2. When do you use a Do Until loop and when do you use a Do-Loop Until loop?
a. Whenever I need to confuse myself on how different loops work.
b. When I get mixed up and use the wrong loop by mistake.
c. I use the Do Until loop if I need to loop zero or more times. I use the Do-loop Until loop if I need to loop at least once.
d. When I can't remember how to use a Do While loop. Can you explain it to me again?
* When to use a Do-Loop Until Loop
Use a Do-Loop Until loop whenever you need to do the following:
Loop at least once
Loop until a certain condition becomes True
The Do-Loop Until loop is equal to the following two Do-Loop While loops:
Do
Instructions
Loop While Not Condition
Do
Instructions
Loop While Condition = False
When creating loops, always make sure that they eventually end and that they do exactly what you want them to do. If anything goes wrong with your program, look to see if your loops are causing the problem first.
++ CHAPTER 25
FOR NEXT LOOPS THAT CAN COUNT
In This Chapter....
Using the For Next loop
Counting backward and forward
Using the Step increment
If you want to loop until a certain condition becomes True or False, use one of the following principles:
Loops at least once:
As long as condition = True - Do Loop While condition
As long as Condition = False - Do loop Until condition
Loops zero or more times:
As long as Condition = True - Do While condition Loop
As long as Condition = False - Do Until condition Loop
All four of these types of loops keep running until a certain condition becomes True or False. But if you already know how many times you want a loop to run, use a For Next loop.
A For Next loop looks like the following:
For Counter = Start to End Step X
Instructions
Next Counter
The counter is a variable that represents an integer. Start represents the first number assigned to the value of the counter. End is the last number assigned to the value of the counter. Step is the interval to count by. If Step is omitted, the interval defaults to 1.
** How the For Next Loop Works
If you want to loop exactly three times, you can use the following code:
For X = 1 To 3
Instructions
Next X
Here's what this code means:
1 . The first line says, "Create a variable called X and set its value equal to 1. Keep looping as long as the value of X is either 1, 2, or 3. The moment the value of X is no longer one of these values, stop looping."
2. The second line is where you can shove one or more instructions (including additional For Next loops if you want).
3. The third line says, "Okay, get the next value of X (by adding 1 to the value of X) and go back to the first line. At this point, the value of X is now 2."
The following line tells Visual Basic to loop three times:
For X = 1 To 3
By default, Visual Basic counts by one. For grins and laughs, you can use any combination of numbers that you want, such as:
For X = 1209 To 1211
Instructions
Next X
This For Next loop also loops three times, although it's not as easy to tell that just by looking at it:
The first time, X = 1209.
Thesecond time, X = 1210.
The third and last time, X = 1211.
You can count by such bizarre numbers if these numbers somehow make sense to your program. For example, you can count by employee numbers:
For EmployeeNumber = 11250 To 11290
' Use the value of EmplyeeNumber to search a database
' of employees and print their background information
Next EmployeeNumber
In this case, the instructions inside your For Next loop use the value of EmployeeNumber to find a specific employee.
If you just need to loop a particular number of times, such as five, use the simplest and most straightforward method, as shown in the following:
For X = 1 To 5
Instructions
Next X
Only if your numbers must be used inside your For Next loop should you resort to bizarre, hard-to-read counting methods, such as:
For Counter = 3492 To 12909
Instructions
Next Counter
** Counting Backward and Forward
Normally, the For Next loop counts forward by 1s. However, if you want to count by 5s, 10s, 13s, or 29s, you can. To count by any number other than 1, you have to specify a Step increment. For example:
For Counter = start To end Step increment
Instructions
Next counter
Adding the Step increment instruction tells Visual Basic, "Instead of counting forward by ones, count by the value of the increment that follows the word Step." If you wanted to count by 16s, you would use the following code:
For X = 0 To 32 Step 16
Instructions
Next X
This For Next loop actually loops just three times:
The first time, X = 0
The second time, X = 16
The third time and last time, X = 32
If you want, you can even count backward. To count backward three times, you could use the following code:
For X = 3 To 1 Step - 1
Instructions
Next X
Here's what the preceding code means:
1. The first line says, "Create a variable called X, set its value to 3, and count backward by -1"
2. The second line contains one or more instructions to follow.
3. The third line says, "Choose the next value of X. Because we're counting backward by -1, the new value of X will be X -1. The second time, X will be 2."
Although Visual Basic doesn't care about how you count, always choose the simplest method whenever possible. That way, you and any other programmers can quickly see how many times a For Next loop keeps looping.
Count backward or by unusual numbers (increments of 3, 5, 16, and so on) only if instructions in a For Next loop need the numbers. Otherwise, you only make your program harder to read.
So what happens if you write a For Next loop like the following?
For J = 1 To 7 Step 5
Instructions
Next J
Here's what this code means:
1. This For Next loop repeats itself twice. The first time, the value of J is 1.
2. The second time, the value of J is 1 + 5 (remember, the value of Step is 5), or 6.
3. Before it can repeat a third time, the loop changes the value of J to 6 + 5, or 11. Because 11 is greater than the specified range of J = 1 To 7, the For Next loop refuses to loop a third time and quits.
** Use Caution When Using a For Next Loop with the Step Increment
A For Next loop must create its own variable to do its counting. For example, the following code creates a variable called X Y Z that counts by 10s:
For XYZ = 1 To 50 Step 10
Instructions
Next XYZ
And the next code creates a variable called TUV that counts by increments of 1.5:
For TUV = 1 To 7 Step 1.5
Instructions
Next TUV
For Next loops usually count by whole numbers, such as 1, 2, 5, or 58, so it's not difficult to determine the number of loops there will be. The first code example just shown counts by 10, so the number of times it will loop is pretty easy to figure out. (It loops five times.)
The second example, however, counts in increments of 1.5; because of this decimal increment, it's harder to tell how many times it will loop. (It loops five times.) When using the Step increment, use whole numbers so that you can see the number of loops more easily.
When using a For Next loop, never (and I repeat, never) change the value of the counting variable within the loop. The loop will get messed up, as the following example illustrates:
For X = 1 To 5
X = 3
Next X
Here's what happens:
1. The first line says, "Create a variable called X and set its value to 1."
2. The second line says, "Assign the value of 3 to a variable called X."
3. The third line says, "Add 1 to the value of X. Because X is equal to 3, now make X equal to 4."
Because X always equals 4 at the end of each loop, this For Next loop becomes an endless loop, which never stops. So when using For Next loops, make sure that none of the instructions inside the loop changes the counting variable. Otherwise, you'll be sorry...
** When to Use a For Next Loop
Use a For Next loop whenever you want to loop a specific number of times.
Just to show you that it is possible, you can also use other types of loops to count. The following two loops will loop exactly six times:
X = 0
Do While X < 6
X = X + 1
Instructions
Loop
For X = 1 To 6
Instructions
Next X
Notice how simple and clean the For Next loop is compared to the Do While loop. There are an infinite number of ways to write a program that works (and an even greater number of ways to write programs that don't work), but the simplest way is usually the best.
* Test your newfound knowledge
1. How many times will the following For Next loop repeat itself?
For ID = 15 To 1 Step -1
Instructions
Next ID
a. Fifteen times.
b. One time, but fifteen times as fast.
c. Zero or more times, or something like that. Wait a minute. I think I'm in the wrong lesson.
d. None, because only history can repeat itself.
2. What is the main advantage of a For Next Loop over a Do While loop?
a. A Do While loop is more complicated to use, and a For Next loop doesn't work at all.
b. It all depends on your of view, man. Like, all things are good if we only love one another and live in peace and harmony.
c. You can specify how many times you want a For Next loop to repeat itself.
d. There is no advantage to learning Visual Basic. You should be learning C++ or Java instead.
** Try a For Next Loop for Yourself
If you don't want to type any code right now, just run and study the source code for this chapter, stored on the enclosed CD-ROM.
This sample program runs through a For Next loop and prints the value of X on the screen each time it loops. To create this program for yourself, use the settings defined in the following table:
Object - Form
Property - Caption
Setting - A For Next Loop Example
Object - Command1
Property - Caption
Setting - Next X
Property - Name
Setting - cmdNext
Type the following in the Code window:
Private Sub cmdNext_Click( )
For X = 1 To 10 Step 2
Print X
Next X
End Sub
This program does nothing more exciting than loop five times and print the value of X on the form each time.
++ CHAPTER 26
NESTED LOOPS AND QUICK EXITS
In This Chapter:
Using nested loops
Making nested loops work
Exiting quickly from loops
For the ultimate in flexibility and complexity, you can jam loops inside other loops to create an endless series of loops. Whenever you have one loop stuffed inside another loop, it's called a nested loop. So which loop runs and completes first? The answer is simple.
** Using Nested Loops
When you have nested loops, the loop inside finishes first as shown in the following example:
Do While Employee = "Supervisor"
For J = 1 To 5
Instructions
Next J
Loop
Now, here's what this code means:
1. The first line says, "Create a variable called Employee and check to make sure its value is equal to "Supervisor". If it is, move to the second line. If it isn't, don't even bother looking at the For Next loop inside; simply skip to the fifth line."
2. The second line says, "Create a variable called J and set its value to 1."
3. The third line says, "Follow these instructions, whatever they may be."
4. The fourth line says, "Increase the value of J by 1 and jump back to the first line again. Keep doing this until the value of J is greater than 5."
5. The fifth line says, "This is the end of the Do While loop. Keep repeating as long as the variable Employee is equal to "Supervisor "."
In the preceding example, the For Next loop finishes before the Do While loop. The For Next loop also runs one complete time through for every thime you go through the Do While loop.
** Making Nested Loops Work
Naturally, Visual Basic gives you the complete freedom to cram as many loops inside one another as you want. When creating nested loops, indent each loop to make it easier to see where each loop begins and ends. For example, notice how confusing the following nested loops look without indentation:
Do While Name = "Sam"
Do
For K = 20 To 50 Step 10
Do
Do Until Sex = "Male"
' Change some variables here
Loop
Loop While Age > 21
Next K
Loop Until LastName = "Doe"
Loop
This is what the same code looks like with indentations:
Do While Name = "Sam"
Do
For K = 20 To 50 Step 10
Do
Do Until Sex = "Male"
' Change some variables here
Loop
Loop While Age > 21
Next K "Doe"
Loop Until LastName = "Doe"
Loop
From the computer's point of view, both nested loops work the same. But from a programmer's point of view, the nested loops using indentation are much easier to read and understand.
With so many nested loops, make sure that the inside loops don't accidentally mess up the conditions or counting variables of the outer loops. Otherwise, you may create an endless loop and have to examine all your loops to find the problem.
Another problem that can prevent nested loops from running is if they are tangled, as in the following example:
For K = 1 To 4
For J = 2 To 20 Step 2
Next K
Next J
In this example, the two For Next loops intertwine because the first For Next loop ends before the second, inner For Next loop can end. Fortunately, if you run the above BASIC code, Visual Basic catches this mistake so this type of problem is easy to correct.
** Quick Exits from Loops
A Do loop continues running until a certain condition becomes True or False. A For Next loop continues running until it finishes counting. But what if you need to exit a loop prematurely? In that case, you can bail out of a loop by using the magic Exit command.
To bail out of a Do loop, use the Exit Do command, as shown in the following example:
X = 0
Do While X < 6
X = X + 1
If X = 4 Then Exit Do
Loop
This Do While loop continues looping as long as the value of a variable called X is less than 6. The moment the value of X equals 4, Visual Basic runs the Exit Do command.
The Exit Do command bails Visual Basic out of the loop, even though the value of X is still less than 6.
To bail out of a For Next loop, use the following:
Exit For
For example:
For Y = 1 To 100
If Y = 50 Then Exit For
Next Y
Normally this For Next loop repeats 100 times but the second line tells Visual Basic to ball out of the For Next loop as soon as the value of Y equals 50 - even if the value of Y is still less than 100.
Usually (notice the emphasis on usually), a good idea is to provide a way to bail out of a loop prematurely, just in case the user needs to do something else. However, make sure that using Exit Do or Exit For doesn't kick you out of the loop before you want it to. Otherwise, you'll have another bug to hunt and track down.
If you use the Exit Do/For commands within a loop nested inside another loop, the Exit Do/For commands exit out of only the current loop and then return control to the outer loop.
* Test your newfound knowledge
1. What is the limit to the number of loops you can nest?
a. Theoretically, the number is infinity. Practically, the number is as many as you feel like typing, although the more nested your loops are, the harder it is to see what each one does.
b. The number of loops is limited to your yearly allotment, as defined by Microsoft when you send in your registration card.
c. Five.
d. Discovered by Einstein, the limit to the number of nested loops is equal to the same value that represents the speed of light.
2. To make nested loops easier to read and understand, what should you do?
a. Avoid using loops, control structures, variables, or anything else that requires,thinking.
b. Absolutely nothing. If people can't understand my nested loops, that's their problem.
C. Avoid indentation, because only amateurs need to rely on such editing tricks to write programs.
d. Use plenty of indentations to make the beginning and ending of each loop easy to find.
++ PART VII
WRITING SUBPROGRAMS (SO YOU DON'T GO CRAZY ALL AT ONCE)
In this part....
Now is the time to find out how to divide your Visual Basic program into smaller programs so you can create programs that are easier to write, modify, and understand. Rather than write one huge, monolithic program (which is like carving a mansion out of a single piece of granite), subprograms let you create miniature programs and paste them together to make one larger program (like using bricks to build a house).
By dividing one large program into several smaller ones, you can test each part of your program before moving on to writing another part of the program. By conquering each task of your program one by one, you can also maintain your sanity so that you don't go nuts trying to create one monster program in one sitting.
++ CHAPTER 27
GENERAL PROCEDURES (SUBPROGRAMS THAT EVERYONE CAN SHARE)
In This Chapter....
Creating general procedures
Naming general procedures
Using general procedures
Pocedures are small programs that make up a single larger program, much like bricks make up an entire wall. Visual Basic has two types of procedures: event procedures and general procedures.
An event procedure is part of a user interface object, such as a command button, check box, or scroll bar. Event procedures run only when a certain event occurs to a certain object, such as clicking the mouse on a command button or a check box.
A general procedure isn't attached or connected to any specific objects on the user interface. A general procedure doesn't do anything until an event procedure (or another general procedure) specifically tells it to get to work.
So do you need event procedures? Yes. Event procedures make your user interface responsive. Do you need general procedures? No. General procedures exist solely for the programmer's convenience.
If two or more event procedures contain nearly identical instructions, typing the same instructions over and over would be repetitive. Even worse, if you needed to modify the instructions, you would have to change these instructions in every event procedure that used those same instructions.
As an alternative, you can use general procedures. The whole purpose of general procedures is to hold commonly used instructions in one place. That way, if you need to modify the instructions, you change them in just one place.
** How to Create a General Procedure
After you've drawn all the objects that make up your user interface, Visual Basic automatically creates empty event procedures for all your user interface objects.
Unfortunately, Visual Basic doesn't create a single general procedure for you; you have to create it yourself. You can create and save general procedures in two types of files:
FRM (form) files
BAS (module) files
When you save a general procedure in an FRM (form) file, that general procedure can be used only by event or general procedures stored in that same FRM file. (The one exception is that any part of your program can use a general procedure that is stored in an FRM form file, but only if that form file is loaded into memory at the time.) When you save a general procedure in a BAS (module) file, however, the general procedure can be used by any event or general procedures that make up your Visual Basic program.
If you save your general procedures in a BAS file, you can create a library of useful general procedures that you can plug into any other Visual Basic programs you write. If your general procedures are useful for only one specific program, store them in an FRM file.
To create and save a general procedure in an FRM (form) file, follow these steps:
1. Click on a form file in the Project Explorer window and then press F7; choose View->Code; or click on the View Code icon to open the Code window.
2. Select (General) In the Object list box.
3. Choose Tools->Add Procedure. Visual Basic displays the Add Procedure dialog box.
4. Select the Sub radio button, type your procedure name in the Name box, and press Enter or click on OK.
Visual Basic displays an empty general procedure.
To create and save a general procedure to a new BAS (module) file, follow these steps:
1. Choose Project->Add Module. An Add Module dialog box appears.
2. Click on the Module icon and then click on Open.
3. Choose Tools->Add Procedure. Visual Basic displays the Add Procedure dialog box.
4. With the Sub radio button selected, type your procedure name in the Name box, and press Enter or click on OK.
Visual Basic displays an empty general procedure.
To create a general procedure and save it to an existing BAS file, follow these steps:
1. Choose View->Project Explorer, press Ctrl+R, or click on the Project Explorer icon on the toolbar.
2. Click on the module file where you want to save your general procedure; then click on the View Code icon.
Visual Basic displays the Code window for that particular BAS module file.
3. Choose Tools->Add Procedure. Visual Basic displays an Add Procedure dialog box.
4. With the Sub radio button selected, type your procedure name in the Name box, and press Enter or click on OK.
Visual Basic displays an empty general procedure.
** How to Name General Procedures
Unlike with event procedure names (which identify the object name and the event), you can name general procedures anything you want, with the following restrictions:
The name must be 40 characters or fewer.
The name must begin with a letter and can consist of only letters, numbers, and the underscore character (_).
The name can't be a reserved word that Visual Basic already uses such as End, Sub, or Private.
Ideally, you want to use names for your general procedures that describe what they do. For example:
CubeRoot
Ask4Password
DisplayWindow
These complete procedure names would appear in the Code window as follows:
Public Sub CubeRoot( )
End Sub
And
Public Sub Ask4Password( )
End Sub
And
Public Sub DisplayWindow( )
End Sub
Notice that a complete general procedure name consists of four parts:
Public (or Private)
Sub
Your general procedure name
A pair of parentheses ( )
The word Public tells Visual Basic that the general procedure is public. So, if the general procedure is stored in a BAS file, any event or general procedures stored in other FRM or BAS files can use it. (If you don't want procedures stored in other files to use your general procedure, just use the word Private instead.)
The word Sub identifies your subroutine as a procedure. (If you replace the word Sub with the word Function, you can create a general function. I explain functions in Chapter 29.) Your procedure name is the name that event procedures and other general procedures use to call your general procedure. Calling a procedure means telling a particular procedure, "Okay, do something now!"
The pair of parentheses is called the argument list (which you can read about in Chapter 28). The simplest general procedures have an empty argument list, represented by an empty pair of parentheses.
** How to Use a General Procedure
A general procedure contains one or more instructions. When another procedure wants to use those instructions that are stored in a general procedure, it calls this general procedure by name.
You can call a procedure in two ways. You can state the procedure's name:
ProcedureName
Or, you can state the procedure's name along with the word Call:
If you want to use a general procedure that is stored in another FRM form file, you can call it by specifying the form name and then the procedure name, such as:
FormName.ProcedureName
Or you can state the procedure's name along with the word Call:
Your computer doesn't care which method you use. But whatever method you choose, use it consistently - your program's easier to read that way.
Stating only the procedure name is simpler than using the word Call, but the latter helps identify all the procedure calls in your program. For example, consider the following general procedure stored in a form called frmMainForm:
Public Sub Warning( )
MsgBox "Your computer will blow up in 3 seconds!", 16,
"Warning!"
End Sub
This general procedure simply displays a dialog box with the message "Your computer will blow up in 3 seconds!".
If an event procedure, stored in a different form, wanted to use or call this general procedure, it would look like the following:
Public Sub cmdAlert_Click( )
frmMainForm.Warning
EndSub
If the event procedure used the Call method of calling a procedure, it would look like this:
Public Sub cmdAlert_Click( )
Call frmMainForm.Warning
End Sub
Both of these event procedures are equivalent to the following:
Public Sub cmdAlert_Click( )
MsgBox "Your computer will blow up in 3 seconds!", 16,
"Warning!"
End Sub
Although this example is simple, you can see that, if you cram two or more instructions inside a general procedure, you won't have to type these same instructions over and over again in multiple places. Instead, you can just call a single general procedure.
* Test your newfound knowledge
1. Why would you need to use a general procedure?
a. Because a more specific procedure won't do the job.
b. To store commonly used instructions in one place so that the procedure is easy to modify.
c. To keep your event procedures from getting lonely.
d. There's no reason to use general procedures - cool programmers have no need of such crutches.
2. What are the two ways to use, or call, a general procedure?
a. Telephone or mail a letter.
b. State the general procedure's name or insert the word Call before the general procedure's name.
c. See your favorite psychic and participate in a seance.
d. Get your local hog-calling contest winner to shout real loud for you.
++ CHAPTER 28
PASSING ARGUMENTS
In This Chapter....
Sending arguments to a procedure
Passing arguments by value
Quitting a procedure early
When a procedure calls a general procedure, it does so by name. Calling a general procedure by name essentially tells that general procedure, "Hey, get busy and start doing something."
Many times, the called general procedure needs no further instructions when its name is called. Sometimes, though, the called general procedure needs additional information before it can do anything.
Any procedure can call a general procedure and give it data to work with. This data is called an argument. Essentially, the first procedure is saying, "Don't argue with me. Here's all the information you need to get busy. Now get to work."
** Why Use Arguments?
An argument is data - numbers, strings, or variables (which represent a number or string) - that a general procedure needs to work with. By using arguments, you can write a single, nonspecific procedure that can replace two or more specialized general procedures.
For example, you can create two general procedures, as follows:
Public Sub DisplayWarning( )
TxtReadMe.Text = "Warning! Nuclear meltdown has occurred!"
End Sub
and
Public Sub DisplayCaution( )
txtReadMe.Text = "Caution! Turn off the reactors now!"
End Sub
To use either procedure, you call them in one of two ways:
DisplayWarning or Call DisplayWarning
DisplayCaution or Call DisplayCaution
However, creating two procedures that do almost the same thing is tedious and wasteful, which makes it a perfect technique to use in a job you don't like.
Instead of writing near-duplicate copies of the same procedure, you can replace both of those procedures with a single one, such as the following:
Public Sub Display(Message As String)
txtReadMe.Text = Message
End Sub
This new procedure says, "Create a variable called Message that holds any string that another procedure will give me. Whatever this value may be, stuff it in the Text property of the text box called txtReadMe.
Using a general procedure, you can choose the type of text by calling the Display procedure like this:
Display ("Warning! Nuclear meltdown has occurred!")
Or
Display ("Caution! Turn off the reactors now!")
** Sending Arguments to a Procedure
To call a procedure and send an argument to it, you can use one of three methods:
ProcedureName Argument
ProcedureName (Argument)
Call ProcedureName (Argument)
Suppose you had the following general procedure:
Public Sub Display(Message As String)
TxtReadMe.Text = Message
End Sub
To call the preceding procedure and display the message "Warning! Nuclear meltdown has occurred!", you can use one of three methods:
Display "Warning! Nuclear meltdown has occurred!"
Display ("Warning! Nuclear meltdown has occurred!")
Call Display ("Warning! Nuclear meltdown has occurred!")
All three methods are equivalent.
Now here's what calling the Display general procedure does:
1. First it tells Visual Basic, "Find a general procedure named Display and send it one argument." In this case, the one argument is the string, "Warning! Nuclear meltdown has occurred!"
2. Visual Basic finds a general procedure called Display. The Display procedure says to assign whatever argument it gets to the variable Message.
3. Then the Display procedure says, "Stuff the value of Message into the Text property of the text box called txtReadMe. Because the value of Message is the string "Warning! Nuclear meltdown has occurred!", stuff this value into the Text property."
If you call the procedure in the following way:
Display ("Caution! Turn off the reactors now!")
the Text property of the text box called txtReadMe gets stuffed with the string "Caution! Turn off the reactors now!"
By using the same procedure but feeding it different arguments, you replace two specialized general procedures with a single general procedure.
** Accepting Arguments
Before a general procedure can accept arguments, you have to define the procedure's argument list. Essentially, this list defines how many arguments the general procedure can take.
For example, to define a general procedure that won't take any arguments, you use a pair of empty parentheses, as follows:
Public Sub NoBackTalk( )
End Sub
To call this procedure, you can choose one of two ways:
NoBackTalk
Call NoBackTalk
To define an argument list that takes one argument, you do the following:
Public Sub BackTalk(Something)
End Sub
In this case, the variable called Something is by default a Variant data type (see Chapter 15 for more information about data types), which can represent a number or string. To call this procedure and give it the argument 4, you can do one of the following:
BackTalk 4
BackTalk (4)
Call BackTalk (4)
To define an argument list that takes two or more arguments, you have to specify a variable for each argument and separate each by a comma. For example, the following defines three arguments:
Public Sub Chatty(Message, Reply, Gossip)
End Sub
This argument list defines three arguments that can represent a number or a string. To call this procedure and give it the arguments 30, "Hello", and 12.9, you must do one of the following:
Chatty 30, "Hello", 12.9
Call Chatty (30, "Hello", 12.9)
There is no practical limit to the number of arguments that a procedure can accept. However, the longer your argument list is, the more complicated your procedure must be and the more likely you may get confused trying to understand exactly what your procedures do.
** Defining argument types
Besides defining the number of arguments in an argument list, you have the option of defining the type of data each argument must represent.
For example, you can define an argument to represent only one of the following data types (see Chapter 15 for more information about data types):
Integer
Long
Single
Double
Currency
String
You have to define argument types in the argument list. For example:
Public Sub Convert(Fahrenheit As Integer, Celsius As Integer)
The preceding line of code defines two arguments, both of which must represent an integer. The following code shows the only procedure call that works:
Public cmdTest_Click( )
Dim X, Y As Integer
Dim A, B As String
Dim M, N As Single
Call Convert(X, Y) ' This would work
Call Convert(A, B) ' Neither A nor B are Integers
Call Convert(M, N) ' Neither M nor N are Integers
Call Convert("Hello", X) ' "Hello" is not an Integer
End Sub
The main reason to specify an argument's type is to prevent your procedure from trying to work with incorrect data. For example, if your procedure expects a string but gets an integer, your program can crash. Even worse, it may run correctly but contain the wrong information. In this case, you have created a bug called a logic error. (For more about logic errors, check out Chapter 20)
Although you don't have to declare an argument's type, doing so is a good idea, just to help prevent any bugs from breeding inside your precious code.
** Problems with sending arguments
Two problems may occur when calling procedures. One problem is when the number of arguments sent doesn't match the number of arguments defined by the procedure. The other problem occurs when the types of arguments sent don't match the types of arguments defined by the procedure.
** Giving the wrong number of arguments:
When you define a procedure with an argument list, the argument list defines the number of arguments it needs to run. If you call this procedure and don't give it the correct number of arguments, the procedure doesn't work. For example:
Public Sub ArgueWithMe(Flame)
End Sub
This procedure expects one argument, which can be a number or a string. None of the following calls to this procedure works, because the number of arguments is not one:
ArgueWithMe
ArgueWithMe 9, "Shut up!"
ArgueWithMe("Why?", "Go away!", 4500, "Okay.")
** Giving the wrong type of arguments:
Likewise, when calling a procedure, always make sure that the arguments have the same data types as the types defined in the argument list. For example:
Public Sub ArgueWithMe(Flame As String)
End Sub
This procedure expects one argument, which must be a String data type. None of the following calls to this procedure work because the arguments are not String data types:
ArgueWithMe(78.909)
ArgueWithMe(9)
Call ArgueWithMe(34)
** Passing arguments by value
To further protect your procedures from messing around with each other's variables, you can pass arguments by value. Normally, when you pass an argument to a procedure, the new procedure can change the argument's value. This is like giving somebody a drink and having them spit in it and hand it back to you.
If another procedure needs certain data, but you don't want that other procedure to risk changing the data used by other parts of your program, you can pass an argument by value. So, you give a procedure an argument, and the procedure can change the argument all it wants. However, any changes it makes to this variable are limited within that particular procedure. This situation is like pouring part of your drink into another cup and giving it to another person. If that person decides to spit in his or her cup, your drink remains uncontaminated.
To define an argument as passed by value, you use the ByVal keyword in the argument list, like this:
Public Sub ShowMe(ByVal Name As String)
You never need to use the ByVal keyword to call a procedure.
Suppose you have an event procedure such as the following:
Private Sub Command1_Click( )
Dim MyString As String
MyString = "John Doe"
ShowMe MyString
txtMessage.Text = MyString
End Sub
and a general procedure such as the following:
Public Sub ShowMe (ByVal Name As String)
Name = UCase(Name)
txtNewMessage.Text = Name
End Sub
Visual Basic follows the instructions in the Command1_Click( )event procedure in the following way:
1. When the user clicks on a command button named Command 1, Visual Basic starts to follow the instructions in the Private Command1_Click( ) event procedure.
2. The second line in the Command1_Click( ) event procedure tells Visual Basic to declare a variable called MyString that can only hold string values.
3. The third line stuffs the string "JohnDoe" into the MyString variable.
4. The fourth line calls the general procedure called ShowMe and sends it the value of the MyString variable as a single argument.
5. Visual Basic immediately jumps to the general procedure called ShowMe. The ByVal keyword tells Visual Basic that any changes it makes to the argument received are isolated within the ShowMe general procedure.
6. The first line of the ShowMe general procedure defines a new variable called Name and defines it to hold a string.
7. The second line of the ShowMe general procedure says to use the UCase keyword to turn all the letters of the Name variable into uppercase and store the result back in the Name variable. In this case, the value of the Name variable is now "JOHN DOE ".
8. The third line of the ShowMe general procedure says to display the Name value in the Text property of a text box named txt NewMessage. In this case, the txtNewMessage text box displays "JOHN DOE " .
9. The fourth line of the ShowMe general procedure says this is the end of the general procedure; now go back to the procedure that was originally called the ShowMe general procedure. This information tells Visual Basic to jump back to the fifth line of the Command1_Click( ) event procedure.
10. The fifth line of the Command1_Click( ) event procedure says to take the value of MyString and stuff it in the Text property of a text box named txtMessage. In this case, the txtMessage text box displays "John Doe".
11. The sixth line of the Command1_Click event procedure tells Visual Basic to stop thinking anymore.
If you remove the ByVal keyword from the ShowMe general procedure, the txtMes sage text box in Step 10 displays "JOHN DOE" . The ByVal keyword simply keeps the general procedure from messing up variables used by other procedures.
To specify that an argument be passed by value, you must put the ByVaI keyword in front of each argument:
Public Sub BlackBox(ByVal X As Integer, Y As Integer)
In the preceding example, only the argument X is passed by value. The Y variable is not. To specify that the Y argument also be passed by value, you have to do the following:
Public Sub BlackBox(ByVal X As Integer, ByVal Y As Integer)
** Quitting a Procedure Prematurely
Normally, a procedure runs until all its instructions are followed. However, you may want to exit a procedure before it finishes.
To exit a procedure prematurely, you have to use the following code:
Exit Sub
For example, you may have a procedure like the one that follows:
Public Sub Endlessloop( )
X = 0
Do
X = X + 1
If (X - 13) Then
Exit Sub
End If
Loop Until X = 25
End Sub
Normally, this loop would keep repeating until the value of X equals 25. However the If-Then statement inside the Do - Loop causes this procedure to end when X equals 13.
** Try Passinig Arguments Yourself
If you want to try your hand at a passing argument, head over to the CD. There you find a sample program that lets you type text in a text box and see how passing a variable by value (using the ByVal keyword) causes the variable's value to change only within that particular procedure. This program is stored under the name CALL.VBP.
* Test your newfound knowledge
1. Explain why argument passing is useful in writing programs.
a. Argument passing is like passing the buck. Programmers do this all the time to avoid taking responsibility when their project is behind schedule.
b. Passing arguments lets you write one general-purpose procedure to replace two or more specialized procedures.
c. Arguments let you give wrong information to your procedures, so they have twice as many chances of wrecking your entire project.
d. Argument passing is like scream therapy. Each side argues for its own point until both sides are exhausted, which prevents people from shooting each other at work.
2. Explain what the following argument list means:
Public Sub Confusion(ByVal Catch As String, X As Integer, Z)
End Sub
a. Crud, I have to review this section all over again.
b. Now I finally understand why programmers look and act the way they do, if they have to spend eight hours a day deciphering cryptic commands like this.
c. Someone didn't comment the code correctly to make this argument easy to understand.
d. The procedure expects three arguments. The first argument is called Catch, is passed by value, and must be a String data type. The second argument is called X and must be an Integer data type. The third argument is called Z and can be any data type.
++ CHAPTER 29
FUNCTIONS, A UNIQUE TYPE OF SUBPROGRAM
In This Chapter....
Calling functions
Defining argument types
Quitting a function early
Functions return a single value. Procedures on the other hand, return zero or more values. When you need to calculate only a single value, use a function. When you need to calculate zero or more than two values, use a procedure.
A typical function looks like the following:
Public Function FunctionName(ArgumentList) As DataType
FunctionName = SomeValue
End Function
The word Public tells Visual Basic that if the function is stored in a BAS file, that this function can be used by all event and general procedures in your Visual Basic program. (If the function is stored in an FRM file, this function can be used only by event and general procedures stored in that same FRM file.)
The word Function defines the subprogram as a function. The FunctionName can be any valid Visual Basic name, preferably one that describes what the function does. The Argument List can containzero or more arguments. The DataType defines what type of data the function returns, such as an integer or string.
Visual Basic includes several built-in functions that you can use in your programs. Table 29-1 lists a few of these built-in functions.
||
TABLE 29-1
SOME COMMONLY USED VISUAL BASIC FUNCTIONS
Built-In Visual Basic Function - Abs(number)
What It Does - Returns the absolute value of a number
Built-In Visual Basic Function - Date
What It Does - Returns the current system date
Built-In Visual Basic Function - Lcase(string)
What It Does - Converts a string to lowercase
Built-In Visual Basic Function - Sqr(number)
What It Does - Returns the square root of a number
** How to Create a Function
You can create and save functions in two types of files:
FRM (form) files
BAS (module) files
When you save a function in an FRM (form) file, that function can be used only by procedures or functions stored in that same FRM file. (The one exception is that any part of your program can use a function stored in an FRM form file, but only if that form file is loaded into memory at the time.) When you save a function in a BAS (module) file, the function can be used by any procedures or functions that make up your Visual Basic program.
If you save your functions in a BAS file, you can create a library of useful functions that you can plug into any other Visual Basic programs you write. If your functions are useful for only one specific program, store them in an FIRM file.
To create and save a function in an FRM (form) file, follow these steps:
1. Click on a form file in the Project Explorer window and then press F7, choose View->Code, or click on the View Code Icon to open the Code window.
2. Select (General) in the Object list box.
3. Choose Tools->Add Procedure. Visual Basic displays the Add Procedure dialog box.
4. Click the Function radio button. Then type your function name In the -Name box, and press Enter or click on OK.
Visual Basic displays an empty function.
To create and save a function to a new BAS (module) file, follow these steps:
1. Choose Project->Add Module. An Add Module dialog box appears.
2. Click on the Module Icon and then click on Open.
3. Choose Tools->Add Procedure. Visual Basic displays the Add Procedure dialog box.
4. Click the function radio button. Then type your function name In the Name box, and press Enter or click on OK. Visual Basic displays an empty function.
To create a function and save it to an existing BAS file, follow these steps:
1. Choose View->Project Explorer, press Ctrl+R, or click on the Project Explorer Icon on the toolbar.
2. Click on the module file where you want to save your general procedure. Then click on the View Code Icon.
Visual Basic displays the Code window for that particular BAS module file.
3. Choose Tools->Add Procedure. Visual Basic displays an Add Procedure dialog box.
4. Click the function radio button. Then type your function name in the Name box, and press Enter or click on OK. Visual Basic displays an empty function.
** Assigning a Value to a Function
Somewhere inside the function, you must assign the function's name to a value or an expression, such as the following:
Public Function YardsToMeters(Yards As Single) As Single
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
If you don't assign a value to a function's name, the function can't return any value - and the whole point of using functions is to return a value.
You can also define the specific data type of the value that a function returns, such as Integer, String, or Currency. For everything you've ever wanted to know about data types, check out Chapter 15.
The three main differences between a function and a procedure are as follows:
A function can return only one value. A procedure can return zero or more values.
Somewhere inside the function, the function's name must be assigned a value. You never have to do this with a procedure.
You can define the data type that a function represents. You cannot define a procedure to represent a data type (but you can define the data types of a procedure's argument list).
** Calling Functions
Calling a function is different than calling a procedure. Because functions represent a single value, you call a function by assigning the function name to a variable:
Public Function YardsToMeters (Yards As Single)
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
Private Sub cmdConvert_Click( )
Dim Meters As Single
Meters = YardsToMeters(CSng(txtYards.Text))
TxtMetric.Text = Cstr(Meters)
End Sub
This event procedure says, "When the user clicks on a command button called cmdConvert, do the following:"
1. Create a variable called Meters and define this variable to hold only Single datatype.
2. Take whatever value is stored in the Text property of a text box called txtYards and use the value as an argument for the YardsToMeters function.
3. The YardsToMeters function takes txtYards.Text as its argument, multiplies the argument by 0.9, and stores this new result in the YardsToMeters function name. The result stored in the YardsTo Meters function name gets stuffed into the Meters variable.
4. The value stored in the Meters variable gets converted into a string and stuffed into the Text property of a text box named txtMetric.
Note the differences in calling procedures and calling functions. When calling a procedure, you can use one of three methods:
ProcedureName ArgumentList
ProcedureName(ArgumentList)
Call ProcedureName(ArgumentList)
You have only one way to call a function:
Variable = FunctionName(ArgumentList)
Because a function name represents a single value, you can use a function name in any mathematical expression, such as:
Variable = FunctionName(ArgumentList) + Variable
So a procedure that calls a function called YardsToMeters may look like this:
Private Sub cmdConvert_Click( )
Dim Meters As Single, NewValue As Single
NewValue = (YardsToMeters (Meters) + 32) * 4
End Sub
** Defining a Function as a Certain Data Type
Because a function returns a single value, you can specify what data type that value represents.
Take a look at the following, for example:
Public Function YardsToMeters (Yards) As Single
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
This defines the value of YardstoMeters as a Single datatype. You can define a function to represent any one of the following:
Integer
Long
Single
Double
Currency
String
No matter what data type a function represents, any variables assigned to the function must be of the same data type. For example:
Public Function YardsToMeters(Yards) As Single
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
Private Sub cmdConvert_Click( )
Dim Meters As Single
Meters = YardsToMeters(CSng(txtYards.Text))
txtMetric.Text = CStr(Meters)
End Sub
In this example, the variable Meters is defined as a Single data type, and the YardsToMeters function is also defined as a Single data type.
If Meters is defined as the following:
Dim Meters As String
the line
Meters = YardsToMeters(CSng(txtYards.Text))
can't work because Meters is a String data type, and YardsToMeters returns a Single data type value. Because Meters expects a string but YardsToMeters gives Meters a number, the program won't work.)
* Defining argument types
Arguments are data (numbers, strings, or variables, which represent a number or a string) that a function needs to be able to work.
In addition to defining the number of arguments in an argument list, you have the option of defining the type of data each argument must represent.
For example, you can define an argument to represent only one of the following:
Integer
Long
Single
Double
Currency
String
To define an argument type, you have to define the type in the argument list:
Public Function Convert(Fahrenheit As Integer, Celsius As Integer)
This example defines two arguments and both must represent an Integer data type. The following shows the only procedure call that can work:
Private Sub cmdTest_Click( )
Dim X, Y, Z As Integer
Dim A, B, C As String
Dim L, M, N As Single
Z = Convert(X, Y) ' This would work
C = Convert(A, B) ' Neither A nor B are Integers
L = Convert(M, N) ' Neither M nor N are Integers
Z = Convert("Hello", X) ' "Hello" is not an Integer
End Sub
The main reason to specify an argument's type is to prevent a function from working with the wrong type of data. For example, if your function expects an integer but gets a string, your program can crash. Even worse, your program may create a logic error. Although you don't have to declare an argument's type, you should - just to prevent any bugs from wrecking your
program.
* Test your newfound knowledge
1. When do you want to use a function and when do you want to use a procedure?
a. In Chapter 27, 1 read about procedures, so that's where you use a procedure. in this chapter, you use only functions, because that's what this chapter covers.
b. Use a function when you need to calculate a single value. Use a procedure to calculate zero or more values.
c. Functions and procedures are identical except that they use different names, have different purposes, and don't look the same.
d. You use a function only if you're too wimpy to use a procedure, like a real programmer does.
2. Which line is a function call and which line is a procedure call?
Private Sub cmdDisplay( )
Dim Alex, Pete, George As Double
Pete = 3
George = 0
Alex =
Clockworkorange(Pete, George) ' Line 5
ConditionedBehavior(Alex) ' Line 6
End Sub
a. line five is a function call, because a variable is assigned to the value represented by the function name. Line six is a procedure call, because the function name is not assigned to a variable.
b. Both lines five and six are procedure calls, because I think this is a trick question and because guessing wrong 99 percent of the time since I started answering these question sections of this book still hasn't convinced me that the right answer is always the most obvious one.
c. Line six is a function call because this line looks different from line five, which is also a function call.
d. I don't know the answer, but this question looks like something organic that's really mechanical, much like a clockwork orange. Have you read the book, with the original ending, by Anthony Burgess?
* Problems sending arguments
Two types of problems may occur when calling functions. One problem is when the number of arguments sent doesn't match the number of arguments defined by the function.
Another problem is when the types of arguments sent don't match the types of arguments defined by the function.
Giving the wrong number of arguments:
When you define a function with an argument list, the list specifies the number of arguments that the function needs to run. If you call this function and don't give the function the right number of arguments, the function isn't going to work. For example:
Public Function Flame(Mail)
End Function
This function expects one argument, which can be a number or a string. None of the following calls to this function work, because the number of arguments is not one:
X = Flame
X = Flame(9, "Shut up!")
X = Flame("Why?", "Go away!", 4500, "Okay.")
Giving the wrong type of arguments:
Likewise, when calling a function, always make sure that the arguments have the same data types as the types defined in the argument list:
Public Function Flame(Mail As String)
End Function
This function expects one argument, which must be a String data type. None of the following calls to this function can work because the arguments are not String data types:
X = Flame (78.909)
X = Flame (9)
X = Flame (34)
* Quitting a Function Prematurely
Normally, your function runs until all the function's instructions have been followed. However, you may want to exit a function before the function is finished.
To exit a function prematurely, you have to use the following code:
Exit Function
Before you exit a function, make sure that you also assign a value to the function name; otherwise, your program may not work correctly.
** Try Using Functions Yourself
The following sample program lets you see how a function works. In this example, the function converts yards to meters.
In case you don't feel like typing everything out, this program is stored on the CD-ROM under the name FUNCTION.VBP.
Object - Form
Property - Caption
Setting - Converts Yards into Meters
Object - Label 1
Property - Caption
Setting - Yards:
Property - Height
Setting - 495
Property - Left
Setting - 360
Property - Top
Setting - 360
Property - Width
Setting - 1215
Object - Textbox 1
Property - Height
Setting - 495
Property - Left
Setting - 1680
Property - Name
Setting - txtYards
Property - Text
Setting - (Empty)
Property - Width
Setting - 1215
Object - Label2
Property - Caption
Setting - Meters:
Property - Height
Setting - 495
Property - Left
Setting - 360
Property - Top
Setting - 1320
Property - Width
Setting - 1215
Object - Textbox2
Property - Height
Setting - 495
Property - Left
Setting - 1680
Property - Name
Setting - txtMetric
Property - Text
Setting - (Empty)
Property - Width
Setting - 1215
Object - Command1
Property - Caption
Setting - &Convert
Property - Height
Setting - 495
Property - Left
Setting - 840
Property - Name
Setting - cmdConvert
Property - Top
Setting - 2400
Property - Width
Setting - 1215
Object - Command2
Property - Caption
Setting - E&xit
Property - Height
Setting - 495
Property - Left
Setting - 2520
Property - Name
Setting - cmdExit
Property - Top
Setting - 2400
Property - Width
Setting - 1215
Type the following in the Code window:
Public Function YardsToMeters(Yards) As Single
Const Conversion = 0.9
YardsToMeters = Yards * Conversion
End Function
Private Sub cmdExit_Click( )
Unload Me
End Sub
Private Sub cmdConvert_Click( )
Dim Meters As Single
Meters = YardsToMeters(CSng(txtYards.Text))
txtMetric.Txt = CStr(Meters)
End Sub
++ CHAPTER 30
GETTING SOME CLASS WITH OBJECT-ORIENTED PROGRAMMING
In This Chapter....
Defining object-oriented programming
Creating objects in Visual Basic
Using Visual Basic objects
In an attempt to keep up with the latest programming fads, Visual Basic gives you the ability to use object-oriented programming to develop your next killer application. Of course, unless you know what object-oriented programming is and what this form of programming can do for you, this latest feature is about as useful as giving a chain saw to a monkey and expecting the monkey to build a tree house.
So before you get all excited about using object-oriented programming with Visual Basic, take some time to find out what the heck object-oriented programming is supposed to do for you in the first place.
**What the Heck Is Object-Oriented Programming?
Despite the rigors of college computer science curriculums, programming Is still more an art than a science, which means that even having a Ph.D. in computer science is no guarantee that you can write better programs than a high school dropout.
To help turn programming into a science, the computer industry has been trying to develop guidelines to help people write bug-free programs as quickly and easily as possible. The first attempt at this "Holy Grail" of programming was something called structured programming, which encouraged you to divide a program into subprograms where each subprogram performed exactly one function.
While structured programming helped people create programs that were easier to write and modify, a problem still remained. Different parts of a program often accessed the same data, such as a file stored on the disk. This meant that if you wanted to change the way the program accessed specific data, you had to exhaustively search through the entire program and find all the commands that accessed that data. Miss one command and you've just introduced a bug into your program. In case you understand pictures better than text.
So the latest focus in computer science revolves around object-oriented programming, often abbreviated as OOP as in "Oops, you just wasted four years of college studying structured programming."
The central idea behind object-oriented programming is to divide your program into isolated parts called (what else?) objects. Each object contains two parts:
Data (called properties in Visual Basic)
Commands for manipulating that data (called methods in Visual Basic)
So when your program wants to access data, your program just gives a command to the object that contains that data. Your main program never directly accesses the data but just gives commands to the object to access the data.
If you ever need to change the way your program accesses specific data, you just have to modify the commands inside that object instead of modifying commands throughout your entire program. In this way, object-Oriented programming helps isolate the commands that affect specific data, thereby reducing the chance of introducing new bugs into your program when you need to modify your program.
Object-oriented programming is being credited with making programs easier to write, modify, and reuse. However, object-oriented programming alone won't make your program useful or bug-free in any way. A poor programmer using object-oriented programming will still be less efficient than a great programmer who doesn't use object-oriented programming. Don't let the hype about object-oriented programming fool you into thinking your programs will run faster, work better, or sell more copies just because you used object-oriented programming.
** Theoretical Stuff about Class Modules
Now that you have a general idea how objects work, you may be curious how objects work in Visual Basic. To create an object in Visual Basic, you have to create something called a class module, which has a file extension of .CLS, such as NOCLASS.CLS.
The class module defines:
The type of data the object can hold (but not any actual data itself)
BASIC commands (the methods) for manipulating the data in that object
The types of data defined by an object is called the properties of an object. The BASIC commands that manipulate the data in an object are called the methods of an object. If the terms properties and methods don't seem to clarify anything, don't worry; you're not the only one who feels that way.
A class module acts like a cookie cutter because this type of module defines the object but doesn't contain any data itself, much like a cookie cutter defines the shape of a cookie but doesn't contain any cookie dough itself.
A typical class module, as shown below, consists of three parts:
Variable declarations
Property declarations
Methods, which are BASIC commands, stored in procedures, that manipulate variables and properties
In the example class module code below, the variable declaration is the line Private mvarMessage As String.
The first property declaration begins with the line Public Property Let Message (ByVal vData As String) and the second property declaration begins with the line Public Property Get Message( ) As String.
The method begins with the line Public Sub CorporateSpeak( ).
Private mvarMessage As String 'local copy
Public Property Let Message(ByVal vData As String)
MvarMessage = vData
End Property
Public Property Get Message( ) As String
Message = vbarMessage
End Property
Public Sub CorporateSpeak( )
Dim NewString As String, FrontString As String, TailString As String
Dim Location As Integer, PickOne As Integer
NewString = ""
TailString = ""
PickOne = Cint((4 * Rnd) + 1
Select Case PickOne
Case 1
NewString = "manufacture high-quality, customer-empowered"
Case 2
NewString = "service and deliver world-class"
Case 3
NewString = "create product-driven, solution-oriented "
Case Else
NewString = "customize functional, creatively-packaged "
End Select
Location = InStr(mvarMessage, "make")
If Location = 0 Then
MvarMessage = "You need to type a mission statement that uses the word 'make' in it. """
Else
FrontString = Left(mvarMessage, Location - 1)
TailString Right(mvarMessage, Len(mvarMessage) - (Location + 4))
mvarMessage = FrontString & NewString & TailString
End If
End Sub
* Declaring your variables
Declaring all variables used by your class at the beginning of your class is a good idea, just so you know what type of information your class is using. If you want to declare a variable that any part of your program can use to store or retrieve information, you can declare a public variable, such as:
Public MoneyStolen As Currency
If you declare a public variable, any part of your program (including other objects) can put data into the variable, which means debugging your program can be extremely difficult. In general, don't declare a public variable unless you have a really good reason to do so.
If you want to declare a variable that only your class needs to use, you can declare a private variable, such as:
Private Counter As Integer
* Defining an object's properties
As far as the rest of your Visual Basic program is concerned, two types of properties exist:
Properties that the program can assign a value to
Properties that the program can retrieve a value from
For example, if you have a property called Direction and you want to assign a value to this property, you have to declare the Direction property in its class by using a property definition like this:
Private mWay As Integer
Property Let Direction(ByVal WhichWay As Integer)
mWay - WhichWay
End Property
Visual Basic interprets the above code as follows:
1. The first line defines a private variable, called mWay, that can hold an integer value.
2. The second line tells Visual Basic, "Let another part of the program assign an integer value to the mWay private variable." To assign a value to an object's property, you use code such as the following:
Set m_Object = New cObject
M_Object.Direction = 5
These two lines of code tell Visual Basic to create an object using the Set and New keywords (you can find out more about these keywords, later in this chapter, in the section "Creating an object"), and then assign a value to the object's properties just like you assign a property to a text box or a label on a form.
3. The third line tells Visual Basic, "Whatever integer value gets assigned to the Direction property, store that value in the mWay private variable."
4. The fourth line tells Visual Basic, "This is the end of the property definition that lets another part of the program assign a value to a property."
To yank out information trapped in an object's property, you have to declare the Direction property in its class by using a property definition like this:
Property Get Direction( ) As Integer
Direction = mWay
End Property
Visual Basic interprets the above code as follows:
1. The first line tells Visual Basic, "The Direction property of this object can only hold integer values."
2. The second line tells Visual Basic, "Assign the integer value stored in the mWay private variable to the Direction property."
3. The third line tells Visual Basic, "This is the end of the property definition so another part of your program can get a value from the property"
To actually yank out information stored in an object's property, you can use BASIC code, such as the following:
TxtDirection.Text = CStr (mObject.Direction)
Note the subtle differences between the two property definitions. To allow another part of your program to define a value for an object's property, you need to:
Use the magical Let keyword.
Define a variable and its data type in parentheses, such as (WhichWay As Integer).
Assign a private variable (declared earlier in the class module) to equal the value of the variable identified in parentheses, such as mWay = WhichWay.
To allow another part of your program to retrieve (or Get) a value from an object's property, you need to:
Use the magical Get keyword and assign a data type to the property such as an integer.
Leave the parentheses empty, such as ( ).
Assign the actual property name (such as Direction) to a private variable, such as Direction = mWay.
You need to use the Property Let and Property Get statements for each of your object's properties.
* Writing an object's methods
After you declare any variables your object may need and define your object's properties, the next step is to write methods (procedures or functions) that do something with the data stored inside of your object.
Writing a procedure or function is fairly straightforward (for more information check out Chapters 27, 28, and 29). The main difference is that instead of using the Private keyword in front of your procedure or function declarations, you use the Public keyword such as:
Public Sub Move( )
' Some useful BASIC code goes'here
End Sub
Public Function XLocation ( ) As Integer
' Some useful BASIC code goes here
End Function
When you want to call an object's methods, you just use the object's name plus the object's method such as:
M_Object.Move
The above code tells Visual Basic, "Find an object called m-Object and call the procedure named Move." As far as your main program is concerned, the program has no idea how the Move procedure works.
* Designing a class on paper
While you could rush right into writing the BASIC code that makes up a class module, you should take some time to design your class module first. What's the best way to design a class module? None. (Now aren't you glad you bought a book that told you that?)
Actually, the optimum design for a class module depends on how you plan to use the class module. The optimum design of a class module for one program may be horrible for another type of a program. To give you some pointers in designing classes, consider the following tips:
* To determine a class module's properties, decide the basic building block of data your program needs to manipulate. If you're writing a program to store information about employees, your class module needs to include properties that contain names, addresses, phone numbers, IQs, or felony records. If you're writing a video game where cartoon aliens pop up on the screen so you can shoot them, your class module may contain the X- and Y-coordinates of your cartoon alien's position on the screen.
* To determine a class module's methods, decide what your main Visual Basic program needs to do with the information stored in your class module. For example, a class module containing employee names and addresses may need methods that allow the main program to search, sort, and print employee data. Likewise, a class module containing cartoon alien X- and Y-coordinates may need methods that allow the main program to move, display, and blow up the alien.
* After you sketch out the type of properties and methods your class module needs to include, then you're ready to create an actual class module.
** Creating a Class Module with the VB Class Builder
Because creating classes can be messy if you don't know what you're doing, Visual Basic provides a handy program called the VB Class Builder. Essentially, the VB Class Builder lets you define your class methods and properties, and then the VB Class Builder writes the BASIC code for you.
Of course, if you like doing things the hard way, you can still create class modules from scratch, but at least with the VB Class Builder, you don't have to if you don't want to waste your time.
Rather than rush right into creating a class module, you should design your object's properties and methods on paper first. Only after you're satisfied that you've designed your objects correctly should you rush into creating a class module.
* Creating a new class
You need to create a separate class module for every different object you want to use in your program. To create a class module (or to edit an existing class module), follow these steps:
1. Choose Project->Add Class Module. An Add Class Module dialog box appears.
2. Click on the VB Class Builder icon and click on Open.
3. Choose File->New->Class or click on the Add New Class icon on the toolbar.
A Class Module Builder dialog box appears. (If you just want to edit an existing class module, skip Steps 3 and 4, and just click on the class name that appears in the class window.)
4. Type a name for your class in the Name text box and click on OK.
If you don't choose a name, Visual Basic names your class something dull and generic like Class1.
The above steps only let you create and name a class module, but you still need to define properties and methods inside your class.
* Creating properties for your classes
To define the properties for a class module, follow these steps (assuming that the VB Class Builder window is already displayed):
1. Click on the class module where you want to define your properties.
2. Choose File->New->Property or click on the Add New Property to Current Class. The Property Builder dialog box
appears.
3. Type the name for your property in the Name text box.
4. Click on the downward-pointing arrow in the Data Type list box and choose a data type (such as Integer or Single) for your property.
5. Click on OK.
* Creating methods for your classes
To define the methods for a class module, follow these steps (assuming that the VB Class Builder window is already displayed):
1. Click on the class module where you want to define your methods.
2. Choose File->New->Method or click on the Add New Method to Current Class. The Method Builder dialog box appears.
3. Type the name for your method in the Name text box.
4. Click on the Add a new argument icon (the plus sign), if your methods need arguments. An argument dialog box appears.
5. Type a name for your argument in the Name text box.
6. Click on a data type (such as Integer or Currency) in the Data Type list box.
7. Click on OK.
8. Click on the downward-pointing arrow in the Return Data Type list box and choose a data type.
(Skip this step if you don't want the method to return a value.
9. Click on OK.
* Quitting the VB Class Builder
When you're done creating or editing your class modules, you can get rid of the VB Class Builder window by one of these methods:
Click on the close box of the VB Class Builder window
Choose File->Exit
If a dialog box appears, asking if you want to update your Visual Basic project, click on Yes. After the VB Class Builder window goes away, you can click on your newly created class module and view the BASIC code that Visual Basic kindly created for you automatically.
After you use the VB Class Builder to create a class module, you must still write code inside your class module to make it do something useful. Then you must write code in your main program to use the class module.
** Using a Class Module in a Visual Basic Program
After going through all this trouble creating a class module, you still have to write BASIC code in your main program to actually use the program. Before you can use an object, you have to create one. After you create an object, then you can use that object's methods or store or retrieve information in that object's properties.
* Creating an object
Even though you may have gone through the trouble of creating and defining your class module, you still have to create an object based on the design of your class module. In the world of object-oriented programming, creating an object is called creating an instance. (Once again, you can see that computer scientists are no better at choosing self-explanatory terms than other experts are in their fields.)
To create an instance of an object, you have to create an object to represent your class module using the Set and New keywords such as:
Set ObjectName = New ClassName
This is how Visual Basic interprets this single line of code:
1. The Set keyword tells Visual Basic, "Get ready to create an object."
2. The 0bjectName variable is the name of your object.
3. The New keyword tells Visual Basic, "Create a new object based on the class module defined by ClassName."
* Using an object
After you create an object, the final step is to use that object to:
Stuff a value into an object's property
Retrieve a value out from an object's property
Use an object's method to do something with the object's data
To stuff a value into an object's property, you just have to use the following code:
ObjectName.Property = Value
To retrieve a value out from an object's property, you have to use the following code:
Variable = ObjectName.Property
To use an object's method, you can use the following code:
ObjectName.Method
** Try Class Modules Yourself
Naturally the best way to understand anything is to do what you don't understand yourself, so in case this entire chapter doesn't make a whole lot of sense, try the following program. This sample program demonstrates how a main program can create an object from a class module, call an object's method to manipulate the data, and then retrieve the data afterwards.
In this particular program, type your company's mission statement in the top text box, making sure that this mission statement includes the word "make," such as "We make cars." Then this program substitutes the word "make" for a more acceptable corporate term that makes no sense such as "manufacture high-quality, customer-empowered."
This program is stored on the CD-ROM so you don't have to type it all in if you don't want to. Just load the MISSION.VBP file.
If you get nothing else out of this chapter, just remember that object-oriented programming is supposed to help you organize your programs so that you have fewer chances of introducing bugs into any programs that you write or modify.
Object - Form
Property - Caption
Setting - Mission Statement Maker
Property - Height
Setting - 3600
Property - Left
Setting - 0
Property - Top
Setting - 0
Property - Width
Setting - 4800
Object - Text1
Property - Height
Setting - 615
Property - Left
Setting - 480
Property - MultiLine
Setting - True
Property - Name
Setting - txtInput
Property - TabIndex
Setting - 0
Property - Text
Setting - (Empty)
Property - Top
Setting - 360
Property - Width
Setting - 3850
Object - Text2
Property - Height
Setting - 615
Property - Left
Setting - 480
Property - MultiLine
Setting - True
Property - Name
Setting - txtOutput
Property - Text
Setting - (Empty)
Property - Top
Setting - 1440
Property - Width
Setting - 3850
Object - Command1
Property - Caption
Setting - &Create
Property - Height
Setting - 495
Property - Left
Setting - 600
Property - Name
Setting - cmdCreate
Property - Top
Setting - 2400
Property - Width
Setting - 1575
Object - Command2
Property - Caption
Setting - E&xit
Property - Height
Setting - 495
Property - Left
Setting - 2640
Property - Name
Setting - cmdExit
Property - Top
Setting - 2400
Property - Width
Setting - 1575
Double-click on the two command buttons on the form and create the following event procedures:
Private Sub cmdCreate-Click( )
Set DoubleSpeak = New CorporateTalk
DoubleSpeak.Message = txtInput.Text
DoubleSpeak.CorporateSpeak
TxtOutput.Text = DoubleSpeak.Message
End Sub
Private Sub cmdExit_Click( )
Unload Me
End Sub
Create a separate class module, give this module a name of CorporateTalk, create a property named Message, and create a method named CorporateSpeak. You will need to type cod in the CorporateSpeak method:
Private mvarMessage As String ' local copy
Public Property Let Message(ByVal vData As String)
MvarMessage = vData
End Property
Public Property Get Message( ) As String
Message = mvarMessage
End Property
Public Sub CorporateSpeak( )
Dim NewString As String, FrontString As String, TailString As String
Dim Location As Integer, PickOne As Integer
NewString = ""
TailString = ""
PickOne = Cint((4 * Rnd) + 1)
Select Case PickOne
Case 1
NewString = "manufacture high-quality, customer-empowered "
Case 2
NewString = "service and deliver world-class "
Case 3
NewString = "create product-driven, solution-oriented "
Case Else
NewSTring = "customize functional, creatively-packaged "
End Select
Location = InStr(mvarMessage, "make")
If Location = 0 Then
MvarMessage = "You need to type a mission statement that uses the word 'make' in it." " "
Else
FrontString = Left(mvarMessage, Location - 1)
TailString = Right(mvarMessaage, Len(mvarMessage) - (Location + 4))
MvarMessage = FrontString & NewString & TailString
End If
End Sub
++ CHAPTER 31
MANAGING FILES
In This Chapter....
Determining how Visual Basic stores a program
Playing with the Project Explorer window
Adding Form, Module, and Class files
Plugging ActiveX controls into your programs
In the old days, a single program consisted of a single file. You just had to modify that one file and you could modify the entire program. But as programs have gotten more complicated, they now consist of two or more files that make up a single program. This chapter explains how Visual Basic manages these multiple files so you can keep track of them all.
** How Visual Basic Stores a Program
When you use a word processor, you can type one sentence or 3,000 pages and your word processor can store all your text in a single file. However, when you use Visual Basic, Visual Basic always (yes, always) saves your program as two or more separate flies. Table 31-1 lists the most common types of Visual Basic files you may see, along with their identifying threeletter file extension.
Every Visual Basic program must contain exactly one Project (.VBP) file. The Project file lists all the separate files (Form files, Module files, ActiveX controls, and Class files) that make up a single Visual Basic program.
A Form file contains one window that makes up the user interface of your program along with the BASIC code that tells any button (check boxes and so on) on the window how to work. Most Visual Basic programs consist of one or more Form (.FRM) files.
A Module file contains BASIC code that performs some sort of calculation independent of the user interface. Visual Basic programs can contain zero or more Module (.BAS) files.
An ActiveX control is a miniature program that you can plug into your own program to add features. Some popular ActiveX controls let you quickly add word processing, charting, or spreadsheet features to your Visual Basic programs. You can buy ActiveX controls or write your own. A Visual Basic program may contain zero or more ActiveX (.OCX) controls.
A Class file contains BASIC code that defines different classes used by your Visual Basic program. A Class is simply a funny way to organize data that your program uses. (For more information about classes, see Chapter 30.) A Visual Basic program may contain zero or more Class (.CLS) files.
You can store your Project file in one folder and your other files (Form, Module, Class, or ActiveX files) in entirely different folders. However, if you don't store all files for a single program in the same folder, you may have difficulty modifying your program later if you can't find all the files that make up your program.
||
TABLE 31-1
TYPICAL VISUAL BASIC FILES AND THEIR FILE EXTENSIONS
File Type - Project File
Three-Letter File Extension - .VBP
Sample File Name - VIRUS.VBP
File Type - Form File
Three-Letter File Extension - .FRM
Sample File Name - MAINMENU.FRM
File Type - Module File
Three-Letter File Extension - .BAS
Sample File Name - LIBRARY.BAS
File Type - ActiveX Control
Three-Letter File Extension - .OCX
Sample File Name - RICHTX32.OCX
File Type - Class File
Three-Letter File Extension - .CLS
Sample File Name - OBJECTS.CLS
** VBP Project
A VBP project file contains a list of all the FRM, BAS, CLS, and OCX custom control files that make up a single Visual Basic program. The Project Explorer window shows you all the files stored in a VBP project file.
To help organize your files, the Project Explorer window displays them as if they were stored within separate folders, such as:
Forms
Modules
Class Modules
The Project Explorer window does not list all the ActiveX controls that your Visual Basic program uses. To see a list of ActiveX controls that a program uses, press Ctrl+T.
If you don't want to see all the files within a folder, double-click on the folder so that a Plus Sign Icon appears to the left. Then, when you want to see a list of your files within a particular folder, double-click on that folder to display all the files again.
To create a new VBP project file, follow these steps:
1. Choose File->New Project or press Ctrl+N. Visual Basic displays a New Project dialog box.
2. Click on the type of program you want to create (such as a Standard EXE or ActiveX DLL) and click on OK.
To load an existing VBP project file, follow these steps:
1. Choose File->Open Project, press Ctrl+O, or click on the Open Project icon. Visual Basic displays an Open Project dialog box.
2. Type or click on the name of the VBP project file you want to load and then click on Open.
Whenever you load a VBP project file, Visual Basic automatically loads all the files listed in the VBP project file.
To save your Visual Basic project for all eternity (or until you erase the project from your hard disk), follow these steps:
1. Choose File->Save Project or click on the Save Project Icon on the toolbar.
If you haven't saved your files (such as FRM form or BAS module flies), a Save File As dialog box appears.
2. Type a name for each of your FRM Form files and click on Save.
After you save all your Form files, Visual Basic displays a Save Project As dialog box.
3. Type a name for your VBP Project file and click on Save.
Be sure to save your flies before you try running your program. A program often has bugs that can crash your computer, which means that if you don't save your program before running it, you are going to lose all (yes, all) your changes if the computer crashes.
If you want Visual Basic to save any changes automatically before you run your program, follow these steps:
1. Choose Tools->Options. An Options dialog box appears.
2. Click on the Environment tab.
3. Click on the Save Changes or the Prompt to Save Changes radio button and then click on OK.
* Adding files to a VBP project file
The more Visual Basic programs you write, the more useful certain parts of them are going to be for future programs you may write. Fortunately, Visual Basic makes it easy to take FRM Form, BAS Module, and CLS Class files from another Visual Basic program and add them to a new Visual Basic program.
To add an FRM Form, BAS Module, or CLS Class file to a VBP project file, choose Project and then choose the type of file you want to add, such as Add Form or Add Module.
If you add a Form, Module, or Class file from another Visual Basic program, make sure that you save the file under another name. If two or more Visual Basic programs share the same Form, Module, or Class file, any changes you make to that file will affect every Visual Basic program that uses that particular file.
* Removing files from a project file
Sometimes you may want to remove a file permanently from a project file. To remove an FRM file, BAS file, or, CLS file from a VBP project file, follow these steps:
1. Press Ctrl+R; choose View->Project Explorer, or click on the Project Explorer Icon in the toolbar.
2. Click on the file (such as a Form, Module, or Class) that you want to remove.
3. Choose Project->Remove.
You can also right-click on a file in Step 2, and then left-click on the Remove command.
When you remove a file from a project file, the removed file still exists on your floppy or hard disk. Removing a file simply tells Visual Basic, "See that file over there? I don't want that file to be part of this particular program anymore, so get rid of it, but keep it saved on disk in case I want it back again." To erase or delete a file from your floppy or hard disk, use the Windows Explorer and erase the file off your floppy or hard disk for good.
** Adding (Or Removing) ActiveX Controls
An ActiveX control is a miniature program that you can plug into your Visual Basic programs to give them added features without writing much BASIC code yourself. Because ActiveX controls can help you write a program quickly and easily, many people's Visual Basic programs are nothing more than globs of ActiveX controls that they've connected by using a little bit of BASIC code.
Although Visual Basic comes with several ActiveX controls, you can buy tons of them by mail order or through the Internet. Just be careful that you don't rely too much on ActiveX controls. If a company wrote an ActiveX control that doesn't work properly, then your Visual Basic program isn't going to work properly either.
Whenever you add a custom control to a V13P project file, the icon for that custom control appears in the Visual Basic Toolbox.
To add an ActiveX control to a VBP project file, follow these steps:
1. Choose Project->Components or press Ctrl+T.
Visual Basic displays a Components dialog box that lets you add custom controls.
2. Click in the check box of the ActiveX control that you want to use and then click on OK.
Visual Basic displays that custom control's Icon in the Visual Basic Toolbox.
After you add an ActiveX control to your program, you still have to draw the ActiveX control on a form and then modify the properties or write BASIC code to make the control do anything.
Believe it or not, you can actually write your own ActiveX controls by using Visual Basic itself. (I explain ActiveX controls in more detail in the latest edition of MORE Visual Basic For Windows For Dummies, written by yours truly and also published by IDG Books Worldwide, Inc.) While writing these controls, just keep in mind that if you create a particularly useful feature in your program that you think others may find useful, you can save the feature as an ActiveX control and give or sell copies to other people.
++ PART VIII
DATABASE FILES AND PRINTING
In this part....
Most people don't care about knowing how to use a database program, and even fewer people care about all the different file formats in which data may be trapped. So to cater to the majority of people, Visual Basic can cheerfully read, write, and modify information stored inside a variety of database file formats including Microsoft Access, dBASE, Paradox, and FoxPro.
In addition to providing employment for hundreds of computer book authors and publishers, the purpose of most programs is to store information and then print or display the information again. After all, what good would a database be if you had to keep typing names and addresses into your computer every time you turned on your computer? So in this part of the book, you find out all about creating, storing, and printing information for your Visual Basic programs.
++ CHAPTER 32
CREATING DATABASE FILES
In This Chapter....
Answering the question, "What the heck is a database?"
Organizing a database
Creating your very own database
In case you need to write a program that stores and retrieves organized information such as names and addresses or inventory part numbers and customer phone numbers, you have two choices. You can either write a Visual Basic program along with tons of BASIC code to store, retrieve, and organize information on your hard disk, or you can take the easy way out and just use a database file instead.
By using database files with Visual Basic, you can spend your time designing your user interface without worrying about the details in saving, retrieving, and sorting your data.
You can use one of two existing ways to create a database file. One, you can rush out and spend hundreds of dollars buying a separate database program with a weird name like Microsoft Access, Paradox, or FoxPro. Two, you can just use the Visual Basic special add-in program called the Visual Data Manager. Because this book is about doing things the easy way, this chapter focuses primarily on using the Visual Data Manager. After all, the program comes free with Visual Basic.
** Databases For Dummies or (What the Heck Is a Database?)
Basically, a database file is nothing more than a special file designed to hold organized information. Just as a word processor file contains sentences and paragraphs, a database file contains information such as names, addresses, phone numbers, Social Security numbers, shoe sizes, secret passwords to sell to foreign agencies, and so on.
Typically a database file consists of the following parts:
One or more tables
Records
Fields
The database is (usually) a single file that contains all your information.
Tables let you organize the information stored within a database. For example, rather than toss names and addresses into a database like throwing clothes in a hamper, tables let you store one set of names and addresses as Customers and another set of names and addresses as Suppliers, Mailing List, Hot Dates, or any other type of classification you can dream up. Every database must have at least one table.
Each table consists of one or more records, where a single record contains all the information about a single item such as a person's name, address, and phone number. In the real world, a person's business card is a record.
Each record consists of one or more fields, where a field contains a single bit of information such as a name, address, hat size, or marital status.
** Organizing the Structure of a Database
Now that you have a general idea about the purpose of each part of a database file, you still need to decide exactly how you want to organize your information. Before creating a database, ask yourself, "What type of information do I need for the future?"
For example, if you're creating a database of business contacts, you may want to store names, phone numbers, e-mail addresses, and fax numbers. If you're creating an inventory list, you may want to store part numbers, part names, quantities left in stock, and the price per item.
After you decide what type of information you want to save, the next step is to create field names to identify each bit of information. Fields have two purposes in life:
To store a chunk of information
To provide a way to sort and search through a database
For example, if you want to store the name "John Doe," you can store the name by using one of the following two methods:
Method - #1
Field Name - Name
Actual Data - John Doe
Method - #2
Field Name - First Name
Actual Data - John
Field Name - Last Name
Actual Data - Doe
The first method stores a person's name as one complete chunk of information, which means that you can't search your database by first or last name.
The second method stores a person's name in two chunks of information, which can be more annoying to type in but allows you to search the database later by first or last name.
Neither method is right or wrong. The method you choose to use all depends on how you want to use your data. After you list all the fields in which you want to store your information, the next step is to decide if you want to divide your database into tables (categories). For example, if you're storing business cards, you may want to organize them according to Personal Contacts, Business Contacts, Contacts Who Owe Me a Favor, and so on.
You don't have to divide your database into two or more tables. A database can consist of just one table if that's what you need.
** Creating a Database File
After you design the structure of your database on paper, you can finally start designing your database on your computer. Although you can use a separate database program for this task (such as Microsoft Access or dBase), you may just want to use the Visual Basic add-in program called the Visual Data Manager.
* Running the Visual Data Manager
To run the Visual Data Manager program, follow these steps:
1. Load Visual Basic.
2. Choose Add-Ins->Visual Data Manager. The Visual Data Manager window appears.
3. Choose File->New. A list of database file formats such as Microsoft Access, Paradox, and FoxPro appears.
4. Choose Microsoft Access->Version 7.0 MDB.
(Unless you need to create database files in another format because everyone else you know uses Paradox or FoxPro database files, use a Microsoft Access 7.0 database.) A dialog box appears.
5. Type the name of your database file and click on Save. The Visual Data manager displays a Database Window.
6. Point the mouse cursor on the Properties Icon In the Database Window and click on the right mouse button. A pop-up menu appears.
7. Click on New Table. The Table Structure dialog box appears.
8. Type a name for your database table in the Table Name text box.
9. Click on the Add Field button. An Add Field dialog box appears.
10. Type the name of your field in the Name box, such as First Name, Employee ID, or Age.
11. Use the pull-down Type list box to choose the type of data you want to store In your field (such as Text, Currency, or Date/Time) and type the size of the data in the Size field); and then click on OK.
12. Repeat Steps 10 and 11 for each field you want to add, and then click on Close when you finish.
13. Click on the Build the Table button. The Database Window displays your table as an icon.
After you create a table, you can always edit it later. To do so, simply point the mouse cursor over the Table icon in the Database Window, click the right mouse button, and choose Design.
* Stuffing information into Your database
After you create a database file, one or more tables, and one or more fields in each table to hold your information, you eventually need to stuff your database full of information such as people's names, phone numbers, or employee ID numbers.
You don't have to stuff a database with information. You may want to leave your database empty so someone using your Visual Basic program can stuff her own information into the database.
To stuff information into a database by using the Visual Data Manager program, make sure that the Visual Data Manager program is running and follow these steps:
1. Move the mouse cursor over the Table lcon in the Database Window, click with the right mouse button, and choose Open.
A dialog box appears.
2. Click on Add.
3. Type the information you want to appear In each field.
4. Click on Update when you finish.
5. Click on Close.
* Exiting the Visual Data Manager
To exit the Visual Data Manager program, just choose File->Exit.
The Visual Data Manager is a simple program for creating databases. If you really need to create complicated databases, you are better off buying a separate copy of Microsoft Access and using that program to create your database files.
++ CHAPTER 33
USING FILES FROM DATABASE PROGRAMS YOU'D RATHER NOT USE
In this Chapter....
Connecting to a database file
Displaying different records
Searching for specific records
If you're planning to store a great deal of data, you may as well store the data in a database file. Visual Basic can store and retrieve data in any one of the following four database formats:
Microsoft Access MDB files
Borland dBASE DBF files
Microsoft FoxPro DBF files
Borland Paradox DB files
If you have no clue as to what any of these files or databases are, consider yourself lucky
Visual Basic comes with a separate program called the Visual Data Manager (see Chapter 32 for more information), which lets you create Microsoft Access database (MDB) files without shelling out the money to buy a real copy of Microsoft Access. If you want to create really sophisticated databases, you are better off buying a separate copy of that database program, such as dBASE, Microsoft Access, or Paradox.
** What Are Database Files?
Whenever one of those fancy database programs such as Paradox, Access, or dBASE saves information, that program stores the information in a disk file. Because the disk file contains a bunch of data that somebody thinks is important, the file is called a database file. Essentially, a database file is like a Rolodex jammed with junk.
To organize the data in the file, a database file is made up of one or more records. A record is like a 3-x-5 index card. Each record contains fields, which contain the specific information (names, addresses, and stuff like that).
To organize this information further, database files let you organize data into tables. A table is a subset of your entire database. The table contains only specific information, such as the names of all the people who live in Oregon or the phone numbers of everyone you can't stand.
** How to Connect to a Database File
When you want a Visual Basic program to store and retrieve information in a database file, you have to specify the following:
Which database file to use
Which recordset type to use
Which database table to use
Which database fields to display
* Connecting to a database manually
To connect your program to a database file, follow these steps:
1. Make sure that the database file to which you want to connect exists.
If not, you need to create the file by using a database program (such as Paradox or FoxPro) or the Visual Data Manager program.
2. Click on the Data Control icon from the Visual Basic Toolbox and draw the data control on your form. Figure 33-2 shows the Data Control icon on the Toolbox and drawn on a form. For more about how to use a data control, check out the section "Manipulating Different Records," later in this chapter.
3. Open the Properties window. (Press F4; choose View->Properties Window; or click on the Properties Window icon on the toolbar.)
4. Double-click on the DatabaseName property. Visual Basic displays the DatabaseName dialog box.
5. Select the database file that you want to use, such as BIBLIO.MDB (which is the sample database that comes with Visual Basic) and click on Open.
6. Click on the Recordset Type property and click on the gray downarrow button in the settings box. Visual Basic displays a list of recordset types to choose from:
0 - Table
1 - Dynaset
2 - Snapshot
7. Click on a recordset type, such as 0 - Table.
8. Click the RecordSource property and click on the gray down-arrow button in the settings box. Visual Basic displays a list of database tables you can choose from.
9. Click on a database table (such as Titles, if you choose the BIBLIO sample database that comes with Visual Basic).
If you're using Microsoft Access MDB files, choose the 0 - Table recordset type in Step 7. If you're using any other type of database, choose the 1 - Dynaset recordset type. If you need to read but not update data, go ahead and choose the 2 - Snapshot recordset type.
* Connecting to a database using the VB Data Form Wizard
In case you'd rather not do everything the hard way, Visual Basic offers a VB Data Form Wizard, which can create a new form specifically for displaying data in your Visual Basic project.
To accept default settings for your data form, you can click the Finish button any time during the following steps.
To use the VB Data Form Wizard, follow these steps:
1. Choose Project->Add Form. An Add Form dialog box appears.
2. Click on the VB Data Form Wizard Icon and click on Open. The Data Form Wizard - Introduction dialog box appears. If you have created a form that you want to base your new form on, click on the down arrow in the list box to choose a profile to use.
3. Click on Next >. The Data Form Wizard - Database Type dialog box appears.
4. Click on a database format (such as Access) and click on Next >. The Data Form Wizard - Database dialog box appears.
5. Click on the Browse button. An Access Database dialog box appears.
6. Click on the database file you want to use, click on Open, and click on Next >. The Data Form Wizard - Form dialog box appears.
7. Type a name for your form and click on one of the following:
* Single record: Creates a form that displays one record at a time.
* Grid (Datasheet): Creates a form that displays data in a spreadsheet (grid) format.
* Master/Detail: Creates a Master record form that displays data in a single record format and a Detail record source in a spreadsheet (grid) format.
* MS HFlexGrid: Creates a form that displays data in a spreadsheet (grid) format that merges and consolidates data by pivoting rows or columns.
* MS Chart: Creates a form that displays your data as a chart.
8. Click on Next >. The Data Form Wizard - Record Source dialog box appears.
9. Click on the downward-pointing arrow in the Record Source list box and choose the record you want to display.
10. Click on a field displayed in the Available Flelds list box and click on the > button for each field you want to display on your form. Click on Next > when you're done. The Data Form Wizard - Control Selection dialog box appears.
11. Click in the check boxes for each type of button you want displayed on your form (such as an Add or Delete button), and then click Next >. The Data Form Wizard - Finished! dialog box appears.
12. Click on the three black dots to the right of the down arrow if you want to save your form as a profile for future use. Click Finish. Visual Basic makes up your form.
13. Click on OK. Your form appears, ready for your editing or modifications.
If you use the VB Application Wizard to create a skeleton program for you, you can create a form for displaying database information. By creating a program that displays database information with the VB Application Wizard, you don't have to go back later and use the VB Data Form Wizard.
** Displaying Data Fields on the Screen
After you connect your program to a database file, the next step is to display the information stored in the fields of the database. (If you use the VB Data Form Wizard to create a form to display your data, you can edit your form and add or delete additional data fields.) Visual Basic gives you several ways to display database fields on a form:
Check boxes
Image and Picture boxes
Labels and Text boxes
List and Combo boxes
Grids
Check boxes display Yes and No or True and False values. If a field has a Yes or True value, the check box appears selected on the screen. If the field has a No or False value, the check box appears blank.
Image and picture boxes enable you to display graphics, as long as the database file itself contains graphics.
Text boxes and labels display information such as names, addresses, phone numbers, and quantities.
List and combo boxes enable you to display multiple choices. The combo box gives users a chance to type in something that's not included in the combo box's list.
Grids let vou display entire records as rows in a spreadsheet format.
Use a text box if you want to give the user the ability to change the displayed data. Use a label when you want to prevent a user from changing data.
To create a text box, label, check box, image box, combo box, list box, grid, or picture box to display a database field, make sure that you draw at least one data control on the form and then follow these steps:
1. Draw the text box, label, check box, image box, combo box, list box, grid, or picture box on the form that contains the data control.
2. Open the Properties window. (Press F4; choose View->Properties Window; or click on the Properties Window icon on the toolbar.)
3. Click on the DataSource property and click on the gray down-arrow button in the settings box. Visual Basic lists the names of all the data controls on the form.
4. Select a data control name (such as Data1).
5. Click on the DataField property and click on the gray down-arrow button In the settings box.
Visual Basic lists all the field names you can display. (Skip this step if you drew a grid in Step 1.)
6. Click on a field name.
When selecting a field name for a check box, make sure that the database field contains only Yes/No or True/False values. When selecting a field name for an image or a picture box, make sure that the database field contains only graphic images. Otherwise, Visual Basic becomes confused and won't know how to handle this data when your program is running.
** Manipulating Different Records
When Visual Basic loads a database, Visual Basic makes the first record of the database the current record. Any commands you issue affect only that first database record.
Choosing different records is like thumbing through a library card catalog. (For those younger people who have never seen a library card catalog, think of thumbing through a CD music collection in a music store.)
Visual Basic provides two ways to make a different record the current record:
Using the data control object
Using BASIC code
* Using the data control object to display the records in a database
The data control object has four arrows that let the user scroll throbgh different records.
The data control object is designed to look like the controls on a tape cassette or VCR. (Then again, how many people know how to program their VCR?)
The arrows on the far left and far right move to the first and last database record. The other two arrows move forward or backward, one record at a time.
The data control object lets a user choose a record. Just make sure that you have a text box or label on the screen to display the data from the current record. If you don't, the user is going to have no idea which record is current and is not going to be able to see or edit the data either.
* Using BASIC code to display the records in a database
To display the first record by using BASIC code, issue the following command:
Data1.Recordset.MoveFirst
In all these commands, Data1 refers to the name of your data control object. If you change the name of the data control, you have to use your new name instead.
To display the last record in the database, use the following command:
Data1.Recordset.MoveLast
To display the next record in the database, use the command that follows:
Data1.Recordset.MoveNext
To display the previous record in the database, use the following command:
Data1.Recordset.MovePrevious
* Adding a record
To add a record to a database, follow these steps:
1. Create a new record, using the following code:
2. Put data in the fields of this newly created record by using the following code:
Data1.Recordset.AddNew
2. Put data in the fields of this newly created record by using the following code:
Data1.Recordset.Fields("FieldName") = "NewData"
FieldName is the specific field you want to change in your database, and yes, you need the quotation marks surrounding the field name. NewData is the new information you want to store in the field, and can be a string or a number. If the information is a string, you need to place quotation marks around the information. If the information is a number, you don't need the quotation marks.
3. Save the record to the database with the following code:
Data1.Recordset.Update
Visual Basic always adds new records to the end of a database file.
* Editing a record
To edit an existing record in a database, follow these steps:
1. Display the record that you want to edit.
2. Use the following command to tell Visual Basic that you want to edit this record:
Data1.Recordset.Edit
3. Assign the new data to the specific field whose contents you want to change:
Data1.Recordset.Fields("FieldName") = "NewData"
4. Save the record to the database:
Data1Recordset.Update
* Deleting a record
To delete a record from a database, find the record that you want to obliterate from the face of the earth and then use the following two BASIC commands:
Data1.Recordset.Delete
Data1.Recordset.MoveNext
This command says, "See the currently displayed record? Delete this record now and then display the next record so that the deleted record is no longer visible."
* Counting the number of existing records
Sometimes knowing how many records a database file contains is handy. To calculate this value, use the commands that follow:
Dim TotalRecords As Long
Data1.Recordset.MoveLast
TotalRecords = Data1.Recordset.RecordCount
1. The first line says, "Create a variable called TotalRecords, and define this variable as a Long data type."
2. The second line says, "Move to the last record in the database."
3. The third line says, "Count the number of records and store this value in the variable called Total Records."
** Finding a Specific Record
To find a specific record in a database, you have to use one of the following commands:
Data1.Recordset.FindFirst "criteria"
Data1.Recordset.FindNext "criteria"
Data1.Recordset.FindPrevious "criteria"
Data1.Recordset.FindLast "criteria"
The search criteria (or criterion, in case you think the singular form of criteria doesn't look silly) specifies which field to search and the specific data you want to find. For example, to find all the records containing information on people with the last name Jefferson, your search criteria may look like the following:
*LastName = 'Jefferson'"
Notice that the data you're searching for ('Jefferson') must appear surrounded by single quotation marks. If you fall to use single quotation marks, Visual Basic isn't going to know what you want to find and isn't going to do anything, out of spite. Of course, this example also assumes that a field called LastName exists.
If you try searching for data in a nonexistent field, you get an error message.
To find the first record that meets your search criteria, use a command like the following:
Data1.Recordset.FindFirst "LastName = 'Jefferson'"
After you find one record that meets your search criteria, you can search for another record that meets your criteria. To find the next record that meets your search criteria, use a command like the following:
Data1.Recordset.FindNext "LastName = 'Jefferson'"
To find the previous record that meets your search criteria, use a command like the following:
Data1.Recordset.FindPrevious "LastName = 'Jefferson'"
To find the last record that meets your search criteria, use a command like this:
Data1.Recordset.FindLast "LastName = 'Jefferson'"
* Yanking data from a database field
Before you yank out information from a database field, find the record that you want by using the FindFirst, FindNext, FindLast, or FindPrevious command. Then yank information from the database field by using one of two methods:
Create a text box or label and set the DataField property to the field you want.
Use BASIC code to yank information out of a database and assign the information to a variable:
Dim Store As String
Store = Data1.Recordset.Fields("Fieldname").Value
1. The first line says, "Create a variable called Store and define this variable as a String data type."
2. The second line says, "Yank the data stored in the field called Fieldname and store the value in the Store variable."
* Changing data in a database
To change the contents of a database field when the field is displayed in a text box, check box, image box, or picture box, just type or paste new information inside.
To change the contents of a database field by using BASIC code, use the following commands:
Data1.Recordset.Edit
Data1.Recordset.Fields("Fieldname").Value = NewValue
Data1.Recordset.Update
1. The first command tells Visual Basic, "I'm getting ready to change some data in a database."
2. The second command replaces the current value of the field " Fieldname " with the value stored in NewValue.
3. The third command says, "Save my changes to the database."
Any time you add, delete, or edit a record in a database using BASIC code, you have to use the magic Update command to update the database file.
* Test your newfound knowledge
1. What are the steps you must follow to make your Visual Basic program use a database file?
a. First, create a database file using a separate database program such as Access or Paradox. Then draw the Data Control icon on a form, choose the database file name for the DatabaseName property of the Data Control icon, and choose a database table for the RecordSource property of the Data Control icon.
b. Buy a computer, take it back to the store because it doesn't work, hire a database programmer, fire the database programmer, and throw up your hands in despair.
c. Visual Basic can use database files? Hey, where can I find out more about this?
d. None. This is a trick question and I refuse to reveal my ignorance by selecting any of these choices.
2. What do the following lines of BASIC code do?
Data1.Recordset.AddNew
Data1.Recordset.Fields("Question") = "Did you ever accept money from overseas governments?"
Data1.Recordset.Fields("Answer") = "Yes, but I made them pay with American money."
Data1.Recordset.Update
a. The first line erases any existing databases on your hard disk, the second and third lines are superfluous, and the fourth line wipes out the rest of the files stored on your hard disk.
b. The first line adds a new record to a database file; the second line stores thestring "Did you ever accept money from overseas governments?" in the database field called "Question ";the third line stores the string "Yes , but I made them pay with American money " in the database field called "Answer"; and the fourth line updates the database with this new information, adding this new record at the end of the database file.
c. The first line confuses Visual Basic, the second line asks Visual Basic a question, the third line displays the Visual Basic answer as given under the advice of a lawyer, and the fourth line ignores the whole problem and hopes that everyone can forget what really happened.
d. Hey! You're making fun of somebody important, aren't you?
* Placing a bookmark on a specific record
You can put a bookmark on a record so that you can quickly jump back to the record at any time. To create a bookmark, you have to create a variable and define this variable as a Variant or String data type. For example:
Dim MyBookMark1 As Variant
Or
Dim MyBookMark2 As String
Each bookmark can point to only one record at a time.
To place a bookmark, display the record where you want to put it and then use the following command:
MyBookMark1 = Data1.Recordset.Bookmark
To jump back to a bookmark, use the following command:
Data1.Recordset.Bookmark = MyBookmark1
Note: Not all database files let you use bookmarks. Microsoft Access MDB files are always bookmarkable (is that a real word?), but other database types may not be, depending on whether an index exists for that particular database. (If you have no idea what an index is and don't care to know, feel free to ignore this whole discussion.) To determine whether a database file supports bookmarks, examine the Bookmarkable property, as in the following example:
If Data1.Recordset.Bookmarkable = True Then ' Can place bookmarks
ElseIf Data1.Recordset.Bookmarkable = False Then ' Cannot use bookmarks
End If
Be careful about database formats. Each time a company such as Microsoft upgrades their database program (such as Access), the new version of that database program may use a different file format. This means that Visual Basic may not be able to use the new, updated file format.
++CHAPTER 34
MAKING YOUR PROGRAM PRINT STUFF
In This Chapter....
Printing your Visual Basic project
Printing a form
Using the printer object to print lines and circles
Counting pages
A program can suck in data, manipulate data, and display data on the screen. But eventually, your program may need to print stuff out. Visual Basic provides two ways to print stuff:
Display data on a form and then print the form
Send data directly to the printer using the Printer object
** Printing a Form
Printing a form is the simplest method for printing out information. To print a form, use the following syntax:
FormName.PrintForm
This tells Visual Basic, "Find the form named FormName and send this form to the printer."
To print a form named frmAbout, use the following command:
frmAbout.PrintForm
You can print any form in your program, including invisible forms and minimized forms. The only drawback with printing forms is that the print resolution may not be very high.
Most screens display a resolution of 96 dots per inch (dpi). Most laser printers can print at resolutions of 300 or 600 dots per inch. For quick printing, the PrintForm command works well. But for higher resolution printing, print by using the Printer object instead.
** Printing with the Printer Object
The Printer object is a temporary storage space that intercepts data, cleans up data, and then sends the clean version to the printer. To print a form using the Printer object, you essentially have to redraw your form on the Printer object, using BASIC code.
The advantage of the Printer object is that this object uses the resolution of your printer. The disadvantage is that you have to write lots of BASIC code just to print a simple form.
Before you print anything using the Printer object, you have to define the top/bottom page margins and the right/left page margins.
To define the top page margin, use the following syntax, where TopValue measures the size of the top margin:
Printer.ScaleTop = TopValue
To define the bottom page margin, use the syntax that follows, where BottomValue measures the size of the bottom margin:
Printer.ScaleBottom = BottomValue
To define the left page margin, use the following syntax, where LeftValue measures the size of the left margin:
Printer.ScaleLeft = LeftValue
To define the right page margin, use the following syntax, where RightValue measures the size of the right margin:
Printer.ScaleRight = RightValue
You may have to experiment with different values for your margins until they look the way you want.
* Printing text on the Printer object
Before you can print text through the Printer object, you have to define the X and Y location where you want the text to appear.
To define this location, use this syntax:
Printer.CurrentX = Xvalue
Printer.CurrentY = Yvalue
These commands say, "Start printing all text at the location defined by the CurrentX and CurrentY properties."
To start printing at the upper-left corner of a page, use the following commands:
Printer.CurrentX = 0
Printer.CurrentY = 0
After you define where to start printing, the next step is to actually print some text. To print text on the Printer object, follow this syntax:
Printer.Print "Text string"
This command says, "Put a text string on the Printer object and print this text string at the location previously defined by the CurrentX and the CurrentY properties."
* Test your newfound knowledge
1. Why would you want to have your Visual Basic program print anything?
a. To waste paper and help contribute to global deforestation in Third World countries.
b So I can justify buying a $1,000 laser printer.
c. To provide hard copies of any important information that my Visual Basic program may create.
d. I can never justify a reason to print 'anything. Hasn't watching American politics taught you to never put down
on paper anything that may implicate you in the future?
2. Why do you need to specify X- and Y- coordinates when printing text or drawing lines or circles?
a.To make you feel like you actually learned something useful in school geometry class.
b. So you can tell Visual Basic exactly where you want your lines, circles, or text to appear on the printed page.
c, The X- and Y-coordinates define the location of your printer in relation to your computer, If you define the X and Y-coordinates incorrectly, your documents may start printing out from your toaster or air conditioner.
* Printing lines and circles on the Printer object
Printing plain text can be boring, so Visual Basic gives you the option to spice up your printouts with lines and circles. If this sounds like Visual Basic is turning your $2,000 computer and $1,000 laser printer into an Etch-ASketch, you're right.
Defining line thicknesses:
Before you start drawing lines and circles, you have to define the draw width. The smaller the draw width, the thinner your lines are going to look.
To define the draw width, use the syntax that follows:
Printer.DrawWidth = Value
To define the skinniest draw width possible, use this command:
Printer.DrawWidth = 1
To define a fatter draw width, use this command:
Printer.DrawWidth = 5
For those who really care, the values of DrawWidth can vary from 1 to 32,767. A value of 1 specifies a line one pixel wide. A value of 32,767 specifies a line of 32,767 pixels wide, which is probably wider than you are going to ever need.
Printing a 1-pixel wide line on a 600 dots-per-inch laser printer may look different than if you print a 1-pixel wide line on a 300-dpi inkjet printer.
Drawing lines on the Printer object:
To draw a line, use the syntax that follows, where x1 and y1 define the starting point of the line and x2 and y2 define the ending point:
Printer.Line (x1,y1) - (x2, y2)
Drawing circles on the Printer object:
To draw a circle, use this syntax, where x and y are the center of the circle and Radius defines the circle's radius:
Printer.Circle(X,y), Radius
* Printing Multiple Pages
Usually Visual Basic keeps printing your data until the printer runs out of room on the page. Then Visual Basic cuts your text off and starts printing a new page automatically.
However, if you want to control when Visual Basic starts printing a new page, use the following command:
Printer.NewPage
This command tells Visual Basic to start printing on a new page. (Wow! What are they going to think of next?)
* Defining the Print Quality
Depending on your printer, you can specify a range of resolutions for printing. The magic command to control print resolution is
In the preceding command, x represents a negative number between -4 and -1, or a positive number representing the specific resolution you want to use, measured in dots per inch. For example, the following list shows the print resolution of the negative numbers -4 to -1:
Printer.PrintOuality = -1 Draft resolution
Printer.PrintOuality = -2 Low resolution
Printer.PrintOuality = -3 Medium resolution
Printer.PrintOuality = -4 High resolution
In case you want to use the predefined Visual Basic constants, you can use them as follows:
Constant - vbPRPQDraft
Value - -1
Print Resolution - Draft resolution
Constant - vbPRPQLow
Value - -2
Print Resolution - Low Resolution
Constant - vbPRPQMedium
Value - -3
Print Resolution - Medium resolution
Constant - vbPRPQHigh
Value - -4
Print Resolution - High resolution
If you're the type who likes to specify the exact print resolution to use, just specify a positive number. To define 300 dpi, which is what most cheap laser printers can produce these days, you want to use the following command:
Printer.PrintQuality = 300
* Keeping a Page Count
When you're printing multiple pages, Visual Basic automatically keeps track of the page count in a property called Page. To use this page count, use the following BASIC command:
Printer.Page
* When You Finish Printing
Use the following command to tell Visual Basic you've finished printing:
Printer.EndDoc
If you neglect to use this command, guess what? Visual Basic assumes this command exists anyway as the last command to the printer, so you really don't need to use this after all. However, for good programming practice, use this command so you know exactly when your program has finished printing.
In case you want to stop printing, you can tell Visual Basic, "Hey, I changed my mind. Stop printing." To stop the printer right away, use the following BASIC command:
Printer.KillDoc
As you can see, trying to torture Visual Basic into printing data can be cumbersome and tedious. If you have the time, write your own printing routines and reuse them for each program you write. As a quicker alternative, buy an ActiveX control called vsView from VideoSoft (www.videosoft.com). This ActiveX control makes it easy to add printing capabilities to your program without writing a lot of BASIC code.
** Try Creating a Print Program Yourself
The following sample program prints a short message when you click the Click to Print command button. To create this program, use the settings specified in the following table:
Object - Form
Property - Caption
Setting - An Example of Printing
Object - Command1
Property - Caption
Setting - &Click to Print
Property - Name
Setting - cmdPrint
Type the following code in the Code window. If you don't want to give your fingers such a strenuous workout, just load the PRINTME.VBP file off the CD-ROM at the back of this book.
Privat Sub cmdPrint_Click( )
Dim TotalPages As Integer
Dim PageCount As String
' Specifies the text position to print
Printer.CurrentX = 100
Printer.CurrentY = 100
Printer.Print "This appears at the top of the page"
' Specifies a line width and location
Printer.DrawWidth = 3
Printer.Line(100, 100) - (10000, 100)
Printer.Line(100, 350) - (10000, 350)
TotalPages = Printer.Page
' Specifies a line width and location
Printer.Circle (2000, 3500), 450
Printer,CurrentX = 2750
Printer.CurrentY = 3500
Printer.Print "This is a circle. Whee!"
' Specifies where to print the page count
Printer.CurrentX = 1000
Printer.CurrentY = 400
PageCount = "TotalPages = " & Str$(TotalPages)
Printer.Print PageCount
Printer.EndDoc
End Sub
** Printing a Visual Basic Project
While this chapter has explained how to make your Visual Basic program print out data while the program is running, you may have another question, "How the heck do I print out the source code to my Visual Basic project itself?"
Fortunately, after you take all the time and effort to write a program in Visual Basic, you can print the program out for posterity to admire. Visual Basic provides three ways to print out a Visual Basic project:
Form Image: Prints your form exactly as the form appears on the screen.
Code: Prints only your BASIC code.
Form As Text: Prints the property values of all the forms and objects that make up your user interface.
To print a Visual Basic project, follow these steps:
1. Choose File->Print or press Ctrl+P. Visual Basic displays the Print dialog box.
2. Click on the Current Module (to print only the highlighted file in the Project Explorer window) or the Current Project radio button (to print the entire Visual Basic project).
3. Click on one or more of the following check boxes: Form Image, Code, or Form As Text.
4. Make sure vour printer Is turned on and click an OK.
++ PART IX
THE PART OF TENS
In this part....
Now that you've made it this far in the book (or maybe you're one of those readers who jumps to the end of a great book), you're looking for a few ideas to help you write the best Visual Basic programs possible without losing your mind in the process.
With the help of this book and Visual Basic, you can now write your own programs or get a job writing programs for others. But no matter what you may plan to do with your programming skills, browse through this part of the book to pick up tips for making the most of your newly acquired Visual Basic programming skills.
++ CHAPTER 35
THE TEN VISUAL BASIC TOPICS THAT DIDN'T FIT ANYWHERE ELSE
In This Chapter....
Finding magazines and newsletters
Visiting Visual Basic Web sites
Joining a user group
Attending technical conferences
Writing your own DLL files
Writing Visual Basic programs for the Macintosh and Linux
Now that you've reached the end of this book (even if you just skipped to this section while browsing through this book in the bookstore), you may be wondering what to do next to continue your Visual Basic education without going through the process of trial and error and driving yourself crazy in the process.
You can pick up the latest edition of MORE Visual Basic For Windows For Dummies (written by yours truly and also published by IDG Books Worldwide, Inc.; order by calling 800-762-2974 or 415-655-3172). You can also use the following list of sources to find out more about Visual Basic and get help in creating the most powerful Visual Basic programs possible.
** Buy, Read, or Steal Visual Basic Programmer's journal
Every month, look for a fresh copy of Visual Basic Programmer's Journal at your favorite magazine stand or bookstore. This magazine comes loaded with articles exploring the intricate details of Visual Basic, reviews of Visual Basic add-ons, and samples of Visual Basic code that you can copy (steal) for your own use.
Unlike other magazines, one of the more useful features is the advertising. If you need an ActiveX control to make your programming task easier, then browse through the ads and you'll probably find what you need. For more information about this magazine, contact
Visual Basic Programmer's Journal
Fawcette Technical Publications
209 Hamilton Avenue
Palo Alto, CA 94301-2500
Tel: 415-833-7100
Fax: 415-853-0230
www.windx.com
** Spend a Bundle of Money and Get a Visual Basic Newsletter
For more detailed information, buy a Visual Basic newsletter. The Cobb Group publishes monthly newsletters for a variety of programs, including Paradox, Microsoft Office, and Visual Basic. Although a year's subscription is about $60, the Inside Visual Basic newsletter provides plenty of source code examples that you can study, modify, and claim you wrote yourself to impress your boss, who doesn't know any better.
Another company, Pinnacle Publishing, Inc., publishes another Visual Basic monthly newsletter, Visual Basic Developer, that comes with source code examples on an enclosed floppy disk. Of course, this newsletter costs a bit more (about $179 a year), but if you can get your company to buy this for you, who cares about the price?
For more information on the two publications discussed in this section, contact:
Inside Visual Basic
The Cobb Group
9420 Bunsen Parkway
Louisville, KY 40220
Tel: 502-493-3300
Fax: 502-491-8050
www.cobb.com
Visual Basic Developer
Pinnacle Publishing, Inc.
P.O. Box 72255
Marietta, GA 30007-2255
Tel: 770-565-1763
Fax: 770-565-8232
www.pinpub.com
** Visit a Web Site Dedicated to visual Basic
If you have access to the Internet, you can often find entire Web sites devoted to Visual Basic programming. While the number of specialized Visual Basic Web sites continue to grow, here's a short list of the more popular Web sites you may want to visit:
Carl and Gary's Visual Basic Home Page at the following Web address:
www.apexsc.com/vb
Chris & Tim's VB Programming Resources at the following Web address:
www.zetnet.co.uk/rad/index.html
VB Tips & Tricks Home Page at the following Web address:
www.vbtt.com
** Attend a Visual Basic Technical Conference
Every few months, Microsoft and Visual Basic Programmer's Journal sponsor a Visual Basic Technical Summit somewhere around the world in the United States, Europe, and Asia. These conferences are great places to learn techniques from real-life Visual Basic programming experts, listen to the latest propaganda talks from Microsoft representatives, buy Visual Basic add-ons cheaply from vendors, and make lots of contacts in the Visual Basic world. For more information about these technical conferences, contact:
Visual Basic Programmer's Journal
Fawcette Technical Publications
209 Hamilton Avenue
Palo Alto, CA 94301-2500
Tel: 415-833-7100
Fax: 415-853-0230
www.windx.com
** Shop from Mail-Order Dealers
Don't buy Visual Basic or any Visual Basic add-on programs direct from the publisher. Most software publishers cheerfully charge full retail price for their programs, which makes as much sense as paying full sticker price for a used car.
Rather than buy direct from the software publisher, shop by mail order instead. Mail-order dealers give you even deeper discounts (up to 50 percent in some cases), with the added advantage of saving you from having to pay sales tax.
Two popular mall-order dealers are VBxtras and ZAC Catalog. Both companies specialize in selling Visual Basic add-ons at substantial discounts, so give them a call and ask for a free catalog.
For more information on these mail-order companies, contact:
ZA Catalog
1090 Kapp Drive
Clearwater, FL 33765-2111
Tel: 813-298-1181
Fax: 813-461-5808
www.zaccatalog.com
VBxtras
1905 Powers Ferry, Suite 100
Atlanta, GA 30339
Tel: 770-952-6356
Fax: 770-952-6388
www.vbxtras.com
** Extend Visual Basic by Writing your Own DLL Files
If you plan to write lots of Visual Basic programs, you need to organize your commonly used procedures in separate BAS module files. That way you can quickly plug in BAS module files to any Visual Basic programs you write.
Unfortunately, BAS module files can contain only Visual Basic commands. Because languages such as C++ and Pascal can be more flexible in digging into the guts of your computer and manipulating individual bits and bytes, many programmers write commonly used procedures in C++ or Pascal and store them in separate Dynamic Link Library (DLL) files. A DLL file contains commonly used procedures that you can share among different programs.
You can write a DLL file using C++, Pascal, or even Visual Basic. But no matter which language you use, a DLL file makes your programs run faster than if you just stored BASIC code in a BAS module file instead. (For more information about writing DLL files, pick up the latest edition of MORE Visual Basic For Windows For Dummies.)
Although using DLL files can be cumbersome, using these files is still easier than your other alternative, which is to use the Windows Application Programming Interface. (See the following section "Extend Visual Basic By Using the Windows API").
** Extend Visual Basic By Using the Windows API
Visual Basic makes programming so easy because it insulates you from all the messy details needed to write Windows programs. But the price you pay for this insulation is a loss of flexibility that C++ programmers love to flaunt (as they slowly go mad, dealing with the complexities of Windows programming).
Eventually, you are likely to find Visual Basic's ease of use frustrating because you can't perform certain tasks. The quickest and easiest alternative is to buy an add-on program to give Visual Basic the features you want. But if you can't find such an add-on program, you are going to have to dig into the guts of Windows itself.
To help programmers write programs for Windows, Windows has many commands called the Microsoft Windows Application Programming Interface (API). Although these commands are complicated to learn and understand, the complexity of these commands gives you the ability to manipulate (and of course, crash) Windows to your heart's content.
If you like living dangerously, take the time to read about the Windows API. (You can find out more about the Windows API in the latest edition of MORE Visual Basic For Windows For Dummies.) Otherwise, just pretend this section never existed and skip to another part of the book.
** Buy a Program to Create Help Files
Every good Windows program has an online help system so that panicky users can browse through a hypertext reference on the screen instead of wading through hundred-page manuals that don't make any sense anyway. If you're serious about writing Visual Basic programs, you are going to need to provide a help system with your programs, too.
Creating a help system isn't difficult, just incredibly dull and tedious. Fortunately, you can get a special program to make the process a little more enjoyable. Two popular help file creation programs are RoboHelp and ForeHelp, which let you design help screens as easily as writing a document in a word processor. When you're finished, each of these programs lets you test your creations by showing you exactly how your help screens are going to look when added to your own Visual Basic programs.
In today's competitive world of software development, a good help file is crucial to your program's professional appearance. Then again, if you don't care about making your programs easier to use, go work for any of the major software companies instead.
For more information on RoboHelp and ForeHelp, contact
RoboHelp
Blue Sky Software
7777 Fay Avenue, Suite 201
La Jolla, CA 92037
Tel: 619-459-6365
Fax: 619-459-6366
www.blue-sky.com
ForeHelp
ForeFront, Inc.
4710 Table Mesa Drive, Suite B
Boulder, CO 80303
Tel: 303-499-9181
Fax: 303-494-5446
www.ff.com
** Buy a Program to Create Installation Disks
After you finish creating a program in Visual Basic, the final step is to distribute your program to others. While you can just copy your Visual Basic program onto a floppy disk and trust that the other person knows how to use the disk, you are better off using a special installation program instead.
Installation programs can guide someone, step-by-step, through the oftentimes complicated process of copying a program onto another computer. Visual Basic comes with an installation program called the Application Setup Wizard. However, you may want to buy a special installation program that offers you the chance to customize the installation with your own corporate logo, advertisements, or sound effects.
Two popular installation programs are InstallShield and PC-Install. Both programs can help you create foolproof installation programs for all your Visual Basic programs.
For more information on these installation programs, contact
PC-Install
20/20 Software, Inc.
8196 SW Hall Blvd., Suite 200
Beaverton, OR 97008
Tel: 503-520-0504
Fax: 503-520-9118
www.twenty.com
InstallShield
InstallShield Corporation
900 National Parkway, Suite 125
Schaumburg, IL 60173-5108
Tel: 847-240-9111
Fax: 847-240-9120
www.installshield.com
** Write Visual Basic Programs for the Macintosh and Linux
When Microsoft first introduced Visual Basic back in 1991, programmers went nuts over it because they could design applications quickly and easily. So Microsoft promised that they would port Visual Basic to other operating systems so you could write Visual Basic programs for other computers.
After releasing Visual Basic for MS-DOS and then letting it die, Microsoft has done nothing but continue to hint of future releases of Visual Basic for other platforms. So if you want to use Visual Basic to write a Macintosh program, you can't.
However, you can use a Macintosh Visual Basic clone called (surprise!) Visual MacStandard Basic. Visual MacStandard Basic lets you visually draw your user interface and then write BASIC code to make it do something useful.
While Visual MacStandard Basic lacks many of the advanced features of Visual Basic 6.0, it does provide you with the ability to write Macintosh programs quickly and easily, just like Visual Basic 1.0 did for Windows back in 1991. So if you know how to program in Visual Basic, you can use your Visual Basic programming skills to write Visual MacStandard Basic programs for the Macintosh.
For the really adventurous, try XBasic, a BASIC compiler that runs under both Windows and Linux. While not as intuitive to use as Visual Basic, XBasic lets you write programs for Windows or Linux without resorting to learning C or C++. Since Linux may be the operating system of tomorrow, you can leverage your BASIC programming skills by writing Linux programs so you can insure job security for the future.
Visual MacStandard Basic
Zcurve Software
8206 Rockville Road #280
Indianapolis, IN 46214
www.zcurve.com
Zbasic
Basmark Corporation
P.O. Box 40450
Cleveland, OH 44140
Tel: 440-871-8855
Fax: 440-871-1715
www.basmark.com
++ APPENDIX A
ABOUT THE CD
Provided that somebody didn't take a razor blade and slice out the plastic envelope attached to the back cover, you should find a compact disc (CD or, more specifically, CD-ROM) embedded inside a plastic envelope like a mosquito trapped in a block of amber. This CD contains the Visual Basic source code from the various exercises contained within the chapters of this book, along with a few bonus demo and trial programs provided by third-party companies. These demo or trial programs are Visual Basic ActiveX programs that you can add to your own programs, which can make your programming much easier.
The sample Visual Basic programs show actual working examples of the BASIC commands that I describe in different chapters. By studying this honest-to-goodness Visual Basic code, you can spend more time learning and less time typing tedious commands into your computer yourself.
** System Requirements
Make sure that your computer meets the minimum system requirements listed below. If your computer doesn't match up to most of these requirements, you may have problems using the contents of the CD.
A PC with a 486 or faster processor.
Microsoft Windows 95/98/NT.
At least 8MB of RAM installed in your computer. For best performance, I recommend that PCs have at least 16MB of RAM installed. (The more RAM, the better.)
A CD-ROM drive - double-speed (2x) or faster.
Visual Basic version 6.0 (earlier versions won't work with the enclosed CD, so there).
If you need more information on the basics, check out PCs For Dummies, 5th Edition, by Dan Gookin, and Windows 98 For Dummies by Andy Rathbone (both published by IDG Books Worldwide, Inc.).
** How to Get to the Good Stuff
If you are running Windows 95, 98 or NT, follow these steps to get to the items on the CD:
1. Insert the CD into your computer's CD-ROM drive.
Give your computer a moment to take a look at the CD.
2. When the light on your CD-ROM drive goes out, double-click on the My Computer Icon (which is probably In the top left corner of your desktop).
This action opens the My Computer window, which shows you all the drives attached to your computer, the Control Panel, and a couple other handy things.
3. Double-click on the !con for your CD-ROM drive.
Another window opens, showing you all the folders and files on the CD.
To use the CD, follow these steps:
1. Double-click on the file called License.txt.
This file contains the end-user license that you agree to by using the CD. When you finish reading the license, close the program (most likely NotePad) that displayed the file.
2. Double-click on the file called Readme.txt.
This file contains instructions about installing the software from this CD. It may be helpful to leave this text file open while you are using the CD.
3. Double-click on the folder for the software you are Interested in.
Be sure to read the descriptions of the programs in the next section this appendix (much of this information also shows up in the Readme file). These descriptions give you more precise information about the programs' folder names and about finding and running the installer program.
4. Find the file called Setup.exe, or Install.exe, or something similar, and double-click on that file.
The program's installer walks you through the process of setting up your new software.
To run some of the programs on the Visual Basic 6 For Dummies CD-ROM, you may need to keep the CD inside your CD-ROM drive. This is a Good Thing. Otherwise, the installed program requires you to install a very large chunk of the program to your hard drive, which may keep you from installing other software.
**What You Find
Sample Visual Basic code appears in separate folders, named according to the chapter they belong to. (See Table A-1 to find out what's in each folder.) For example, to find all the Visual Basic source code for Chapter 3, look for a folder on the CD named Chapter 3. You can load and run the source code directly off the CD or you can copy it to your hard disk. (Just remember that if you run the source code off the CD, you can't modify it.)
||
Table A-1
Folder Contents
Folder - Chapter 3
What's In It - Contains two programs, Hello1 and Hello2. By loading these files off the disk, you don't have to go through the tedium of typing BASIC code directly out of the book.
Folder - Chapter 4
What's In It - Contains the final Hello program for you to examine and enjoy.
Folder - Chapter 5
What's In It - Contains the Caption program that demonstrates how BASIC code can change the caption of an object displayed on the user interface.
Folder - Chapter 6
What's In It - Contains the Forms and Buttons program that demonstrates how to use command buttons to modify the appearance of a form. (As a bonus, this sample program also demonstrates how to use dialog boxes, which you can find out about in Chapter 13).
Folder - Chapter 7
What's In It - Contains the Boxes and Buttons program that demonstrates how to use check boxes, radio buttons, combo boxes, and list boxes.
Folder - Chapter 8
What's In It - Contains the TextBoxes program that demonstrates how to use a password text box and an ordinary text box.
Folder - Chapter 9
What's In It - Contains the ScrollBars program that demonstrates how to use scroll bars and display the scroll bar value on the screen.
Folder - Chapter 10
What's In It - Contains the Shapes program that shows how to change a circle during run-time from the user interface.
Folder - Chapter 13
What's In It - Contains the DialogBox program that shows how to use the built-in Visual Basic Common Dialog Box object.
Folder - Chapter 15
What's In It - Contains the Variables program that shows how to change the properties of objects stored on different forms.
Folder - Chapter 16
What's In It - Contains the ListBoxes programs that shows you how to use a Visual Basic list box.
Folder - Chapter 17
What's In It - Contains the Precedence program that shows you how precedence works when using parentheses and when omitting parentheses while calculating a numeric result.
Folder - Chapter 18
What's In It - Contains the ELIZA program that demonstrates how to manipulate strings. ELIZA mimics a psychotherapist who simply echoes back anything you type, giving the illusion that the computer is actually responding to you.
Folder - Chapter 23
What's In It - Contains the DoWhile program that demonstrates how a simple Do While loop works.
Folder - Chapter 25
What's In It - Contains the ForNext program that demonstrates how a simple For Next loop works.
Folder - Chapter 28
What's In It - Contains the Arguments program that shows how you can pass arguments among procedures.
Folder - Chapter 29
What's In It - Contains the Functions program that shows you how a functions that converts yards to meters works.
Folder - Chapter 30
What's In It - Contains the Mission program, which demonstrates how class modules work and how to use them within a Visual Basic program.
Folder - Chapter 34
What's In It - Contains the PrintExample program that shows how you can print stuff from within a Visual Basic program.
** What You Find in the ActiveX folder
The ActiveX folder contains all the trial ActiveX programs (see table A-2 for details) kindly provided by third-party companies eager to show you their wares and sell you the full-featured versions of their programs. While you can experiment with these trial versions, you can't use them within your own Visual Basic programs that you may distribute for sale.
Each trial ActiveX program comes with its own setup program (usually called SETUP.EXE). Just run this setup program by double-clicking on the SETUP.EXE file within Windows Explorer, or by clicking the Start button on the Windows taskbar, choosing Run, and choosing Browse until you find the SETUP.EXE program you want to run.
Some of the programs included on the Visual Basic 6 For Dummies CD were developed prior to the release of Visual Basic 6. These programs are noted as working with Visual Basic 4 and/or Visual Basic 5 only. However, all software included on the CD had been tested and does install and run with Visual Basic 6. Please visit the manufacturer Web site for each program (as noted in the Readme file on the CD) to cheek for Visual Basic 6 updates.
||
Table A-2
ActiveX Folder Contents
Program - Crescent Internet ToolPak 4.1
What It Does - If you want to create Internet applications, try this demo version of Crescent Internet ToolPak, which helps VB developers create applications that can browse the World Wide Web, send e-mail, execute FTP file transfers, and more. The Crescent Internet ToolPak simplifies accessing the alphabet soup of Internet standards such as IMAP4, TCP/IP Server, and FTP proxy support.
Program - ElasticLight 6.0
What It Does - This demo version can make your programs automatically adjust to any screen resolution. So if you wrote your program on a super VGA monitor displaying an 800 x 600 screen resolution, but someone runs your program on a plain old VGA monitor at 640 x 480 resolution, your program isn't cut off by the screen. This ActiveX control also adjusts the size of objects on your form (such as command buttons and radio buttons) if the user resizes a form.
Program - HASHcipher
What It Does - If you need secrecy, try this demo version of HASHcipher. This ActiveX control uses the Secure Hash Standard (SHS), developed by the U.S. Government's National Security Agency (NSA), to provide security through authentication. The algorithm specified by the Standard, the Secure Hash Algorithm, (SHA-1) is considered by many cryptographers to be the strongest hash algorithm available today and can be used in any application that requires authentication of a file or message.
Program - InstallShield Express 2.02
What It Does - Provides a more sophisticated installation program than the feeble one included with Visual Basic. InstallShield is the most popular installation program that commercial developers everywhere use, so join the bandwagon and try this trial version of InstallShield Express today!
Program - True DBList
What It Does - The trial version of True DBList provides a set of customizable list and combo boxes for accessing information trapped in database files. Supports multiple columns; split group heading; multiple lines per record; alternate row color or style; in-cell graphics and text; data-sensitive color and font; and tons of other features that will make absolutely no sense to you until you try to torture yourself by accessing data with Visual Basic's built-in features.
Program - True DBInput
What It Does - The trial version of True DBInput provides dataaware ActiveX input controls for creating Visual Basic database applications. True DBInput includes scores of features to handle data entry and validation; intuitive customized display of dates, time, text, and numbers; plus other features to make sure your users don't type in invalid information into your database files.
Program - VBAssist 5.0
What It Does - The trial version of VBAssist contains various tools for helping you write Visual Basic programs faster and easier than before. With VBAssist you can quickly assign Tab Order to controls on a form, assign shortcut keys or accelerators to controls, place controls into exact positions on a form, along with other time-saving features.
Program - VBPartner
What It Does - This trial edition of VBPartner is an integrated add-in for Microsoft Visual Basic that includes 11 unique timesaving "Partners" that assist you with routine application design, error handling, and more by simplifying tedious development tasks.
Program - VSData
What It Does - This demo version of VSData enables your Visual Basic applications to access database files without using Visual Basic's own bulky database files that gobble up disk space and may offer way more features than your program needs.
Program - VSDirect
What It Does - This demo version of VSDirect provides routines for accessing Microsoft's DirectX technology for creating graphics, sound, and animation. Designed mostly for creating multimedia applications, VSDirect also includes routines for playing games over a modem link or network.
Program - VSDocX
What It Does - This demo version of VSDocX helps you create help files and printed documentation for your Visual Basic applications. Takes the hassle out of writing your program's documentation yourself, a task that most every programmer despises, as proven by the poor quality of computer manuals all over the world.
Program - VSFlex 3.0
What It Does - Visual Basic comes with an ActiveX control called MSFlexGrid, which is an older version of VSFlex. Like MSFlexGrid, this demo version of VSFlex lets you sort, merge, and group data in a grid. VSFlex also includes a string pattern matching ActiveX control for evaluating mathematical expressions or adding natural language capabilities to your programs.
Program - VS-OCX 6.0
What It Does - Provides notebook-style tabs along with a string parsing ActiveX control for advanced string manipulation. This demo version of VS-OCX also includes a more sophisticated screen resolution adapting control (similar to ElasticLight).
Program - VSReports
What It Does - VSReports enables your Visual Basic programs to create and print reports from Microsoft Access database files. By using this demo version of VSReports, you can avoid using Visual Basic's own bulky and cumbersome database reporting files.
Program - VSSpell
What It Does - Provides a spell-checker and a thesaurus for your Visual Basic applications. Perfect for creating the next generation of word processors to put Microsoft Word out of business.
** If You Have Problems (of the CD Kind)
Because computers are notoriously finicky, picky, and downright unreliable at crucial times in our lives, you may be one of the unfortunate few plagued with problems while trying to use the enclosed CD.
If your computer has trouble reading the files off the CD, you may have a faulty CD. Despite our best efforts, occasionally a CD decides to follow the "dark side of the Force," turns corrupt, and tries to frighten people into thinking their computer problems are actually their fault.
If your computer can read other CDs okay but not the enclosed Visual Basic CD, please call the Hungry Minds Customer Care phone number, at 800-762-2974, and request a replacement CD.
If you don't have Visual Basic 6.0, you can't load any of the sample programs from the CD, even if you have an older version of Visual Basic, such as version 5.0 or 4.0. So check your version of Visual Basic by choosing Help->About Visual Basic. A dialog box appears to show you which version of Visual Basic you have.
The Basics
C.1 The Basics
To use phpMyAdmin to access your database, you need a username and password that are valid for connections from your web server. Your web server might be on the same machine as your MySQL server, in which case, obviously, the user only needs to be able to access the server from localhost.
Once you have logged in using a valid user account, you will see something that looks like Figure C-1.
Figure C-1. phpMyAdmin start page
As you can see in Figure C-2, there are some links to basic server information. Via the Status link, phpMyAdmin provides a way to see the status of your server without logging into it and issuing commands via a command-line interface.
Figure C-2. phpMyAdmin Runtime Information screen
To drill into a specific database, the first step is to select the database name from the pull-down menu on the left menu bar. phpMyAdmin then displays all the tables within that database, as shown in Figure C-3. This page is extremely useful at a quick glance for checking the relative sizes of your tables, which storage engine is used for each table, and the number of records contained in each.
Figure C-3. phpMyAdmin after selecting a database
A step-by-step tutorial in how to use phpMyAdmin is outside the scope of this book, but we'd like to show you some common examples of where you might find it useful to have phpMyAdmin installed because it can make your job as the database administrator significantly easier, or at least faster. It can also allow you to grant people access to issue raw SQL commands and perform maintenance without actually giving them a login on the machine or requiring them to use the MySQL command-line interface.
9.4 Rolling Your Own Backup Script
There are always circumstances in which the standard tools aren't enough to get the job done. Perhaps they're not flexible enough, they're too slow, or they just don't work the way you'd like. The solution, of course, is to build your own tool. In doing so, you may decide to use the existing utilities or to just do your own thing.
Let's look at writing a simple MySQL backup script in Perl. While it isn't the most powerful or flexible script in the world, it can serve as a starting point for building a custom solution.
The script (mysnap.pl) solves the following problem. You have a MySQL server that keeps all its data on a volume with snapshot capabilities. Every 12 hours, you'd like to perform the following tasks to make a good snapshot and gather a list of tables and their sizes:
Flush and lock all MyISAM tables.
Assemble a list of every table and its size.
Initiate a snapshot.
Unlock the tables.
Output the list of table sizes.
The script's output can be captured and automatically mailed to a backup administrator. A cron entry like this does the job nicely if you're using Vixie cron (common on Linux and FreeBSD):
MAILTO=backup-admin@example.com
00 */12 * * * /usr/local/bin/mysnap.pl
Otherwise, you can use the more traditional format:
00 0,12 * * * /usr/local/bin/mysnap.pl | mail backup-admin@example.com
You'll find the complete script listed here.
#!/usr/bin/perl -w
#
# mysnap.pl - snapshot mysql and mail stats to backup admins
use strict;
use DBIx::DWIW;
$|=1; # unbuffer output
my $db_user = 'backup_user';
my $db_pass = 'backup_pass';
my $db_name = 'mysql';
my $db_host = 'localhost';
my $command = '/usr/local/bin/snapshot';
my $conn = DBIx::DWIW->Connect(DB => $db_name, User => $db_user,
Pass => $db_pass, Host => $db_host);
my @table_sizes;
# flush and lock all tables
$conn->Execute("FLUSH TABLES WITH READ LOCK");
# gather stats on the tables
my @db_list = $conn->FlatArray("SHOW DATABASES");
for my $db (@db_list)
{
$conn->Execute("USE $db") or die "$!";
my @table_info = $conn->Hashes("SHOW TABLE STATUS");
for my $table (@table_info)
{
my $name = $table->{Name};
my $size = $table->{Data_length};
push @table_sizes, ["$db.$name", $size];
}
}
# run the snapshot
system($command);
# unlock the tables
$conn->Execute("UNLOCK TABLES");
$conn->Disconnect;
# sort by size and print
for my $info (sort { $b->[1] cmp $a->[1] } @table_sizes)
{
printf "%-10s %s\n", $info->[1], $info->[0];
}
exit;
_ _END_ _
Let's walk through the basic flow. The first thing to notice is that the script requires a module from CPAN. DBIx::DWIW simplifies most Perl work with MySQL.[7] After using the necessary modules, define the necessary variables for the connection to MySQL. Then you execute a FLUSH TABLES WITH READ LOCK to make sure all changes are on disk and that no further changes will happen.
[7] The DWIW stands for Do What I Want, a play on Perl's Do What I Mean.
Once the tables have all been flushed and locked, the script collects a list of all the databases on the server and iterates through them. In each database, the script gets the status of all the tables using SHOW TABLE STATUS, which produces records that look like this:
mysql> SHOW TABLE STATUS \G
*************************** 1. row ***************************
Name: journal
Type: MyISAM
Row_format: Dynamic
Rows: 417
Avg_row_length: 553
Data_length: 230848
Max_data_length: 4294967295
Index_length: 5120
Data_free: 0
Auto_increment: NULL
Create_time: 2001-12-09 23:18:06
Update_time: 2002-06-16 22:20:13
Check_time: 2002-05-19 17:03:35
Create_options:
Comment:
The script grabs the Name and Data_length fields for each table and stores them in the @table_sizes list. Once that data has been gathered, the script calls the snapshot command. Finally, it unlocks the tables and prints the list of tables and sizes (sorted by size).
Running mysnap.pl produces output like this:
$ mysnap.pl
9300388448 Datascope.SymbolHistory
1458868716 Chart.SymbolHistory
773481608 logs.pfs
749644404 IDX.LinkLog
457454228 SEC.SEC_Filings
442951712 IDX.BusinessWireArticles
343099968 Datascope.Symbols
208388096 IDX.Headlines
...
As expected, the largest tables are listed first—regardless of which databases they reside in.
There are many ways mysnap.pl can be improved or enhanced. It could:
Perform more error checking.
Compare the current table sizes with those from the previous run.
Notice whether a table has grown beyond a preset threshold.
Ignore Heap tables, since they don't reside on disk.
None of those enhancements are particularly difficult. With even a basic grasp of Perl and a bit of time, you can transform that script to something custom-tailored for your needs.
Let's look at writing a simple MySQL backup script in Perl. While it isn't the most powerful or flexible script in the world, it can serve as a starting point for building a custom solution.
The script (mysnap.pl) solves the following problem. You have a MySQL server that keeps all its data on a volume with snapshot capabilities. Every 12 hours, you'd like to perform the following tasks to make a good snapshot and gather a list of tables and their sizes:
Flush and lock all MyISAM tables.
Assemble a list of every table and its size.
Initiate a snapshot.
Unlock the tables.
Output the list of table sizes.
The script's output can be captured and automatically mailed to a backup administrator. A cron entry like this does the job nicely if you're using Vixie cron (common on Linux and FreeBSD):
MAILTO=backup-admin@example.com
00 */12 * * * /usr/local/bin/mysnap.pl
Otherwise, you can use the more traditional format:
00 0,12 * * * /usr/local/bin/mysnap.pl | mail backup-admin@example.com
You'll find the complete script listed here.
#!/usr/bin/perl -w
#
# mysnap.pl - snapshot mysql and mail stats to backup admins
use strict;
use DBIx::DWIW;
$|=1; # unbuffer output
my $db_user = 'backup_user';
my $db_pass = 'backup_pass';
my $db_name = 'mysql';
my $db_host = 'localhost';
my $command = '/usr/local/bin/snapshot';
my $conn = DBIx::DWIW->Connect(DB => $db_name, User => $db_user,
Pass => $db_pass, Host => $db_host);
my @table_sizes;
# flush and lock all tables
$conn->Execute("FLUSH TABLES WITH READ LOCK");
# gather stats on the tables
my @db_list = $conn->FlatArray("SHOW DATABASES");
for my $db (@db_list)
{
$conn->Execute("USE $db") or die "$!";
my @table_info = $conn->Hashes("SHOW TABLE STATUS");
for my $table (@table_info)
{
my $name = $table->{Name};
my $size = $table->{Data_length};
push @table_sizes, ["$db.$name", $size];
}
}
# run the snapshot
system($command);
# unlock the tables
$conn->Execute("UNLOCK TABLES");
$conn->Disconnect;
# sort by size and print
for my $info (sort { $b->[1] cmp $a->[1] } @table_sizes)
{
printf "%-10s %s\n", $info->[1], $info->[0];
}
exit;
_ _END_ _
Let's walk through the basic flow. The first thing to notice is that the script requires a module from CPAN. DBIx::DWIW simplifies most Perl work with MySQL.[7] After using the necessary modules, define the necessary variables for the connection to MySQL. Then you execute a FLUSH TABLES WITH READ LOCK to make sure all changes are on disk and that no further changes will happen.
[7] The DWIW stands for Do What I Want, a play on Perl's Do What I Mean.
Once the tables have all been flushed and locked, the script collects a list of all the databases on the server and iterates through them. In each database, the script gets the status of all the tables using SHOW TABLE STATUS, which produces records that look like this:
mysql> SHOW TABLE STATUS \G
*************************** 1. row ***************************
Name: journal
Type: MyISAM
Row_format: Dynamic
Rows: 417
Avg_row_length: 553
Data_length: 230848
Max_data_length: 4294967295
Index_length: 5120
Data_free: 0
Auto_increment: NULL
Create_time: 2001-12-09 23:18:06
Update_time: 2002-06-16 22:20:13
Check_time: 2002-05-19 17:03:35
Create_options:
Comment:
The script grabs the Name and Data_length fields for each table and stores them in the @table_sizes list. Once that data has been gathered, the script calls the snapshot command. Finally, it unlocks the tables and prints the list of tables and sizes (sorted by size).
Running mysnap.pl produces output like this:
$ mysnap.pl
9300388448 Datascope.SymbolHistory
1458868716 Chart.SymbolHistory
773481608 logs.pfs
749644404 IDX.LinkLog
457454228 SEC.SEC_Filings
442951712 IDX.BusinessWireArticles
343099968 Datascope.Symbols
208388096 IDX.Headlines
...
As expected, the largest tables are listed first—regardless of which databases they reside in.
There are many ways mysnap.pl can be improved or enhanced. It could:
Perform more error checking.
Compare the current table sizes with those from the previous run.
Notice whether a table has grown beyond a preset threshold.
Ignore Heap tables, since they don't reside on disk.
None of those enhancements are particularly difficult. With even a basic grasp of Perl and a bit of time, you can transform that script to something custom-tailored for your needs.
Minggu, 2008 September 28
2.5 The Storage Engines
Now it's time to look at each of MySQL's storage engines in more detail. Table 2-5 summarizes some of the high-level characteristics of the handlers. The following sections provide some basic highlights and background about each table handler as well as any unusual characteristics and interesting features.
Before going further, it's worth noting that this isn't an exhaustive discussion of MySQL's storage engines. We assume that you've read (or at least know where to find) the information in the MySQL Reference Manual.
Table 2-5. Storage engine features in MySQL Attribute
MyISAM
Heap
BDB
InnoDB
Transactions
No
No
Yes
Yes
Lock granularity
Table
Table
Page (8 KB)
Row
Storage
Split files
In-memory
Single file per table
Tablespace(s)
Isolation levels
None
None
Read committed
All
Portable format
Yes
N/A
No
Yes
Referential integrity
No
No
No
Yes
Primary key with data
No
No
Yes
Yes
MySQL caches data records
No
Yes
Yes
Yes
Availability
All versions
All versions
MySQL-Max
All Versions[3]
[3] Prior to MySQL 4.0, InnoDB was available in MySQL-Max only.
Most of MySQL's disk-based tables have some basic things in common. Each database in MySQL is simply a subdirectory of MySQL's data directory in the underlying filesystem.[4] Whenever you create a table, MySQL stores the table definition in a .frm file with the same name as the table. Thus, when you create a table named MyTable, MySQL stores the table definition in MyTable.frm.
[4] In MySQL 5.0, the term "database" will likely morph into "schema."
To determine the type of a table, use the SHOW TABLE STATUS command. For example, to examine the user table in the mysql database, you execute the following:
mysql> SHOW TABLE STATUS LIKE 'user' \G
*************************** 1. row ***************************
Name: user
Type: MyISAM
Row_format: Dynamic
Rows: 6
Avg_row_length: 59
Data_length: 356
Max_data_length: 4294967295
Index_length: 2048
Data_free: 0
Auto_increment: NULL
Create_time: 2002-01-24 18:07:17
Update_time: 2002-01-24 21:56:29
Check_time: NULL
Create_options:
Comment: Users and global privileges
1 row in set (0.06 sec)
Notice that it's a MyISAM table. You might also notice a lot of other information and statistics in the output. Let's briefly look at what each line means:
Name
The table's name.
Type
The table's type. Again, in some versions of MySQL, this may say "Engine" rather than "Type."
Row_format
Dynamic, Fixed, or Compressed. Dynamic rows vary in length because they contain variable-length fields such as VARCHAR or BLOB. Fixed rows, which are always the same size, are made up of fields that don't vary in length, such as CHAR and INTEGER. Compressed rows exist only in compressed tables (see the later section "Compressed MyISAM").
Rows
The number of rows in the table. For non-transactional tables, this number is always accurate. For transactional tables, it is usually an estimate.
Avg_row_length
How many bytes the average row contains.
Data_length
How much data (in bytes) the entire table contains.
Max_data_length
The maximum amount of data this table can hold. In a MyISAM table with dynamic (variable length) rows, the index file for a table (tablename.MYI) stores row locations using 32-bit pointers into the data file (tablename.MYD). That means it can address only up to 4 GB of space by default. See Section 2.5.1 for more details. For MyISAM tables with fixed-length rows, the limit is just under 4.3 billion rows.
Index_length
How much space is consumed by index data.
Data_free
The amount of space that has been allocated but is currently unused.
Auto_increment
The next AUTO_INCREMENT value.
Create_time
When the table was first created.
Update_time
When data in the table last changed.
Check_time
When the table was last checked using CHECK TABLE or myisamchk.
Create_options
Any other options that were specified when the table was created.
Comment
The comments, if any, that were set when the table was created.
2.5.1 MyISAM Tables
As MySQL's default storage engine, MyISAM provides a good compromise between performance and useful features. Versions of MySQL prior to 3.23 used the Index Sequential Access Method (ISAM) table format. In Version 3.23, ISAM tables were deprecated in favor of MyISAM, an enhanced ISAM format.[5] MyISAM tables don't provide transactions or a very granular locking model, but they do have full-text indexing (see Chapter 4), compression, and more.
[5] ISAM tables may be used in MySQL 4.0 and 4.1. Presumably they'll vanish sometime in the 5.x release cycle. If you're still using ISAM tables, it's time to upgrade to MyISAM!
2.5.1.1 Storage
In MyISAM storage, there are typically two files: a data file and an index file. The two files bear .MYD and .MYI extensions, respectively. The MyISAM format is platform-neutral, meaning you can copy the data and index files from an Intel-based server to a Macintosh PowerBook or Sun SPARC without any trouble.
MyISAM tables can contain either dynamic or static (fixed-length) rows. MySQL decides which format to use based on the table definition. The number of rows a MyISAM table can hold is limited primarily by the available disk space on your database server and the largest file your operating system will let you create. Some (mostly older) operating systems have been known to cut you off at 2 GB, so check your local documentation.
However, MyISAM files with variable-length rows, are set up by default to handle only 4 GB of data, mainly for efficiency. The index uses 32-bit pointers to the data records. To create a MyISAM table that can hold more than 4 GB, you must specify values for the MAX_ROWS and AVG_ROW_LENGTH options that represent ballpark figures for the amount of space you need:
CREATE TABLE mytable (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
) MAX_ROWS = 1000000000 AVG_ROW_LENGTH = 32;
In the example, we've told MySQL to be prepared to store at least 32 GB of data in the table. To find out what MySQL decided to do, simply ask for the table status:
mysql> SHOW TABLE STATUS LIKE 'mytable' \G
*************************** 1. row ***************************
Name: mytable
Type: MyISAM
Row_format: Fixed
Rows: 0
Avg_row_length: 0
Data_length: 0
Max_data_length: 98784247807
Index_length: 1024
Data_free: 0
Auto_increment: NULL
Create_time: 2002-02-24 17:36:57
Update_time: 2002-02-24 17:36:57
Check_time: NULL
Create_options: max_rows=1000000000 avg_row_length=32
Comment:
1 row in set (0.05 sec)
As you can see, MySQL remembers the create options exactly as specified. And it chose a representation capable of holding 91 GB of data!
2.5.1.2 Other stuff
As one of the oldest storage engines included in MySQL, MyISAM tables have a number of features that have been developed over time specifically to fill niche needs uncovered through years of use:
Locking and concurrency
Locking in MyISAM tables is performed at the table level. Readers obtain shared (read) locks on all tables they need to read. Writers obtain exclusive (write) locks.
Automatic repair
If MySQL is started with the --myisam-recover option, the first time it opens a MyISAM table, it examines the table to determine whether it was closed properly. If it was not (probably because of a hardware problem or power outage), MySQL scans the table for problems and repairs them. The downside, of course, is that your application must wait while a table it needs is being repaired.
Manual repair
You can use the CHECK TABLE mytable and REPAIR TABLE mytable commands to check a table for errors and repair them. The myisamchk command-line tool can also be used to check and repair tables when the server is offline.
Concurrency improvements
If a MyISAM table has no deleted rows, you can insert rows into the table while select queries are running against it.
Index features
BLOB and TEXT columns in a MyISAM table can be indexed. MyISAM tables have a limit of 500 bytes on each key, however, so the index uses only the first few hundred bytes of a BLOB or TEXT field. MyISAM tables also allow you to index columns that may contain NULL values. You can find more information on MyISAM indexes in Chapter 4.
Delayed key writes
MyISAM tables marked with the DELAY_KEY_WRITE create option don't have index changes written to disk as they are made. Instead, the changes are made to the in-memory key buffer only and flushed to disk when the associated blocks are pruned from the key buffer or when the table is closed. This can yield quite a performance boost on heavily used tables that change frequently.
2.5.2 Compressed MyISAM Tables
For circumstances in which the data never changes, such as CD-ROM- or DVD-ROM-based applications, or in some embedded environments, MyISAM tables can be compressed (or packed) using the myisampack utility. Compressed tables can't be modified, but they generally take far less space and are faster as a result. Having smaller tables means fewer disk seeks are required to find records.
On relatively modern hardware, the overhead involved in decompressing the data is insignificant for most applications. The individual rows are compressed, so MySQL doesn't need to unpack an entire table (or even a page) just to fetch a single row.
2.5.3 RAID MyISAM Tables
While they're not really a separate table type, MyISAM RAID tables do serve a particular niche. To use them, you need to compile your own copy of MySQL from source or use the MySQL-Max package. RAID tables are just like MyISAM tables except that the data file is split into several data files. Despite the reference to RAID in the name, these data files don't have to be stored on separate disks, although it is easy to do so. Writes to the table are striped across the data files, much like RAID-0 would do across physical disks. This can be helpful in two circumstances. If you have an operating system that limits file sizes to 2 or 4 GB but you need larger tables, using RAID will get you past the limit. If you're have an I/O bound table that is read from and written to very frequently, you might achieve better performance by storing each of the RAID files on a separate physical disk.
To create a RAID table, you must supply some additional options at table-creation time:
CREATE TABLE mytable (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
) RAID_TYPE = STRIPED RAID_CHUNKS = 4 RAID_CHUNKSIZE = 16;
The RAID_TYPE option, while required, must be STRIPED or RAID0, which are synonymous. No other RAID algorithms are available. The RAID_CHUNKS parameter tells MySQL how many data files to break the table into. The RAID_CHUNKSIZE option specifies how many kilobytes of data MySQL will write in each file before moving to the next.
In the previous example, MySQL would create four subdirectories named 00, 01, 02, and 03 in which it would store a file named mytable.MYD. When writing data to the table, it would write 16 KB of data to one file and then move to the next one. Once created, RAID tables are transparent. You can use them just as you would normal MyISAM tables.
With the availability of inexpensive RAID controllers and the software RAID features of some operating systems, there isn't much need for using RAID tables in MySQL. Also, it's important to realize that RAID tables split only the data file, not the indexes. If you're trying to overcome file size limits, keep an eye on the size of your index files.
2.5.4 MyISAM Merge Tables
Merge tables are the final variation of MyISAM tables that MySQL provides. Where a RAID table is a single table split into smaller pieces, a Merge table is the combination of several similar tables into one virtual table.
This is particularly useful when MySQL is used in logging applications. Imagine you store web server logs in MySQL. For ease of management, you might create a table for each month. However, when it comes time to generate annual statistics, it would be easier if all the records were in a single table. Using Merge tables, that's possible. You can create 12 normal MyISAM tables, log_2004_01, log_2004_02, ... log_2004_12, and then a Merge table named log_2004.
Queries for a particular month can be run against the specific table that holds the data. But queries that may need to cross month boundaries can be run against the Merge table log_2004 as if it was a table that contained all the data in the underlying twelve tables.
The requirements for a Merge table are that the underlying tables must:
Have exactly the same definition
Be MyISAM tables
Exist in the same database (this limitation is removed in MySQL Versions 4.1.1 and higher, however)
Interestingly, it's possible for some underlying tables to be compressed MyISAM tables. That means you can compress tables as they get old (since they're no longer being written to anyway), but still use them as part of a Merge table. Just make sure to remove the table from the Merge table before compressing it, then re-add it after it has been compressed.
Using the example table from earlier, let's create several identical tables and a Merge table that aggregates them:
CREATE TABLE mytable0 (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
);
CREATE TABLE mytable1 (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
);
CREATE TABLE mytable2 (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
);
CREATE TABLE mytable (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
) TYPE = MERGE UNION = (mytable0, mytable1, mytable2) INSERT_METHOD = LAST;
The only difference between the Merge table and the underlying tables is that it has a few extra options set at creation time. The type, of course, is MERGE. The UNION option specifies the tables that make up the Merge table. Order is important if you plan to insert into the Merge table rather than the underlying tables. The INSERT_METHOD option, which can be NO, FIRST, or LAST, tells MySQL how to handle inserts to the Merge table. If the method is NO, inserts aren't allowed. Otherwise, inserts will always go to either the first or last of the underlying tables based on the value of INSERT_METHOD.
The order of the tables is also important for unique-key lookups because as soon as the record is found, MySQL stops looking. Thus, the earlier in the list the table is, the better. In most logging applications where you'll be doing searches on the Merge table, it might make sense to put the tables in reverse chronological order. The order is also important for making ORDER BY as fast as possible because the required merge-sort will be faster when the rows are nearly in order already. If you don't specify INSERT_METHOD, the default is NO.
As with other tables, you can use SHOW TABLE STATUS to get information about a Merge table:
mysql> SHOW TABLE STATUS LIKE 'mytable' \G
*************************** 1. row ***************************
Name: mytable
Type: MRG_MyISAM
Row_format: Fixed
Rows: 2
Avg_row_length: 23
Data_length: 46
Max_data_length: NULL
Index_length: 0
Data_free: 0
Auto_increment: NULL
Create_time: NULL
Update_time: NULL
Check_time: NULL
Create_options:
Comment:
1 row in set (0.01 sec)
Not all of the data is available. MySQL doesn't keep track of the creation, update, and check times for merge tables. It also doesn't store the create options that you might expect. However, you can retrieve that information using SHOW CREATE TABLE:
mysql> SHOW CREATE TABLE mytable \G
*************************** 1. row ***************************
Table: mytable
Create Table: CREATE TABLE `mytable` (
`a` int(11) NOT NULL default '0',
`b` char(18) NOT NULL default '',
PRIMARY KEY (`a`)
) TYPE=MRG_MyISAM INSERT_METHOD=LAST UNION=(mytable0,mytable1,mytable2)
1 row in set (0.00 sec)
This demonstrates that Merge tables really aren't full-fledged tables. In fact, Merge tables have some important limitations and surprising behavior:
REPLACE queries don't work on them.
AUTO_INCREMENT columns aren't updated on insert. They are updated if you insert directly into one of the underlying tables.
DROP TABLE mytable will drop only the virtual table, not the underlying tables. This may or may not be what you'd expect.
2.5.5 InnoDB Tables
The InnoDB table handler is the newest addition to the MySQL family. Developed by Heikki Tuuri of Innobase Oy in Helsinki, Finland, InnoDB was designed with transaction processing in mind and modeled largely after Oracle.
2.5.5.1 Storage
The InnoDB table handler breaks from MySQL tradition and stores all its data in a series of one or more data files that are collectively known as a tablespace. A tablespace is essentially a black box that is completely managed by InnoDB. If a tablespace if composed of several underlying files, you can't choose or influence which of the underlying files will contain the data for any particular database or table.
InnoDB can also use raw disk partitions in building its tablespace, but that's not very common. Using disk partitions makes it more difficult to back up InnoDB's data, and the resulting performance boost is on the order of a few percent on most operating systems.
As of MySQL 4.1, you have the option of slightly more MyISAM-like storage for InnoDB. You can enable multiple tablespace support by adding innodb_file_per_table to my.cnf; this makes InnoDB create one tablespace file per newly created InnoDB table. The filename will be of the form tablename.ibd. In all other respects, they're simply dynamically sized InnoDB tablespace files. Each one just happens to contain data for only one specific table.
2.5.5.2 Locking and concurrency
InnoDB uses MVCC to achieve very high concurrency. InnoDB defaults to the repeatable read isolation level, and as of MySQL Version 4.0.5, it implements all four levels: read uncommitted, read committed, repeatable read, and serializable.
In an InnoDB transaction, You may explicitly obtain either exclusive or shared locks on rows using the MySQL statements: SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE.
2.5.5.3 Special features
Besides its excellent concurrency, InnoDB's next most popular feature is referential integrity in the form of foreign key constraints. This means that given the following schema:
CREATE TABLE master (
id INTEGER NOT NULL PRIMARY KEY,
stuff TEXT NOT NULL
) TYPE = InnoDB;
CREATE TABLE detail (
master_id INTEGER NOT NULL,
detail1 VARCHAR(80) NOT NULL,
detail2 VARCHAR(20) NOT NULL,
INDEX master_idx (master_id),
FOREIGN KEY (master_id) REFERENCES master(id)
) TYPE = InnoDB;
InnoDB doesn't allow you to insert add records to the detail table until there is a corresponding record in the master table. Attempting to do so yields an error:
mysql> INSERT INTO detail VALUES (10, 'blah', 'blah');
ERROR 1216: Cannot add a child row: a foreign key constraint fails
InnoDB also provides lightning fast record lookups for queries that use a primary key. Its clustered index system (described in more detail in Chapter 4) explains how it works.
2.5.6 Heap (In-Memory) Tables
MySQL provides in-memory Heap tables for applications in which you need incredibly fast access to data that either never changes or doesn't need to persist after a restart. Using a Heap table means that a query can complete without even waiting for disk I/O. This makes sense for lookup or mapping tables, such as area code to city/state name, or for caching the results of periodically aggregated data.
2.5.6.1 Limitations
While Heap tables are very fast, they often don't work well as replacements for disk-based tables. Until MySQL Version 4.1, Heap tables used only hash-based indexes rather than B-tree indexes (which MyISAM uses). Hash indexes are suited to only a subset of queries. Section 4.3.2 in Chapter 4 covers this in more detail.
2.5.7 Berkeley DB (BDB) Tables
MySQL's first transaction-safe storage engine, BDB is built on top of the Berkeley DB database library, which is now maintained and developed by Sleepycat Software. In fact, the original work to integrate the Berkeley DB technology with MySQL was performed jointly by MySQL AB and Sleepycat Software. Other than transactions, the BDB table handler's other main feature is that it uses page-level locking to achieve higher concurrency than MyISAM tables.
Though BDB tables have been available in MySQL since Version 3.23, they haven't proven very popular among users. Many users looking for transactions in MySQL were also looking for row-level locking or MVCC. Further dampening interest in BDB, by the time the BDB code had stabilized, word of InnoDB began to circulate. This prompted many users to hold out for the real thing and use MyISAM tables a bit longer.
If nothing else, the inclusion of BDB tables in MySQL served as a stepping stone in many ways. It prompted the MySQL developers to put the transaction-handling infrastructure into MySQL, while at the same time proving to the skeptics that MySQL wasn't a toy.
Before going further, it's worth noting that this isn't an exhaustive discussion of MySQL's storage engines. We assume that you've read (or at least know where to find) the information in the MySQL Reference Manual.
Table 2-5. Storage engine features in MySQL Attribute
MyISAM
Heap
BDB
InnoDB
Transactions
No
No
Yes
Yes
Lock granularity
Table
Table
Page (8 KB)
Row
Storage
Split files
In-memory
Single file per table
Tablespace(s)
Isolation levels
None
None
Read committed
All
Portable format
Yes
N/A
No
Yes
Referential integrity
No
No
No
Yes
Primary key with data
No
No
Yes
Yes
MySQL caches data records
No
Yes
Yes
Yes
Availability
All versions
All versions
MySQL-Max
All Versions[3]
[3] Prior to MySQL 4.0, InnoDB was available in MySQL-Max only.
Most of MySQL's disk-based tables have some basic things in common. Each database in MySQL is simply a subdirectory of MySQL's data directory in the underlying filesystem.[4] Whenever you create a table, MySQL stores the table definition in a .frm file with the same name as the table. Thus, when you create a table named MyTable, MySQL stores the table definition in MyTable.frm.
[4] In MySQL 5.0, the term "database" will likely morph into "schema."
To determine the type of a table, use the SHOW TABLE STATUS command. For example, to examine the user table in the mysql database, you execute the following:
mysql> SHOW TABLE STATUS LIKE 'user' \G
*************************** 1. row ***************************
Name: user
Type: MyISAM
Row_format: Dynamic
Rows: 6
Avg_row_length: 59
Data_length: 356
Max_data_length: 4294967295
Index_length: 2048
Data_free: 0
Auto_increment: NULL
Create_time: 2002-01-24 18:07:17
Update_time: 2002-01-24 21:56:29
Check_time: NULL
Create_options:
Comment: Users and global privileges
1 row in set (0.06 sec)
Notice that it's a MyISAM table. You might also notice a lot of other information and statistics in the output. Let's briefly look at what each line means:
Name
The table's name.
Type
The table's type. Again, in some versions of MySQL, this may say "Engine" rather than "Type."
Row_format
Dynamic, Fixed, or Compressed. Dynamic rows vary in length because they contain variable-length fields such as VARCHAR or BLOB. Fixed rows, which are always the same size, are made up of fields that don't vary in length, such as CHAR and INTEGER. Compressed rows exist only in compressed tables (see the later section "Compressed MyISAM").
Rows
The number of rows in the table. For non-transactional tables, this number is always accurate. For transactional tables, it is usually an estimate.
Avg_row_length
How many bytes the average row contains.
Data_length
How much data (in bytes) the entire table contains.
Max_data_length
The maximum amount of data this table can hold. In a MyISAM table with dynamic (variable length) rows, the index file for a table (tablename.MYI) stores row locations using 32-bit pointers into the data file (tablename.MYD). That means it can address only up to 4 GB of space by default. See Section 2.5.1 for more details. For MyISAM tables with fixed-length rows, the limit is just under 4.3 billion rows.
Index_length
How much space is consumed by index data.
Data_free
The amount of space that has been allocated but is currently unused.
Auto_increment
The next AUTO_INCREMENT value.
Create_time
When the table was first created.
Update_time
When data in the table last changed.
Check_time
When the table was last checked using CHECK TABLE or myisamchk.
Create_options
Any other options that were specified when the table was created.
Comment
The comments, if any, that were set when the table was created.
2.5.1 MyISAM Tables
As MySQL's default storage engine, MyISAM provides a good compromise between performance and useful features. Versions of MySQL prior to 3.23 used the Index Sequential Access Method (ISAM) table format. In Version 3.23, ISAM tables were deprecated in favor of MyISAM, an enhanced ISAM format.[5] MyISAM tables don't provide transactions or a very granular locking model, but they do have full-text indexing (see Chapter 4), compression, and more.
[5] ISAM tables may be used in MySQL 4.0 and 4.1. Presumably they'll vanish sometime in the 5.x release cycle. If you're still using ISAM tables, it's time to upgrade to MyISAM!
2.5.1.1 Storage
In MyISAM storage, there are typically two files: a data file and an index file. The two files bear .MYD and .MYI extensions, respectively. The MyISAM format is platform-neutral, meaning you can copy the data and index files from an Intel-based server to a Macintosh PowerBook or Sun SPARC without any trouble.
MyISAM tables can contain either dynamic or static (fixed-length) rows. MySQL decides which format to use based on the table definition. The number of rows a MyISAM table can hold is limited primarily by the available disk space on your database server and the largest file your operating system will let you create. Some (mostly older) operating systems have been known to cut you off at 2 GB, so check your local documentation.
However, MyISAM files with variable-length rows, are set up by default to handle only 4 GB of data, mainly for efficiency. The index uses 32-bit pointers to the data records. To create a MyISAM table that can hold more than 4 GB, you must specify values for the MAX_ROWS and AVG_ROW_LENGTH options that represent ballpark figures for the amount of space you need:
CREATE TABLE mytable (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
) MAX_ROWS = 1000000000 AVG_ROW_LENGTH = 32;
In the example, we've told MySQL to be prepared to store at least 32 GB of data in the table. To find out what MySQL decided to do, simply ask for the table status:
mysql> SHOW TABLE STATUS LIKE 'mytable' \G
*************************** 1. row ***************************
Name: mytable
Type: MyISAM
Row_format: Fixed
Rows: 0
Avg_row_length: 0
Data_length: 0
Max_data_length: 98784247807
Index_length: 1024
Data_free: 0
Auto_increment: NULL
Create_time: 2002-02-24 17:36:57
Update_time: 2002-02-24 17:36:57
Check_time: NULL
Create_options: max_rows=1000000000 avg_row_length=32
Comment:
1 row in set (0.05 sec)
As you can see, MySQL remembers the create options exactly as specified. And it chose a representation capable of holding 91 GB of data!
2.5.1.2 Other stuff
As one of the oldest storage engines included in MySQL, MyISAM tables have a number of features that have been developed over time specifically to fill niche needs uncovered through years of use:
Locking and concurrency
Locking in MyISAM tables is performed at the table level. Readers obtain shared (read) locks on all tables they need to read. Writers obtain exclusive (write) locks.
Automatic repair
If MySQL is started with the --myisam-recover option, the first time it opens a MyISAM table, it examines the table to determine whether it was closed properly. If it was not (probably because of a hardware problem or power outage), MySQL scans the table for problems and repairs them. The downside, of course, is that your application must wait while a table it needs is being repaired.
Manual repair
You can use the CHECK TABLE mytable and REPAIR TABLE mytable commands to check a table for errors and repair them. The myisamchk command-line tool can also be used to check and repair tables when the server is offline.
Concurrency improvements
If a MyISAM table has no deleted rows, you can insert rows into the table while select queries are running against it.
Index features
BLOB and TEXT columns in a MyISAM table can be indexed. MyISAM tables have a limit of 500 bytes on each key, however, so the index uses only the first few hundred bytes of a BLOB or TEXT field. MyISAM tables also allow you to index columns that may contain NULL values. You can find more information on MyISAM indexes in Chapter 4.
Delayed key writes
MyISAM tables marked with the DELAY_KEY_WRITE create option don't have index changes written to disk as they are made. Instead, the changes are made to the in-memory key buffer only and flushed to disk when the associated blocks are pruned from the key buffer or when the table is closed. This can yield quite a performance boost on heavily used tables that change frequently.
2.5.2 Compressed MyISAM Tables
For circumstances in which the data never changes, such as CD-ROM- or DVD-ROM-based applications, or in some embedded environments, MyISAM tables can be compressed (or packed) using the myisampack utility. Compressed tables can't be modified, but they generally take far less space and are faster as a result. Having smaller tables means fewer disk seeks are required to find records.
On relatively modern hardware, the overhead involved in decompressing the data is insignificant for most applications. The individual rows are compressed, so MySQL doesn't need to unpack an entire table (or even a page) just to fetch a single row.
2.5.3 RAID MyISAM Tables
While they're not really a separate table type, MyISAM RAID tables do serve a particular niche. To use them, you need to compile your own copy of MySQL from source or use the MySQL-Max package. RAID tables are just like MyISAM tables except that the data file is split into several data files. Despite the reference to RAID in the name, these data files don't have to be stored on separate disks, although it is easy to do so. Writes to the table are striped across the data files, much like RAID-0 would do across physical disks. This can be helpful in two circumstances. If you have an operating system that limits file sizes to 2 or 4 GB but you need larger tables, using RAID will get you past the limit. If you're have an I/O bound table that is read from and written to very frequently, you might achieve better performance by storing each of the RAID files on a separate physical disk.
To create a RAID table, you must supply some additional options at table-creation time:
CREATE TABLE mytable (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
) RAID_TYPE = STRIPED RAID_CHUNKS = 4 RAID_CHUNKSIZE = 16;
The RAID_TYPE option, while required, must be STRIPED or RAID0, which are synonymous. No other RAID algorithms are available. The RAID_CHUNKS parameter tells MySQL how many data files to break the table into. The RAID_CHUNKSIZE option specifies how many kilobytes of data MySQL will write in each file before moving to the next.
In the previous example, MySQL would create four subdirectories named 00, 01, 02, and 03 in which it would store a file named mytable.MYD. When writing data to the table, it would write 16 KB of data to one file and then move to the next one. Once created, RAID tables are transparent. You can use them just as you would normal MyISAM tables.
With the availability of inexpensive RAID controllers and the software RAID features of some operating systems, there isn't much need for using RAID tables in MySQL. Also, it's important to realize that RAID tables split only the data file, not the indexes. If you're trying to overcome file size limits, keep an eye on the size of your index files.
2.5.4 MyISAM Merge Tables
Merge tables are the final variation of MyISAM tables that MySQL provides. Where a RAID table is a single table split into smaller pieces, a Merge table is the combination of several similar tables into one virtual table.
This is particularly useful when MySQL is used in logging applications. Imagine you store web server logs in MySQL. For ease of management, you might create a table for each month. However, when it comes time to generate annual statistics, it would be easier if all the records were in a single table. Using Merge tables, that's possible. You can create 12 normal MyISAM tables, log_2004_01, log_2004_02, ... log_2004_12, and then a Merge table named log_2004.
Queries for a particular month can be run against the specific table that holds the data. But queries that may need to cross month boundaries can be run against the Merge table log_2004 as if it was a table that contained all the data in the underlying twelve tables.
The requirements for a Merge table are that the underlying tables must:
Have exactly the same definition
Be MyISAM tables
Exist in the same database (this limitation is removed in MySQL Versions 4.1.1 and higher, however)
Interestingly, it's possible for some underlying tables to be compressed MyISAM tables. That means you can compress tables as they get old (since they're no longer being written to anyway), but still use them as part of a Merge table. Just make sure to remove the table from the Merge table before compressing it, then re-add it after it has been compressed.
Using the example table from earlier, let's create several identical tables and a Merge table that aggregates them:
CREATE TABLE mytable0 (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
);
CREATE TABLE mytable1 (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
);
CREATE TABLE mytable2 (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
);
CREATE TABLE mytable (
a INTEGER NOT NULL PRIMARY KEY,
b CHAR(18) NOT NULL
) TYPE = MERGE UNION = (mytable0, mytable1, mytable2) INSERT_METHOD = LAST;
The only difference between the Merge table and the underlying tables is that it has a few extra options set at creation time. The type, of course, is MERGE. The UNION option specifies the tables that make up the Merge table. Order is important if you plan to insert into the Merge table rather than the underlying tables. The INSERT_METHOD option, which can be NO, FIRST, or LAST, tells MySQL how to handle inserts to the Merge table. If the method is NO, inserts aren't allowed. Otherwise, inserts will always go to either the first or last of the underlying tables based on the value of INSERT_METHOD.
The order of the tables is also important for unique-key lookups because as soon as the record is found, MySQL stops looking. Thus, the earlier in the list the table is, the better. In most logging applications where you'll be doing searches on the Merge table, it might make sense to put the tables in reverse chronological order. The order is also important for making ORDER BY as fast as possible because the required merge-sort will be faster when the rows are nearly in order already. If you don't specify INSERT_METHOD, the default is NO.
As with other tables, you can use SHOW TABLE STATUS to get information about a Merge table:
mysql> SHOW TABLE STATUS LIKE 'mytable' \G
*************************** 1. row ***************************
Name: mytable
Type: MRG_MyISAM
Row_format: Fixed
Rows: 2
Avg_row_length: 23
Data_length: 46
Max_data_length: NULL
Index_length: 0
Data_free: 0
Auto_increment: NULL
Create_time: NULL
Update_time: NULL
Check_time: NULL
Create_options:
Comment:
1 row in set (0.01 sec)
Not all of the data is available. MySQL doesn't keep track of the creation, update, and check times for merge tables. It also doesn't store the create options that you might expect. However, you can retrieve that information using SHOW CREATE TABLE:
mysql> SHOW CREATE TABLE mytable \G
*************************** 1. row ***************************
Table: mytable
Create Table: CREATE TABLE `mytable` (
`a` int(11) NOT NULL default '0',
`b` char(18) NOT NULL default '',
PRIMARY KEY (`a`)
) TYPE=MRG_MyISAM INSERT_METHOD=LAST UNION=(mytable0,mytable1,mytable2)
1 row in set (0.00 sec)
This demonstrates that Merge tables really aren't full-fledged tables. In fact, Merge tables have some important limitations and surprising behavior:
REPLACE queries don't work on them.
AUTO_INCREMENT columns aren't updated on insert. They are updated if you insert directly into one of the underlying tables.
DROP TABLE mytable will drop only the virtual table, not the underlying tables. This may or may not be what you'd expect.
2.5.5 InnoDB Tables
The InnoDB table handler is the newest addition to the MySQL family. Developed by Heikki Tuuri of Innobase Oy in Helsinki, Finland, InnoDB was designed with transaction processing in mind and modeled largely after Oracle.
2.5.5.1 Storage
The InnoDB table handler breaks from MySQL tradition and stores all its data in a series of one or more data files that are collectively known as a tablespace. A tablespace is essentially a black box that is completely managed by InnoDB. If a tablespace if composed of several underlying files, you can't choose or influence which of the underlying files will contain the data for any particular database or table.
InnoDB can also use raw disk partitions in building its tablespace, but that's not very common. Using disk partitions makes it more difficult to back up InnoDB's data, and the resulting performance boost is on the order of a few percent on most operating systems.
As of MySQL 4.1, you have the option of slightly more MyISAM-like storage for InnoDB. You can enable multiple tablespace support by adding innodb_file_per_table to my.cnf; this makes InnoDB create one tablespace file per newly created InnoDB table. The filename will be of the form tablename.ibd. In all other respects, they're simply dynamically sized InnoDB tablespace files. Each one just happens to contain data for only one specific table.
2.5.5.2 Locking and concurrency
InnoDB uses MVCC to achieve very high concurrency. InnoDB defaults to the repeatable read isolation level, and as of MySQL Version 4.0.5, it implements all four levels: read uncommitted, read committed, repeatable read, and serializable.
In an InnoDB transaction, You may explicitly obtain either exclusive or shared locks on rows using the MySQL statements: SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE.
2.5.5.3 Special features
Besides its excellent concurrency, InnoDB's next most popular feature is referential integrity in the form of foreign key constraints. This means that given the following schema:
CREATE TABLE master (
id INTEGER NOT NULL PRIMARY KEY,
stuff TEXT NOT NULL
) TYPE = InnoDB;
CREATE TABLE detail (
master_id INTEGER NOT NULL,
detail1 VARCHAR(80) NOT NULL,
detail2 VARCHAR(20) NOT NULL,
INDEX master_idx (master_id),
FOREIGN KEY (master_id) REFERENCES master(id)
) TYPE = InnoDB;
InnoDB doesn't allow you to insert add records to the detail table until there is a corresponding record in the master table. Attempting to do so yields an error:
mysql> INSERT INTO detail VALUES (10, 'blah', 'blah');
ERROR 1216: Cannot add a child row: a foreign key constraint fails
InnoDB also provides lightning fast record lookups for queries that use a primary key. Its clustered index system (described in more detail in Chapter 4) explains how it works.
2.5.6 Heap (In-Memory) Tables
MySQL provides in-memory Heap tables for applications in which you need incredibly fast access to data that either never changes or doesn't need to persist after a restart. Using a Heap table means that a query can complete without even waiting for disk I/O. This makes sense for lookup or mapping tables, such as area code to city/state name, or for caching the results of periodically aggregated data.
2.5.6.1 Limitations
While Heap tables are very fast, they often don't work well as replacements for disk-based tables. Until MySQL Version 4.1, Heap tables used only hash-based indexes rather than B-tree indexes (which MyISAM uses). Hash indexes are suited to only a subset of queries. Section 4.3.2 in Chapter 4 covers this in more detail.
2.5.7 Berkeley DB (BDB) Tables
MySQL's first transaction-safe storage engine, BDB is built on top of the Berkeley DB database library, which is now maintained and developed by Sleepycat Software. In fact, the original work to integrate the Berkeley DB technology with MySQL was performed jointly by MySQL AB and Sleepycat Software. Other than transactions, the BDB table handler's other main feature is that it uses page-level locking to achieve higher concurrency than MyISAM tables.
Though BDB tables have been available in MySQL since Version 3.23, they haven't proven very popular among users. Many users looking for transactions in MySQL were also looking for row-level locking or MVCC. Further dampening interest in BDB, by the time the BDB code had stabilized, word of InnoDB began to circulate. This prompted many users to hold out for the real thing and use MyISAM tables a bit longer.
If nothing else, the inclusion of BDB tables in MySQL served as a stepping stone in many ways. It prompted the MySQL developers to put the transaction-handling infrastructure into MySQL, while at the same time proving to the skeptics that MySQL wasn't a toy.
2.4 Selecting the Right Engine
When designing MySQL-based applications, you should decide which engine to use for storing your data. If you don't think about it during the design phase, you will likely face complications later in the process. You might find that the default engine doesn't provide a feature you need, such as transactions. Or maybe the mix of read and write queries your application generates will require more granular locking than MyISAM's table locks.
Because you can make the choice on a table-by-table basis, you'll need a clear idea of how each table is used and the data it stores. Of course, it also helps to have a good understanding of the application as a whole and its potential for growth. Armed with this information, you can begin to make good choices about which table engines can do the job.
2.4.1 Considerations
While there are many factors that can affect your decision, it usually boils down to just a few considerations: transactions and concurrency, backups, and special features.
2.4.1.1 Transactions and concurrency
When it comes to transactions and concurrency, consider the following guidelines:
If your application requires transactions and high read/write concurrency, InnoDB is probably your best bet.
If your application requires transactions but only moderate read/write concurrency, either BDB or InnoDB tables should work fine.
If your application doesn't require transactions and issues primarily SELECT or primarily INSERT/UPDATE queries, MyISAM is a good choice. Many web applications fall into this category.
2.4.1.2 Backups
The need to perform regular backups may also influence your table choices. If your server can be shut down at regular intervals for backups, the storage engines are equally easy to deal with. However, if you need to perform online backups in one form or another, the choices become less clear. Chapter 9 deals with this topic in more detail.
Another way of looking at this is simplicity. As you'll see in Chapter 9, using multiple storage engines increases the complexity of backups and server tuning. You may decide that it's just easier to use a single storage engine rather than those that are theoretically best.
2.4.1.3 Special features
Finally, you sometimes find that an application relies on particular features or optimizations that are provided by only some of MySQL's storage engines. For example, not all tables provide a quick answer to queries like the following:
SELECT COUNT(*) FROM mytable
If your application depends on accurate and fast row counts, MyISAM is the answer. InnoDB must actually count up all the rows, but the MyISAM storage engine always knows the exact row count of a table without the need to do any work.
If your application requires referential integrity with foreign keys, you're limited to just InnoDB tables. Do you need full-text search capabilities? Only MyISAM tables provide it.
Keep this in mind as you read the more detailed information about each table's features. There will come a time when you find that the feature you really, really need is available only in one table engine. When that happens, you need to either compromise or break a table into multiple tables of different types.
2.4.2 Practical Examples
These issues may seem rather abstract without some sort of real-world context. So let's consider some common uses for tables in various database applications. For each table, we'll look at which engine best matches with the table's needs. The details of each engine are covered in the next section.
2.4.2.1 Logging
Suppose you want to use MySQL to log a record of every telephone call from a central telephone switch in real time. Or maybe you've installed mod_log_sql for Apache so you can log all visits to your web site directly in a table. In such an application, speed is probably the most important goal; you don't want the database to be the bottleneck. Using MyISAM tables works very well because they have very low overhead and can handle inserting thousands of records per second.
Things will get interesting if you decide it's time to start running reports to summarize the data you've logged. Depending on the queries you use, there's a good chance you will significantly slow the process of inserting records while gathering data for the report. What can you do?
You can use MySQL's built-in replication (Chapter 7) to clone the data onto a second (slave) server. You can then run your time- and CPU-intensive queries against the data on the slave. This keeps the master free to insert records as fast as it possibly can while also giving you the freedom to run any query you want without worrying about how it could affect the real-time logging.
Another option is to use a MyISAM Merge table. Rather than always logging to the same table, adjust the application to log to a table that contains the name or number of the month in its name, such as web_logs_2004_01 or web_logs_2004_jan. Then define a Merge table that contains the data you'd like to summarize and use it in your queries. If you need to summarize data daily or weekly, the same strategy works; you just need to create tables with more specific names, such as web_logs_2004_01_01. While you're busy running queries against tables that are no longer being written to, your application can log records to its current table uninterrupted. Merge tables are discussed in the later section "MyISAM Merge tables."
A final possibility is simply to switch to using a table that has more granular locking than MyISAM does. Either BDB or InnoDB works well in this case. Non-MyISAM tables will generally use more CPU and disk space, but that may be a reasonable tradeoff in this case. Also, in the event of a crash, MyISAM tables may take quite a long time to check and repair while InnoDB tables should recover quickly.
2.4.2.2 Read-only or read-mostly tables
Tables that contain the data used to construct a catalog or listing of some sort (jobs, auctions, real estate, etc.) are usually read from far more often than they are written to. This makes them great candidates for MyISAM.
2.4.2.3 Order processing
When you deal with any sort of order processing, transactions are a requirement. Half-completed orders aren't going to endear customers to your service. Using transaction-safe table types (InnoDB or BDB), these unfortunate "data surprises" can be avoided. Considering that BDB tables use—at best—locking at the page level, applications with high transaction volumes should consider InnoDB tables.
In the case of order processing, InnoDB has a distinct advantage because it supports referential integrity through the use of foreign keys. These keys allow a field in one table to have an enforced relationship to the key of another table (e.g., an Order record contains a CustomerID field that "points" to the primary key of the Customer table). Foreign keys effectively point to those other tables and indicate that data is maintained in them, and they help you keep data consistent across your tables. (Keep in mind that a foreign key in an InnoDB table must reference another InnoDB table. Currently they can't cross storage engines.)
You might want to design your tables so that customers can't be removed without also removing all their orders. Or maybe you'd like to ensure that products aren't deleted from the catalog table before the orders that reference those products are archived. With InnoDB's foreign keys, you can.
2.4.2.4 Stock quotes
If you're collecting stock quotes for your own analysis, MyISAM tables work great. However, if you're running a high-traffic web service that has a real-time quote feed and thousands of users, a query should never have to wait. At any time, there could be many clients attempting to read and write to the table, so the row-level locking provided by InnoDB tables is the way to go.
If you have sufficient memory, MySQL's in-memory Heap tables might be an option, too. However, their indexes have some interesting restrictions you need to investigate first. See Section 4.3.2 in Chapter 4 for more details.
2.4.2.5 Bulletin boards and threaded discussion forums
Threaded discussions are an interesting problem for MySQL users. There are hundreds of freely available PHP and Perl-based systems available that provide threaded discussions. Many of them aren't written with database efficiency in mind, so they tend to perform a large number of queries for each request they serve, as well as updates to counters and usage statistics about the various discussions. Many of the systems also use a small number of monolithic tables to store all their data. As a result, a few central tables become the focus of heavy read and write activity, and the locks required to enforce concurrency become a substantial source of contention.
Despite their design shortcomings, most of the systems work well for small and medium loads. However, if a web site grows large enough and generates a significant amount of traffic, it may begin to get very slow. The obvious solution is to switch to a different table type that can handle the heavy read/write volume. Users who have attempted this are sometimes surprised to find that the system runs even more slowly than it did before!
What they don't realize is that the system is using a particular query, normally something like:
SELECT COUNT(*) FROM table WHERE ...
The problem is that not all engines can run that query quickly. MyISAM tables keep accurate row counts available, so they can. But BDB and InnoDB must actually scan the data to count all the rows. The developers of the popular web site Slashdot (http://slashdot.org/) ran into this problem when they moved their system from MyISAM to InnoDB tables. They spent time going through their code to eliminate all those queries.
MySQL's query cache, which we'll cover in more detail in Chapter 5, can often be a big help in situations in which an application issues the same query over and over with the same parameters.
2.4.2.6 CD-ROM applications
If you ever need to distribute a CD-ROM- or DVD-ROM-based application that uses MySQL data files, consider using MyISAM or Compressed MyISAM tables. They can be easily isolated and copied to other media. Compressed MyISAM tables take far less space than uncompressed ones, but they are read-only. Since the data is going to be on read-only media anyway, there's little reason not to use compressed tables.
2.4.3 Table Conversions
Several techniques are available to convert one table type to another, each with advantages and disadvantages. In the following sections, we cover three of the most common.
2.4.3.1 ALTER TABLE
The easiest way to move a table from one engine to another is by using an ALTER TABLE statement. The following command converts mytable to BDB:
ALTER TABLE mytable TYPE = BDB;
As of MySQL Versions 4.0.18 and 4.1.2, you may use ENGINE instead of TYPE. In a later version of MySQL (probably in the 5.x series), support for TYPE will be removed entirely.
The previous syntax works for all storage engines, but there's a catch: it can take a lot of time. MySQL will perform a row-by-row copy of your old table into your new table. During that time, you'll probably be using all the server's disk I/O capacity, and the original table will be locked while the conversion runs. So take care before trying this technique on a busy table. Instead, you can use one of the following methods, which involve making a copy of the table first.
2.4.3.2 Dump and reimport
To gain more control over the process, you might choose to dump the table to a text file using the mysqldump utility. Once the table is dumped, simply edit the dump file to adjust the CREATE TABLE statement it contains. Be sure to change the table name as well as its type because you can't have two tables with the same name in the same database even if they are of different types.
If you import into InnoDB or BDB, be sure to use the --no-autocommit option to disable AUTOCOMMIT mode. Otherwise each individual insert will be performed in its own transaction.
The downside of using mysqldump is that it isn't terribly fast and uses far more disk space. Not only will the dump file contain all the data from the table, it will also contain all the SQL necessary to repopulate the table. Also, you won't be able to delete the dump file until the new table has been created.
Furthermore, if the dump file happens to be quite large, editing it can be a challenge. You can't simply load a 6-GB file into vi or emacs on most systems.[2] Instead, you'll need to craft a Perl or sed script to do the job.
[2] Maybe you can, but it'll be pretty painful.
2.4.3.3 CREATE and SELECT
The third technique is a compromise between the speed of the first mechanism and the safety of the second. Rather than dumping the entire table or converting it all at once, you create the new table and use MySQL's INSERT INTO ... SELECT syntax to populate it incrementally. If, for example, you have a MyISAM table called myisam_table that you'd like to convert to an InnoDB table named innodb_table, you need to run queries like this:
BEGIN;
INSERT INTO innodb_table SELECT * FROM myisam_table WHERE id BETWEEN x AND y;
COMMIT;
Assuming that id is the primary key, you run that query using larger values of x and y each time until all the data has been copied to the new table. After doing so, you are left with the original table, which you can drop after you're done with it, and the new table, which is now fully populated.
Alternatively, if you use MySQL 4.1 or newer, you can create the new table and copy the table in two steps:
CREATE TABLE newtable LIKE mytable;
INSERT INTO newtable SELECT * FROM mytable;
Whichever method you use, if you're dealing with a large volume of data, it's often more efficient to copy the data before adding indexes to the new table.
Because you can make the choice on a table-by-table basis, you'll need a clear idea of how each table is used and the data it stores. Of course, it also helps to have a good understanding of the application as a whole and its potential for growth. Armed with this information, you can begin to make good choices about which table engines can do the job.
2.4.1 Considerations
While there are many factors that can affect your decision, it usually boils down to just a few considerations: transactions and concurrency, backups, and special features.
2.4.1.1 Transactions and concurrency
When it comes to transactions and concurrency, consider the following guidelines:
If your application requires transactions and high read/write concurrency, InnoDB is probably your best bet.
If your application requires transactions but only moderate read/write concurrency, either BDB or InnoDB tables should work fine.
If your application doesn't require transactions and issues primarily SELECT or primarily INSERT/UPDATE queries, MyISAM is a good choice. Many web applications fall into this category.
2.4.1.2 Backups
The need to perform regular backups may also influence your table choices. If your server can be shut down at regular intervals for backups, the storage engines are equally easy to deal with. However, if you need to perform online backups in one form or another, the choices become less clear. Chapter 9 deals with this topic in more detail.
Another way of looking at this is simplicity. As you'll see in Chapter 9, using multiple storage engines increases the complexity of backups and server tuning. You may decide that it's just easier to use a single storage engine rather than those that are theoretically best.
2.4.1.3 Special features
Finally, you sometimes find that an application relies on particular features or optimizations that are provided by only some of MySQL's storage engines. For example, not all tables provide a quick answer to queries like the following:
SELECT COUNT(*) FROM mytable
If your application depends on accurate and fast row counts, MyISAM is the answer. InnoDB must actually count up all the rows, but the MyISAM storage engine always knows the exact row count of a table without the need to do any work.
If your application requires referential integrity with foreign keys, you're limited to just InnoDB tables. Do you need full-text search capabilities? Only MyISAM tables provide it.
Keep this in mind as you read the more detailed information about each table's features. There will come a time when you find that the feature you really, really need is available only in one table engine. When that happens, you need to either compromise or break a table into multiple tables of different types.
2.4.2 Practical Examples
These issues may seem rather abstract without some sort of real-world context. So let's consider some common uses for tables in various database applications. For each table, we'll look at which engine best matches with the table's needs. The details of each engine are covered in the next section.
2.4.2.1 Logging
Suppose you want to use MySQL to log a record of every telephone call from a central telephone switch in real time. Or maybe you've installed mod_log_sql for Apache so you can log all visits to your web site directly in a table. In such an application, speed is probably the most important goal; you don't want the database to be the bottleneck. Using MyISAM tables works very well because they have very low overhead and can handle inserting thousands of records per second.
Things will get interesting if you decide it's time to start running reports to summarize the data you've logged. Depending on the queries you use, there's a good chance you will significantly slow the process of inserting records while gathering data for the report. What can you do?
You can use MySQL's built-in replication (Chapter 7) to clone the data onto a second (slave) server. You can then run your time- and CPU-intensive queries against the data on the slave. This keeps the master free to insert records as fast as it possibly can while also giving you the freedom to run any query you want without worrying about how it could affect the real-time logging.
Another option is to use a MyISAM Merge table. Rather than always logging to the same table, adjust the application to log to a table that contains the name or number of the month in its name, such as web_logs_2004_01 or web_logs_2004_jan. Then define a Merge table that contains the data you'd like to summarize and use it in your queries. If you need to summarize data daily or weekly, the same strategy works; you just need to create tables with more specific names, such as web_logs_2004_01_01. While you're busy running queries against tables that are no longer being written to, your application can log records to its current table uninterrupted. Merge tables are discussed in the later section "MyISAM Merge tables."
A final possibility is simply to switch to using a table that has more granular locking than MyISAM does. Either BDB or InnoDB works well in this case. Non-MyISAM tables will generally use more CPU and disk space, but that may be a reasonable tradeoff in this case. Also, in the event of a crash, MyISAM tables may take quite a long time to check and repair while InnoDB tables should recover quickly.
2.4.2.2 Read-only or read-mostly tables
Tables that contain the data used to construct a catalog or listing of some sort (jobs, auctions, real estate, etc.) are usually read from far more often than they are written to. This makes them great candidates for MyISAM.
2.4.2.3 Order processing
When you deal with any sort of order processing, transactions are a requirement. Half-completed orders aren't going to endear customers to your service. Using transaction-safe table types (InnoDB or BDB), these unfortunate "data surprises" can be avoided. Considering that BDB tables use—at best—locking at the page level, applications with high transaction volumes should consider InnoDB tables.
In the case of order processing, InnoDB has a distinct advantage because it supports referential integrity through the use of foreign keys. These keys allow a field in one table to have an enforced relationship to the key of another table (e.g., an Order record contains a CustomerID field that "points" to the primary key of the Customer table). Foreign keys effectively point to those other tables and indicate that data is maintained in them, and they help you keep data consistent across your tables. (Keep in mind that a foreign key in an InnoDB table must reference another InnoDB table. Currently they can't cross storage engines.)
You might want to design your tables so that customers can't be removed without also removing all their orders. Or maybe you'd like to ensure that products aren't deleted from the catalog table before the orders that reference those products are archived. With InnoDB's foreign keys, you can.
2.4.2.4 Stock quotes
If you're collecting stock quotes for your own analysis, MyISAM tables work great. However, if you're running a high-traffic web service that has a real-time quote feed and thousands of users, a query should never have to wait. At any time, there could be many clients attempting to read and write to the table, so the row-level locking provided by InnoDB tables is the way to go.
If you have sufficient memory, MySQL's in-memory Heap tables might be an option, too. However, their indexes have some interesting restrictions you need to investigate first. See Section 4.3.2 in Chapter 4 for more details.
2.4.2.5 Bulletin boards and threaded discussion forums
Threaded discussions are an interesting problem for MySQL users. There are hundreds of freely available PHP and Perl-based systems available that provide threaded discussions. Many of them aren't written with database efficiency in mind, so they tend to perform a large number of queries for each request they serve, as well as updates to counters and usage statistics about the various discussions. Many of the systems also use a small number of monolithic tables to store all their data. As a result, a few central tables become the focus of heavy read and write activity, and the locks required to enforce concurrency become a substantial source of contention.
Despite their design shortcomings, most of the systems work well for small and medium loads. However, if a web site grows large enough and generates a significant amount of traffic, it may begin to get very slow. The obvious solution is to switch to a different table type that can handle the heavy read/write volume. Users who have attempted this are sometimes surprised to find that the system runs even more slowly than it did before!
What they don't realize is that the system is using a particular query, normally something like:
SELECT COUNT(*) FROM table WHERE ...
The problem is that not all engines can run that query quickly. MyISAM tables keep accurate row counts available, so they can. But BDB and InnoDB must actually scan the data to count all the rows. The developers of the popular web site Slashdot (http://slashdot.org/) ran into this problem when they moved their system from MyISAM to InnoDB tables. They spent time going through their code to eliminate all those queries.
MySQL's query cache, which we'll cover in more detail in Chapter 5, can often be a big help in situations in which an application issues the same query over and over with the same parameters.
2.4.2.6 CD-ROM applications
If you ever need to distribute a CD-ROM- or DVD-ROM-based application that uses MySQL data files, consider using MyISAM or Compressed MyISAM tables. They can be easily isolated and copied to other media. Compressed MyISAM tables take far less space than uncompressed ones, but they are read-only. Since the data is going to be on read-only media anyway, there's little reason not to use compressed tables.
2.4.3 Table Conversions
Several techniques are available to convert one table type to another, each with advantages and disadvantages. In the following sections, we cover three of the most common.
2.4.3.1 ALTER TABLE
The easiest way to move a table from one engine to another is by using an ALTER TABLE statement. The following command converts mytable to BDB:
ALTER TABLE mytable TYPE = BDB;
As of MySQL Versions 4.0.18 and 4.1.2, you may use ENGINE instead of TYPE. In a later version of MySQL (probably in the 5.x series), support for TYPE will be removed entirely.
The previous syntax works for all storage engines, but there's a catch: it can take a lot of time. MySQL will perform a row-by-row copy of your old table into your new table. During that time, you'll probably be using all the server's disk I/O capacity, and the original table will be locked while the conversion runs. So take care before trying this technique on a busy table. Instead, you can use one of the following methods, which involve making a copy of the table first.
2.4.3.2 Dump and reimport
To gain more control over the process, you might choose to dump the table to a text file using the mysqldump utility. Once the table is dumped, simply edit the dump file to adjust the CREATE TABLE statement it contains. Be sure to change the table name as well as its type because you can't have two tables with the same name in the same database even if they are of different types.
If you import into InnoDB or BDB, be sure to use the --no-autocommit option to disable AUTOCOMMIT mode. Otherwise each individual insert will be performed in its own transaction.
The downside of using mysqldump is that it isn't terribly fast and uses far more disk space. Not only will the dump file contain all the data from the table, it will also contain all the SQL necessary to repopulate the table. Also, you won't be able to delete the dump file until the new table has been created.
Furthermore, if the dump file happens to be quite large, editing it can be a challenge. You can't simply load a 6-GB file into vi or emacs on most systems.[2] Instead, you'll need to craft a Perl or sed script to do the job.
[2] Maybe you can, but it'll be pretty painful.
2.4.3.3 CREATE and SELECT
The third technique is a compromise between the speed of the first mechanism and the safety of the second. Rather than dumping the entire table or converting it all at once, you create the new table and use MySQL's INSERT INTO ... SELECT syntax to populate it incrementally. If, for example, you have a MyISAM table called myisam_table that you'd like to convert to an InnoDB table named innodb_table, you need to run queries like this:
BEGIN;
INSERT INTO innodb_table SELECT * FROM myisam_table WHERE id BETWEEN x AND y;
COMMIT;
Assuming that id is the primary key, you run that query using larger values of x and y each time until all the data has been copied to the new table. After doing so, you are left with the original table, which you can drop after you're done with it, and the new table, which is now fully populated.
Alternatively, if you use MySQL 4.1 or newer, you can create the new table and copy the table in two steps:
CREATE TABLE newtable LIKE mytable;
INSERT INTO newtable SELECT * FROM mytable;
Whichever method you use, if you're dealing with a large volume of data, it's often more efficient to copy the data before adding indexes to the new table.
2.3 Transactions
You can't examine the more advanced features of a database system for very long before transactions enter the mix. A transaction is a group of SQL queries that are treated atomically, as a single unit of work. Either the entire group of queries is applied to a database, or none of them are. Little of this section is specific to MySQL. If you're already familiar with ACID transactions, feel free to skip ahead to the section "Transactions in MySQL."
A banking application is the classic example of why transactions are necessary. Imagine a bank's database with a two tables: checking and savings. To move $200 from Jane's checking account to her savings account, you need to perform at least three steps:
Make sure her checking account balance is greater than $200.
Subtract $200 from her checking account balance.
Add $200 to her savings account balance.
The entire operation should be wrapped in a transaction so that if any one of the steps fails, they can all be rolled back.
A transaction is initiated (or opened) with the BEGIN statement and applied with COMMIT or rolled back (undone) with ROLLBACK. So the SQL for the transaction might look like this:
BEGIN;
[step 1] SELECT balance FROM checking WHERE customer_id = 10233276;
[step 2] UPDATE checking SET balance = balance - 200.00 WHERE customer_id = 10233276;
[step 3] UPDATE savings SET balance = balance + 200.00 WHERE customer_id = 10233276;
COMMIT;
But transactions alone aren't the whole story. What happens if the database server crashes while performing step 3? Who knows? The customer probably just lost $200. What if another process comes along between Steps 2 and 3 and removes the entire checking account balance? The bank has given the customer a $200 credit without even knowing it.
Simply having transactions isn't sufficient unless the database server passes what is known as the ACID test. ACID is an acronym for Atomicity, Consistency, Isolation, and Durability—four tightly related criteria that are required in a well-behaved transaction processing system. Transactions that meet those four criteria are often referred to as ACID transactions.
Atomicity
Transactions must function as a single indivisible unit of work. The entire transaction is either applied or rolled back. When transactions are atomic, there is no such thing as a partially completed transaction: it's all or nothing.
Consistency
The database should always move from one consistent state to the next. Consistency ensures that a crash between Steps 2 and 3 doesn't result in $200 missing from the checking account. Because the transaction is never committed, none of the transaction's changes are ever reflected in the database.
Isolation
The results of a transaction are usually invisible to other transactions until the transaction is complete. This ensures that if a bank account summary runs after Step 2, but before Step 3, it still sees the $200 in the checking account. When we discuss isolation levels, you'll understand why we said usually invisible.
Durability
Once committed, the results of a transaction are permanent. This means that the changes must be recorded in such a way that system crashes won't lose the data. Of course, if the database server's disks fail, all bets are off. That's a hardware problem. We'll talk more about how you can minimize the effects of hardware failures in Chapter 6.
2.3.1 Benefits and Drawbacks
ACID transactions ensure that banks don't lose your money. By wrapping arbitrarily complex logic into single units of work, the database server takes some of the burden off application developers. The database server's ACID properties offer guarantees that reduce the need for code guarding against race conditions and handling crash recovery.
The downside of this extra security is that the database server has to do more work. It also means that a database server with ACID transactions will generally require more CPU power, memory, and disk space than one without them. As mentioned earlier, this is where MySQL's modularity comes into play. Because you can decide on a per-table basis if you need ACID transactions or not, you don't need to pay the performance penalty on a table that really won't benefit from transactions.
2.3.2 Isolation Levels
The previous description of isolation was a bit simplistic. Isolation is more complex than it might first appear because of some peculiar cases that can occur. The SQL standard defines four isolation levels with specific rules for which changes are and aren't visible inside and outside a transaction. Let's look at each isolation level and the type of problems that can occur.
2.3.2.1 Read uncommitted
In the read uncommitted isolation level, transactions can view the results of uncommitted transactions. At this level, many problems can occur unless you really, really know what you are doing and have a good reason for doing it. Read uncommitted is rarely used in practice. Reading uncommitted data is also known as a dirty read.
2.3.2.2 Read committed
The default isolation level for most database systems is read committed. It satisfies the simple definition of isolation used earlier. A transaction will see the results only of transactions that were already committed when it began, and its changes won't be visible to others until it's committed.
However, there are problems that can occur using that definition. To visualize the problems, refer to the sample data for the Stock and StockPrice tables as shown in Table 2-2 and Table 2-3.
Table 2-2. The Stock table id
Ticker
Name
1
MSFT
Microsoft
2
EBAY
eBay
3
YHOO
Yahoo!
4
AMZN
Amazon
Table 2-3. The StockPrice table stock_id
date
open
high
low
close
1
2002-05-01
21.25
22.30
20.18
21.30
2
2002-05-01
10.01
10.20
10.01
10.18
3
2002-05-01
18.23
19.12
18.10
19.00
4
2002-05-01
45.55
46.99
44.87
45.71
1
2002-05-02
21.30
21.45
20.02
20.21
2
2002-05-02
10.18
10.55
10.10
10.35
3
2002-05-02
19.01
19.88
19.01
19.22
4
2002-05-02
45.69
45.69
44.03
44.30
Imagine you have a Perl script that runs nightly to fetch price data about your favorite stocks. For each stock, it fetches the data and adds a record to the StockPrice table with the day's numbers. So to update the information for Amazon.com, the transaction might look like this:
BEGIN;
SELECT @id := id FROM Stock WHERE ticker = 'AMZN';
INSERT INTO StockPrice VALUES (@id, '2002-05-03', 20.50, 21.10, 20.08, 21.02);
COMMIT;
But what if, between the select and insert, Amazon's id changes from 4 to 17 and a new stock is added with id 4? Or what if Amazon is removed entirely? You'll end up inserting a record with the wrong id in the first case. And in the second case, you've inserted a record for which there is no longer a corresponding row in the Stock table. Neither of these is what you intended.
The problem is that you have a nonrepeatable read in the query. That is, the data you read in the SELECT becomes invalid by the time you execute the INSERT. The repeatable read isolation level exists to solve this problem.
2.3.2.3 Repeatable read
At the repeatable read isolation level, any rows that are read during a transaction are locked so that they can't be changed until the transaction finishes. This provides the perfect solution to the problem mentioned in the previous section, in which Amazon's id can change or vanish entirely. However, this isolation level still leaves the door open to another tricky problem: phantom reads.
Using the same data, imagine that you have a script that performs some analysis based on the data in the StockPrice table. And let's assume it does this while the nightly update is also running.
The analysis script does something like this:
BEGIN;
SELECT * FROM StockPrice WHERE close BETWEEN 10 and 20;
// think for a bit
SELECT * FROM StockPrice WHERE close BETWEEN 10 and 20;
COMMIT;
But the nightly update script inserts between those two queries new rows that happen to match the close BETWEEN 10 and 20 condition. The second query will find more rows that the first one! These additional rows are known as phantom rows (or simply phantoms). They weren't locked the first time because they didn't exist when the query ran.
Having said all that, we need to point out that this is a bit more academic than you might think. Phantom rows are such a common problem that InnoDB's locking (known as next-key locking) prevents this from happening. Rather than locking only the rows you've touched in a query, InnoDB actually locks the slot following them in the index structure as well.
2.3.2.4 Serializable
The highest level of isolation, serializable, solves the phantom read problem by ordering transactions so that they can't conflict. At this level, a lot of timeouts and lock contention may occur, but the needs of your application may bring you to accept the decreased performance in favor of the data stability that results.
Table 2-4 summarizes the various isolation levels and the drawbacks associated with each one. Keep in mind that as you move down the list, you're sacrificing concurrency and performance for increased safety.
Table 2-4. ANSI SQL isolation levels Isolation level
Dirty reads possible
Non-repeatable reads possible
Phantom reads possible
Read uncommitted
Yes
Yes
Yes
Read committed
No
Yes
Yes
Repeatable read
No
No
Yes
Serializable
No
No
No
2.3.3 Deadlocks
Whenever multiple transactions obtain locks, there is the danger of encountering a deadlock condition. Deadlocks occur when two transactions attempt to obtain conflicting locks in a different order.
For example, consider these two transactions running against the StockPrice table:
Transaction #1:
BEGIN;
UPDATE StockPrice SET close = 45.50 WHERE stock_id = 4 and date = '2002-05-01';
UPDATE StockPrice SET close = 19.80 WHERE stock_id = 3 and date = '2002-05-02';
COMMIT;
Transaction #2:
BEGIN;
UPDATE StockPrice SET high = 20.12 WHERE stock_id = 3 and date = '2002-05-02';
UPDATE StockPrice SET high = 47.20 WHERE stock_id = 4 and date = '2002-05-01';
COMMIT;
If you're unlucky, each transaction will execute its first query and update a row of data, locking it in the process. Each transaction will then attempt to update its second row only to find that it is already locked. Left unchecked, the two transactions will wait for each other to complete—forever.
To combat this problem, database systems implement various forms of deadlock detection and timeouts. The more sophisticated systems, such as InnoDB, will notice circular dependencies like the previous example and return an error. Others will give up after the query exceeds a timeout while waiting for a lock. InnoDB's default timeout is 50 seconds. In either case, applications that use transactions need to be able to handle deadlocks and possibly retry transactions.
2.3.4 Transaction Logging
Some of the overhead involved with transactions can be mitigated through the use of a transaction log. Rather than directly updating the tables on disk each time a change occurs, the system can update the in-memory copy of the data (which is very fast) and write a record of the change to a transaction log on disk. Then, at some later time, a process (or thread) can actually apply the changes that the transaction log recorded. The serial disk I/O required to append events to the log is much faster than the random seeks required to update data in various places on disk.
As long as events are written to the transaction log before a transaction is considered committed, having the changes in a log will not affect the durability of the system. If the database server crashes before all changes have been applied from the transaction log, the database will continue applying changes from the transaction log when it is restarted and before it accepts new connections.
2.3.5 Transactions in MySQL
MySQL provides two transaction-safe storage engines: Berkeley DB (BDB) and InnoDB. Their specific properties are discussed in next section. Each one offers the basic BEGIN/COMMIT/ROLLBACK functionality. They differ in their supported isolation levels, locking characteristics, deadlock detection, and other features.
2.3.5.1 AUTOCOMMIT
By default MySQL operates in AUTOCOMMIT mode. This means that unless you've explicitly begun a transaction, it automatically executes each query in a separate transaction. You can enable AUTOCOMMIT for the current connection by running:
SET AUTOCOMMIT = 1;
Disable it by executing:
SET AUTOCOMMIT = 0;
Changing the value of AUTOCOMMIT has no effect on non-transaction-safe tables such as MyISAM or HEAP.
2.3.5.2 Implicit commits
Certain commands, when issued during an open transaction, cause MySQL to commit the transaction before they execute. Typically these are commands that make significant changes, such as removing or renaming a table.
Here is the list of commands for which MySQL implicitly commits a transaction:
ALTER TABLE
BEGIN
CREATE INDEX
DROP DATABASE
DROP TABLE
RENAME TABLE
TRUNCATE
LOCK TABLES
UNLOCK TABLES
As additional features are added to MySQL, it is possible that other commands will be added to the list, so be sure to check the latest available documentation.
2.3.5.3 Isolation levels
MySQL allows you to set the isolation level using the SET TRANSACTION ISOLATION LEVEL command. Unless otherwise specified, the isolation level is changed beginning with the next transaction.
To set the level for the whole session (connection), use:
SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED
Here's how to set the global level:
SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE
MySQL recognizes all four ANSI standard isolation levels, and as of Version 4.0.5 of MySQL, InnoDB supports all of them:
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
The default isolation level can also be set using the --transaction-isolation command-line option when starting the server or set via my.cnf.
2.3.5.4 Mixing storage engines in transactions
Transaction management in MySQL is currently handled by the underlying storage engines, not at a higher level. Thus, you can't reliably mix tables stored in transactional engines (such as InnoDB and BDB) in a single transaction. A higher-level transaction management service may someday be added to MySQL, making it safe to mix and match transaction-safe tables in a transaction. Until then, don't expect it to work.
If you mix transaction-safe and non-transaction-safe tables (such as InnoDB and MyISAM) in a transaction, the transaction will work properly if all goes well. However, if a rollback is required, the changes to the non-transaction-safe table won't be undone. This leaves the database in an inconsistent state that may be difficult to recover from (and renders the entire point of transactions moot).
2.3.5.5 Simulating transactions
At times you may need the behavior of transactions when you aren't using a transaction-safe table. You can achieve something like transactions using MySQL's LOCK TABLES and UNLOCK TABLES commands. If you lock the tables that will be involved in the transaction and keep track of any changes that you make (in case you need to simulate a rollback), you'll have something equivalent to running at the serializable isolation level. But the process is kludgy and error prone, so if you really need transactions, we recommend using a transactional storage engine.
A banking application is the classic example of why transactions are necessary. Imagine a bank's database with a two tables: checking and savings. To move $200 from Jane's checking account to her savings account, you need to perform at least three steps:
Make sure her checking account balance is greater than $200.
Subtract $200 from her checking account balance.
Add $200 to her savings account balance.
The entire operation should be wrapped in a transaction so that if any one of the steps fails, they can all be rolled back.
A transaction is initiated (or opened) with the BEGIN statement and applied with COMMIT or rolled back (undone) with ROLLBACK. So the SQL for the transaction might look like this:
BEGIN;
[step 1] SELECT balance FROM checking WHERE customer_id = 10233276;
[step 2] UPDATE checking SET balance = balance - 200.00 WHERE customer_id = 10233276;
[step 3] UPDATE savings SET balance = balance + 200.00 WHERE customer_id = 10233276;
COMMIT;
But transactions alone aren't the whole story. What happens if the database server crashes while performing step 3? Who knows? The customer probably just lost $200. What if another process comes along between Steps 2 and 3 and removes the entire checking account balance? The bank has given the customer a $200 credit without even knowing it.
Simply having transactions isn't sufficient unless the database server passes what is known as the ACID test. ACID is an acronym for Atomicity, Consistency, Isolation, and Durability—four tightly related criteria that are required in a well-behaved transaction processing system. Transactions that meet those four criteria are often referred to as ACID transactions.
Atomicity
Transactions must function as a single indivisible unit of work. The entire transaction is either applied or rolled back. When transactions are atomic, there is no such thing as a partially completed transaction: it's all or nothing.
Consistency
The database should always move from one consistent state to the next. Consistency ensures that a crash between Steps 2 and 3 doesn't result in $200 missing from the checking account. Because the transaction is never committed, none of the transaction's changes are ever reflected in the database.
Isolation
The results of a transaction are usually invisible to other transactions until the transaction is complete. This ensures that if a bank account summary runs after Step 2, but before Step 3, it still sees the $200 in the checking account. When we discuss isolation levels, you'll understand why we said usually invisible.
Durability
Once committed, the results of a transaction are permanent. This means that the changes must be recorded in such a way that system crashes won't lose the data. Of course, if the database server's disks fail, all bets are off. That's a hardware problem. We'll talk more about how you can minimize the effects of hardware failures in Chapter 6.
2.3.1 Benefits and Drawbacks
ACID transactions ensure that banks don't lose your money. By wrapping arbitrarily complex logic into single units of work, the database server takes some of the burden off application developers. The database server's ACID properties offer guarantees that reduce the need for code guarding against race conditions and handling crash recovery.
The downside of this extra security is that the database server has to do more work. It also means that a database server with ACID transactions will generally require more CPU power, memory, and disk space than one without them. As mentioned earlier, this is where MySQL's modularity comes into play. Because you can decide on a per-table basis if you need ACID transactions or not, you don't need to pay the performance penalty on a table that really won't benefit from transactions.
2.3.2 Isolation Levels
The previous description of isolation was a bit simplistic. Isolation is more complex than it might first appear because of some peculiar cases that can occur. The SQL standard defines four isolation levels with specific rules for which changes are and aren't visible inside and outside a transaction. Let's look at each isolation level and the type of problems that can occur.
2.3.2.1 Read uncommitted
In the read uncommitted isolation level, transactions can view the results of uncommitted transactions. At this level, many problems can occur unless you really, really know what you are doing and have a good reason for doing it. Read uncommitted is rarely used in practice. Reading uncommitted data is also known as a dirty read.
2.3.2.2 Read committed
The default isolation level for most database systems is read committed. It satisfies the simple definition of isolation used earlier. A transaction will see the results only of transactions that were already committed when it began, and its changes won't be visible to others until it's committed.
However, there are problems that can occur using that definition. To visualize the problems, refer to the sample data for the Stock and StockPrice tables as shown in Table 2-2 and Table 2-3.
Table 2-2. The Stock table id
Ticker
Name
1
MSFT
Microsoft
2
EBAY
eBay
3
YHOO
Yahoo!
4
AMZN
Amazon
Table 2-3. The StockPrice table stock_id
date
open
high
low
close
1
2002-05-01
21.25
22.30
20.18
21.30
2
2002-05-01
10.01
10.20
10.01
10.18
3
2002-05-01
18.23
19.12
18.10
19.00
4
2002-05-01
45.55
46.99
44.87
45.71
1
2002-05-02
21.30
21.45
20.02
20.21
2
2002-05-02
10.18
10.55
10.10
10.35
3
2002-05-02
19.01
19.88
19.01
19.22
4
2002-05-02
45.69
45.69
44.03
44.30
Imagine you have a Perl script that runs nightly to fetch price data about your favorite stocks. For each stock, it fetches the data and adds a record to the StockPrice table with the day's numbers. So to update the information for Amazon.com, the transaction might look like this:
BEGIN;
SELECT @id := id FROM Stock WHERE ticker = 'AMZN';
INSERT INTO StockPrice VALUES (@id, '2002-05-03', 20.50, 21.10, 20.08, 21.02);
COMMIT;
But what if, between the select and insert, Amazon's id changes from 4 to 17 and a new stock is added with id 4? Or what if Amazon is removed entirely? You'll end up inserting a record with the wrong id in the first case. And in the second case, you've inserted a record for which there is no longer a corresponding row in the Stock table. Neither of these is what you intended.
The problem is that you have a nonrepeatable read in the query. That is, the data you read in the SELECT becomes invalid by the time you execute the INSERT. The repeatable read isolation level exists to solve this problem.
2.3.2.3 Repeatable read
At the repeatable read isolation level, any rows that are read during a transaction are locked so that they can't be changed until the transaction finishes. This provides the perfect solution to the problem mentioned in the previous section, in which Amazon's id can change or vanish entirely. However, this isolation level still leaves the door open to another tricky problem: phantom reads.
Using the same data, imagine that you have a script that performs some analysis based on the data in the StockPrice table. And let's assume it does this while the nightly update is also running.
The analysis script does something like this:
BEGIN;
SELECT * FROM StockPrice WHERE close BETWEEN 10 and 20;
// think for a bit
SELECT * FROM StockPrice WHERE close BETWEEN 10 and 20;
COMMIT;
But the nightly update script inserts between those two queries new rows that happen to match the close BETWEEN 10 and 20 condition. The second query will find more rows that the first one! These additional rows are known as phantom rows (or simply phantoms). They weren't locked the first time because they didn't exist when the query ran.
Having said all that, we need to point out that this is a bit more academic than you might think. Phantom rows are such a common problem that InnoDB's locking (known as next-key locking) prevents this from happening. Rather than locking only the rows you've touched in a query, InnoDB actually locks the slot following them in the index structure as well.
2.3.2.4 Serializable
The highest level of isolation, serializable, solves the phantom read problem by ordering transactions so that they can't conflict. At this level, a lot of timeouts and lock contention may occur, but the needs of your application may bring you to accept the decreased performance in favor of the data stability that results.
Table 2-4 summarizes the various isolation levels and the drawbacks associated with each one. Keep in mind that as you move down the list, you're sacrificing concurrency and performance for increased safety.
Table 2-4. ANSI SQL isolation levels Isolation level
Dirty reads possible
Non-repeatable reads possible
Phantom reads possible
Read uncommitted
Yes
Yes
Yes
Read committed
No
Yes
Yes
Repeatable read
No
No
Yes
Serializable
No
No
No
2.3.3 Deadlocks
Whenever multiple transactions obtain locks, there is the danger of encountering a deadlock condition. Deadlocks occur when two transactions attempt to obtain conflicting locks in a different order.
For example, consider these two transactions running against the StockPrice table:
Transaction #1:
BEGIN;
UPDATE StockPrice SET close = 45.50 WHERE stock_id = 4 and date = '2002-05-01';
UPDATE StockPrice SET close = 19.80 WHERE stock_id = 3 and date = '2002-05-02';
COMMIT;
Transaction #2:
BEGIN;
UPDATE StockPrice SET high = 20.12 WHERE stock_id = 3 and date = '2002-05-02';
UPDATE StockPrice SET high = 47.20 WHERE stock_id = 4 and date = '2002-05-01';
COMMIT;
If you're unlucky, each transaction will execute its first query and update a row of data, locking it in the process. Each transaction will then attempt to update its second row only to find that it is already locked. Left unchecked, the two transactions will wait for each other to complete—forever.
To combat this problem, database systems implement various forms of deadlock detection and timeouts. The more sophisticated systems, such as InnoDB, will notice circular dependencies like the previous example and return an error. Others will give up after the query exceeds a timeout while waiting for a lock. InnoDB's default timeout is 50 seconds. In either case, applications that use transactions need to be able to handle deadlocks and possibly retry transactions.
2.3.4 Transaction Logging
Some of the overhead involved with transactions can be mitigated through the use of a transaction log. Rather than directly updating the tables on disk each time a change occurs, the system can update the in-memory copy of the data (which is very fast) and write a record of the change to a transaction log on disk. Then, at some later time, a process (or thread) can actually apply the changes that the transaction log recorded. The serial disk I/O required to append events to the log is much faster than the random seeks required to update data in various places on disk.
As long as events are written to the transaction log before a transaction is considered committed, having the changes in a log will not affect the durability of the system. If the database server crashes before all changes have been applied from the transaction log, the database will continue applying changes from the transaction log when it is restarted and before it accepts new connections.
2.3.5 Transactions in MySQL
MySQL provides two transaction-safe storage engines: Berkeley DB (BDB) and InnoDB. Their specific properties are discussed in next section. Each one offers the basic BEGIN/COMMIT/ROLLBACK functionality. They differ in their supported isolation levels, locking characteristics, deadlock detection, and other features.
2.3.5.1 AUTOCOMMIT
By default MySQL operates in AUTOCOMMIT mode. This means that unless you've explicitly begun a transaction, it automatically executes each query in a separate transaction. You can enable AUTOCOMMIT for the current connection by running:
SET AUTOCOMMIT = 1;
Disable it by executing:
SET AUTOCOMMIT = 0;
Changing the value of AUTOCOMMIT has no effect on non-transaction-safe tables such as MyISAM or HEAP.
2.3.5.2 Implicit commits
Certain commands, when issued during an open transaction, cause MySQL to commit the transaction before they execute. Typically these are commands that make significant changes, such as removing or renaming a table.
Here is the list of commands for which MySQL implicitly commits a transaction:
ALTER TABLE
BEGIN
CREATE INDEX
DROP DATABASE
DROP TABLE
RENAME TABLE
TRUNCATE
LOCK TABLES
UNLOCK TABLES
As additional features are added to MySQL, it is possible that other commands will be added to the list, so be sure to check the latest available documentation.
2.3.5.3 Isolation levels
MySQL allows you to set the isolation level using the SET TRANSACTION ISOLATION LEVEL command. Unless otherwise specified, the isolation level is changed beginning with the next transaction.
To set the level for the whole session (connection), use:
SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED
Here's how to set the global level:
SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE
MySQL recognizes all four ANSI standard isolation levels, and as of Version 4.0.5 of MySQL, InnoDB supports all of them:
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
The default isolation level can also be set using the --transaction-isolation command-line option when starting the server or set via my.cnf.
2.3.5.4 Mixing storage engines in transactions
Transaction management in MySQL is currently handled by the underlying storage engines, not at a higher level. Thus, you can't reliably mix tables stored in transactional engines (such as InnoDB and BDB) in a single transaction. A higher-level transaction management service may someday be added to MySQL, making it safe to mix and match transaction-safe tables in a transaction. Until then, don't expect it to work.
If you mix transaction-safe and non-transaction-safe tables (such as InnoDB and MyISAM) in a transaction, the transaction will work properly if all goes well. However, if a rollback is required, the changes to the non-transaction-safe table won't be undone. This leaves the database in an inconsistent state that may be difficult to recover from (and renders the entire point of transactions moot).
2.3.5.5 Simulating transactions
At times you may need the behavior of transactions when you aren't using a transaction-safe table. You can achieve something like transactions using MySQL's LOCK TABLES and UNLOCK TABLES commands. If you lock the tables that will be involved in the transaction and keep track of any changes that you make (in case you need to simulate a rollback), you'll have something equivalent to running at the serializable isolation level. But the process is kludgy and error prone, so if you really need transactions, we recommend using a transactional storage engine.
2.2 Locking and Concurrency
The first of those problems is how to deal with concurrency and locking. In any data repository you have to be careful when more than one person, process, or client needs to change data at the same time. Consider, for example, a classic email box on a Unix system. The popular mbox file format is incredibly simple. Email messages are simply concatenated together, one after another. This simple format makes it very easy to read and parse mail messages. It also makes mail delivery easy: just append a new message to the end of the file.
But what happens when two processes try to deliver messages at the same time to the same mailbox? Clearly that can corrupt the mailbox, leaving two interleaved messages at the end of the mailbox file. To prevent corruption, all well-behaved mail delivery systems implement a form of locking to prevent simultaneous delivery from occurring. If a second delivery is attempted while the mailbox is locked, the second process must wait until it can acquire the lock before delivering the message.
This scheme works reasonably well in practice, but it provides rather poor concurrency. Since only a single program may make any changes to the mailbox at any given time, it becomes problematic with a high-volume mailbox, one that receives thousands of messages per minute. This exclusive locking makes it difficult for mail delivery not to become backlogged if someone attempts to read, respond to, and delete messages in that same mailbox. Luckily, few mailboxes are actually that busy.
2.2.1 Read/Write Locks
Reading from the mailbox isn't as troublesome. There's nothing wrong with multiple clients reading the same mailbox simultaneously. Since they aren't making changes, nothing is likely to go wrong. But what happens if someone tries to delete message number 25 while programs are reading the mailbox? It depends. A reader could come away with a corrupted or inconsistent view of the mailbox. So to be safe, even reading from a mailbox requires special care.
Database tables are no different. If you think of each mail message as a record and the mailbox itself as a table, it's easy to see that the problem is the same. In many ways, a mailbox is really just a simple database table. Modifying records in a database table is very similar to removing or changing the content of messages in a mailbox file.
The solution to this classic problem is rather simple. Systems that deal with concurrent read/write access typically implement a locking system that consists of two lock types. These locks are usually known as shared locks and exclusive locks, or read locks and write locks.
Without worrying about the actual locking technology, we can describe the concept as follows. Read locks on a resource are shared: many clients may read from the resource at the same time and not interfere with each other. Write locks, on the other hand, are exclusive, because it is safe to have only one client writing to the resource at given time and to prevent all reads when a client is writing. Why? Because the single writer is free to make any changes to the resource—even deleting it entirely.
In the database world, locking happens all the time. MySQL has to prevent one client from reading a piece of data while another is changing it. It performs this lock management internally in a way that is transparent much of the time.
2.2.2 Lock Granularity
One way to improve the concurrency of a shared resource is to be more selective about what is locked. Rather than locking the entire resource, lock only the part that contains the data you need to change. Better yet, lock only the exact piece of data you plan to change. By decreasing the amount of data that is locked at any one time, more changes can occur simultaneously—as long as they don't conflict with each other.
The downside of this is that locks aren't free. There is overhead involved in obtaining a lock, checking to see whether a lock is free, releasing a lock, and so on. All this business of lock management can really start to eat away at performance because the system is spending its time performing lock management instead of actually storing and retrieving data. (Similar things happen when too many managers get involved in a software project.)
To achieve the best performance overall, some sort of balance is needed. Most commercial database servers don't give you much choice: you get what is known as row-level locking in your tables. MySQL, on the other hand, offers a choice in the matter. Among the storage engines you can choose from in MySQL, you'll find three different granularities of locking. Let's have a look at them.
2.2.2.1 Table locks
The most basic and low-overhead locking strategy available is a table lock, which is analogous to the mailbox locks described earlier. The table as a whole is locked on an all-or-nothing basis. When a client wishes to write to a table (insert, delete, or update, etc.), it obtains a write lock that keeps all other read or write operations at bay for the duration of the operation. Once the write has completed, the table is unlocked to allow those waiting operations to continue. When nobody is writing, readers obtain read locks that allow other readers to do the same.
For a long time, MySQL provided only table locks, and this caused a great deal of concern among database geeks. They warned that MySQL would never scale up beyond toy projects and work in the real world. However, MySQL is so much faster than most commercial databases that table locking doesn't get in the way nearly as much as the naysayers predicted it would.
Part of the reason MySQL doesn't suffer as much as expected is because the majority of applications for which it is used consist primarily of read queries. In fact, the MyISAM engine (MySQL's default) was built assuming that 90% of all queries run against it will be reads. As it turns out, MyISAM tables perform very well as long as the ratio of reads to writes is very high or very low.
2.2.2.2 Page locks
A slightly more expensive form of locking that offers greater concurrency than table locking, a page lock is a lock applied to a portion of a table known as a page. All the records that reside on the same page in the table are affected by the lock. Using this scheme, the main factor influencing concurrency is the page size; if the pages in the table are large, concurrency will be worse than with smaller pages. MySQL's BDB (Berkeley DB) tables use page-level locking on 8-KB pages.
The only hot spot in page locking is the last page in the table. If records are inserted there at regular intervals, the last page will be locked frequently.
2.2.2.3 Row locks
The locking style that offers the greatest concurrency (and carries the greatest overhead) is the row lock. In most applications, it's relatively rare for several clients to need to update the exact same row at the same time. Row-level locking, as it's commonly known, is available in MySQL's InnoDB tables. InnoDB doesn't use a simple row locking mechanism, however. Instead it uses row-level locking in conjunction with a multiversioning scheme, so let's have a look at that.
2.2.3 Multi-Version Concurrency Control
There is a final technique for increasing concurrency: Multi-Version Concurrency Control (MVCC). Often referred to simply as versioning, MVCC is used by Oracle, by PostgreSQL, and by MySQL's InnoDB storage engine. MVCC can be thought of as a new twist on row-level locking. It has the added benefit of allowing nonlocking reads while still locking the necessary records only during write operations. Some of MVCC's other properties will be of particular interest when we look at transactions in the next section.
So how does this scheme work? Conceptually, any query against a table will actually see a snapshot of the data as it existed at the time the query began—no matter how long it takes to execute. If you've never experienced this before, it may sound a little crazy. But give it a chance.
In a versioning system, each row has two additional, hidden values associated with it. These values represent when the row was created and when it was expired (or deleted). Rather than storing the actual time at which these events occur, the database stores the version number at the time each event occurred. The database version (or system version) is a number that increments each time a query[1] begins. We'll call these two values the creation id and the deletion id.
[1] That's not quite true. As you'll see when we start talking about transactions later, the version number is incremented for each transaction rather than each query.
Under MVCC, a final duty of the database server is to keep track of all the running queries (with their associated version numbers). Let's see how this applies to particular operations:
SELECT
When records are selected from a table, the server must examine each row to ensure that it meets several criteria:
Its creation id must be less than or equal to the system version number. This ensures that the row was created before the current query began.
Its deletion id, if not null, must be greater than the current system version. This ensures that the row wasn't deleted before the current query began.
Its creation id can't be in the list of running queries. This ensures that the row wasn't added or changed by a query that is still running.
Rows that pass all of these tests may be returned as the result of the query.
INSERT
When a row is added to a table, the database server records the current version number along with the new row, using it as the row's creation id.
DELETE
To delete a row, the database server records the current version number as the row's deletion id.
UPDATE
When a row is modified, the database server writes a new copy of the row, using the version number as the new row's creation id. It also writes the version number as the old row's deletion id.
The result of all this extra record keeping is that read queries never lock tables, pages, or rows. They simply read data as fast as they can, making sure to select only rows that meet the criteria laid out earlier. The drawbacks are that the server has to store a bit more data with each row and do a bit more work when examining rows. Table 2-1 summarizes the various locking models and concurrency in MySQL.
Table 2-1. Locking models and concurrency in MySQL Locking strategy
Concurrency
Overhead
Engines
Table socks
Lowest
Lowest
MyISAM, Heap, Merge
Page locks
Modest
Modest
BDB
Multiversioning
Highest
High
InnoDB
But what happens when two processes try to deliver messages at the same time to the same mailbox? Clearly that can corrupt the mailbox, leaving two interleaved messages at the end of the mailbox file. To prevent corruption, all well-behaved mail delivery systems implement a form of locking to prevent simultaneous delivery from occurring. If a second delivery is attempted while the mailbox is locked, the second process must wait until it can acquire the lock before delivering the message.
This scheme works reasonably well in practice, but it provides rather poor concurrency. Since only a single program may make any changes to the mailbox at any given time, it becomes problematic with a high-volume mailbox, one that receives thousands of messages per minute. This exclusive locking makes it difficult for mail delivery not to become backlogged if someone attempts to read, respond to, and delete messages in that same mailbox. Luckily, few mailboxes are actually that busy.
2.2.1 Read/Write Locks
Reading from the mailbox isn't as troublesome. There's nothing wrong with multiple clients reading the same mailbox simultaneously. Since they aren't making changes, nothing is likely to go wrong. But what happens if someone tries to delete message number 25 while programs are reading the mailbox? It depends. A reader could come away with a corrupted or inconsistent view of the mailbox. So to be safe, even reading from a mailbox requires special care.
Database tables are no different. If you think of each mail message as a record and the mailbox itself as a table, it's easy to see that the problem is the same. In many ways, a mailbox is really just a simple database table. Modifying records in a database table is very similar to removing or changing the content of messages in a mailbox file.
The solution to this classic problem is rather simple. Systems that deal with concurrent read/write access typically implement a locking system that consists of two lock types. These locks are usually known as shared locks and exclusive locks, or read locks and write locks.
Without worrying about the actual locking technology, we can describe the concept as follows. Read locks on a resource are shared: many clients may read from the resource at the same time and not interfere with each other. Write locks, on the other hand, are exclusive, because it is safe to have only one client writing to the resource at given time and to prevent all reads when a client is writing. Why? Because the single writer is free to make any changes to the resource—even deleting it entirely.
In the database world, locking happens all the time. MySQL has to prevent one client from reading a piece of data while another is changing it. It performs this lock management internally in a way that is transparent much of the time.
2.2.2 Lock Granularity
One way to improve the concurrency of a shared resource is to be more selective about what is locked. Rather than locking the entire resource, lock only the part that contains the data you need to change. Better yet, lock only the exact piece of data you plan to change. By decreasing the amount of data that is locked at any one time, more changes can occur simultaneously—as long as they don't conflict with each other.
The downside of this is that locks aren't free. There is overhead involved in obtaining a lock, checking to see whether a lock is free, releasing a lock, and so on. All this business of lock management can really start to eat away at performance because the system is spending its time performing lock management instead of actually storing and retrieving data. (Similar things happen when too many managers get involved in a software project.)
To achieve the best performance overall, some sort of balance is needed. Most commercial database servers don't give you much choice: you get what is known as row-level locking in your tables. MySQL, on the other hand, offers a choice in the matter. Among the storage engines you can choose from in MySQL, you'll find three different granularities of locking. Let's have a look at them.
2.2.2.1 Table locks
The most basic and low-overhead locking strategy available is a table lock, which is analogous to the mailbox locks described earlier. The table as a whole is locked on an all-or-nothing basis. When a client wishes to write to a table (insert, delete, or update, etc.), it obtains a write lock that keeps all other read or write operations at bay for the duration of the operation. Once the write has completed, the table is unlocked to allow those waiting operations to continue. When nobody is writing, readers obtain read locks that allow other readers to do the same.
For a long time, MySQL provided only table locks, and this caused a great deal of concern among database geeks. They warned that MySQL would never scale up beyond toy projects and work in the real world. However, MySQL is so much faster than most commercial databases that table locking doesn't get in the way nearly as much as the naysayers predicted it would.
Part of the reason MySQL doesn't suffer as much as expected is because the majority of applications for which it is used consist primarily of read queries. In fact, the MyISAM engine (MySQL's default) was built assuming that 90% of all queries run against it will be reads. As it turns out, MyISAM tables perform very well as long as the ratio of reads to writes is very high or very low.
2.2.2.2 Page locks
A slightly more expensive form of locking that offers greater concurrency than table locking, a page lock is a lock applied to a portion of a table known as a page. All the records that reside on the same page in the table are affected by the lock. Using this scheme, the main factor influencing concurrency is the page size; if the pages in the table are large, concurrency will be worse than with smaller pages. MySQL's BDB (Berkeley DB) tables use page-level locking on 8-KB pages.
The only hot spot in page locking is the last page in the table. If records are inserted there at regular intervals, the last page will be locked frequently.
2.2.2.3 Row locks
The locking style that offers the greatest concurrency (and carries the greatest overhead) is the row lock. In most applications, it's relatively rare for several clients to need to update the exact same row at the same time. Row-level locking, as it's commonly known, is available in MySQL's InnoDB tables. InnoDB doesn't use a simple row locking mechanism, however. Instead it uses row-level locking in conjunction with a multiversioning scheme, so let's have a look at that.
2.2.3 Multi-Version Concurrency Control
There is a final technique for increasing concurrency: Multi-Version Concurrency Control (MVCC). Often referred to simply as versioning, MVCC is used by Oracle, by PostgreSQL, and by MySQL's InnoDB storage engine. MVCC can be thought of as a new twist on row-level locking. It has the added benefit of allowing nonlocking reads while still locking the necessary records only during write operations. Some of MVCC's other properties will be of particular interest when we look at transactions in the next section.
So how does this scheme work? Conceptually, any query against a table will actually see a snapshot of the data as it existed at the time the query began—no matter how long it takes to execute. If you've never experienced this before, it may sound a little crazy. But give it a chance.
In a versioning system, each row has two additional, hidden values associated with it. These values represent when the row was created and when it was expired (or deleted). Rather than storing the actual time at which these events occur, the database stores the version number at the time each event occurred. The database version (or system version) is a number that increments each time a query[1] begins. We'll call these two values the creation id and the deletion id.
[1] That's not quite true. As you'll see when we start talking about transactions later, the version number is incremented for each transaction rather than each query.
Under MVCC, a final duty of the database server is to keep track of all the running queries (with their associated version numbers). Let's see how this applies to particular operations:
SELECT
When records are selected from a table, the server must examine each row to ensure that it meets several criteria:
Its creation id must be less than or equal to the system version number. This ensures that the row was created before the current query began.
Its deletion id, if not null, must be greater than the current system version. This ensures that the row wasn't deleted before the current query began.
Its creation id can't be in the list of running queries. This ensures that the row wasn't added or changed by a query that is still running.
Rows that pass all of these tests may be returned as the result of the query.
INSERT
When a row is added to a table, the database server records the current version number along with the new row, using it as the row's creation id.
DELETE
To delete a row, the database server records the current version number as the row's deletion id.
UPDATE
When a row is modified, the database server writes a new copy of the row, using the version number as the new row's creation id. It also writes the version number as the old row's deletion id.
The result of all this extra record keeping is that read queries never lock tables, pages, or rows. They simply read data as fast as they can, making sure to select only rows that meet the criteria laid out earlier. The drawbacks are that the server has to store a bit more data with each row and do a bit more work when examining rows. Table 2-1 summarizes the various locking models and concurrency in MySQL.
Table 2-1. Locking models and concurrency in MySQL Locking strategy
Concurrency
Overhead
Engines
Table socks
Lowest
Lowest
MyISAM, Heap, Merge
Page locks
Modest
Modest
BDB
Multiversioning
Highest
High
InnoDB
2.1 MySQL Architecture
It will greatly aid your thinking about storage engines and the capabilities they bring to MySQL if you have a good mental picture of where they fit. Figure 2-1 provides a logical view of MySQL. It doesn't necessarily reflect the low-level implementation, which is bound to be more complicated and less clear cut. However, it does serve as a guide that will help you understand how storage engines fit in to MySQL. (The NDB storage engine was added to MySQL just before this book was printed. Watch for it in the second edition.)
Figure 2-1. A logical view of MySQL's architecture
The topmost layer is composed of the services that aren't unique to MySQL. They're services most network-based client/server tools or servers need: connection handling, authentication, security, etc.
The second layer is where things get interesting. Much of the brains inside MySQL live here, including query parsing, analysis, optimization, caching, and all the built-in functions (dates, times, math, encryption, etc.). Any functionality provided across storage engines lives at this level. Stored procedures, which will arrive in MySQL 5.0, also reside in this layer.
The third layer is made up of storage engines. They're responsible for the storage and retrieval of all data stored "in" MySQL. Like the various filesystems available for Linux, each storage engine has its own benefits and drawbacks. The good news is that many of the differences are transparent at the query layer.
The interface between the second and third layers is a single API not specific to any given storage engine. This API is made up of roughly 20 low-level functions that perform operations such as "begin a transaction" or "fetch the row that has this primary key" and so on. The storage engines don't deal with SQL or communicate with each other; they simply respond to requests from the higher levels within MySQL.
It will greatly aid your thinking about storage engines and the capabilities they bring to MySQL if you have a good mental picture of where they fit. Figure 2-1 provides a logical view of MySQL. It doesn't necessarily reflect the low-level implementation, which is bound to be more complicated and less clear cut. However, it does serve as a guide that will help you understand how storage engines fit in to MySQL. (The NDB storage engine was added to MySQL just before this book was printed. Watch for it in the second edition.)
Figure 2-1. A logical view of MySQL's architecture
The topmost layer is composed of the services that aren't unique to MySQL. They're services most network-based client/server tools or servers need: connection handling, authentication, security, etc.
The second layer is where things get interesting. Much of the brains inside MySQL live here, including query parsing, analysis, optimization, caching, and all the built-in functions (dates, times, math, encryption, etc.). Any functionality provided across storage engines lives at this level. Stored procedures, which will arrive in MySQL 5.0, also reside in this layer.
The third layer is made up of storage engines. They're responsible for the storage and retrieval of all data stored "in" MySQL. Like the various filesystems available for Linux, each storage engine has its own benefits and drawbacks. The good news is that many of the differences are transparent at the query layer.
The interface between the second and third layers is a single API not specific to any given storage engine. This API is made up of roughly 20 low-level functions that perform operations such as "begin a transaction" or "fetch the row that has this primary key" and so on. The storage engines don't deal with SQL or communicate with each other; they simply respond to requests from the higher levels within MySQL.
1.3 The SHOW Commands
MySQL users often wonder how to find out what their server is actually doing at any point in time—usually when things start to slow down or behave strangely. You can look at operating system statistics to figure out how busy the server is, but that really doesn't reveal much. Knowing that the CPU is at 100% utilization or that there's a lot of disk I/O occurring provides a high-level picture of what is going on, but MySQL can tell far more.
Several SHOW commands provide a window into what's going on inside MySQL. They provide access to MySQL's configuration variables, ongoing statistics, and counters, as well as a description of what each client is doing.
1.3.1 SHOW VARIABLES
The easiest way to verify that configuration changes have taken effect is to ask MySQL for its current variable settings. The SHOW VARIABLES command does just that. Executing it produces quite a bit of output, which looks something like this:
mysql> SHOW VARIABLES;
+---------------------------------+------------------------------------------+
| Variable_name | Value |
+---------------------------------+------------------------------------------+
| back_log | 20 |
| basedir | mysql |
| binlog_cache_size | 32768 |
| character_set | latin1 |
| concurrent_insert | ON |
| connect_timeout | 5 |
| datadir | /home/mysql/data/ |
The output continues from there, covering over 120 variables in total. The variables are listed in alphabetical order, which is convenient for reading, but sometimes related variables aren't anywhere near each other in the output. The reason for this is because as MySQL evolves, new variables are added with more descriptive names, but the older variable names aren't changed; it would break compatibility for any program that expects them.[3]
[3] In the rare event they do change, MySQL retains the old names as aliases for the new ones.
Many of the variables in the list may be adjusted by a set-variable entry in any of MySQL's configuration files. Some of them are compiled-in values that can not be changed. They're really constants (not variables), but they still show up in the output of SHOW VARIABLES. Still others are boolean flags.
Notice that the output of SHOW VARIABLES (and all of the SHOW commands, for that matter) looks just like the output of any SQL query. It's tabular data. MySQL returns the output in a structured format, making it easy to write tools that can summarize and act on the output of these commands. We'll put that to good use in later chapters.
1.3.2 SHOW PROCESSLIST
The other SHOW command we'll look at is SHOW PROCESSLIST. It outputs a list of what each thread is doing at the time you execute the command.[4] It's roughly equivalent to the ps or top commands in Unix or the Task Manager in Windows.
[4] Not all threads appear in the SHOW PROCESSLIST output. The thread that handles incoming network connections, for example, is never listed.
Executing it produces a process list in tabular form:
mysql> SHOW PROCESSLIST;
+----+---------+-----------+------+-------------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+---------+-----------+------+-------------+------+-------+------------------+
| 17 | jzawodn | localhost | NULL | Query | 0 | NULL | show processlist |
+----+---------+-----------+------+-------------+------+-------+------------------+
It's common for the State and Info columns to contain more information that produces lines long enough to wrap onscreen. So it's a good idea to use the \G escape in the mysql command interpreter to produce vertical output rather than horizontal output:
mysql> SHOW PROCESSLIST \G
*************************** 1. row ***************************
Id: 17
User: jzawodn
Host: localhost
db: NULL
Command: Query
Time: 0
State: NULL
Info: show processlist
No matter which way you look at it, the same fields are included:
Id
The number that uniquely identifies this process. Since MySQL is a multi-threaded server, it really identifies the thread (or connection) and is unrelated to process IDs the operating system may use. As the operating system does with processes, MySQL starts numbering the threads at 1 and gives each new thread an ID one higher than the previous thread.
User
The name of the MySQL user connected to this thread.
Host
The name of the host or IP address from which the user is connected.
db
The database currently selected. This may be NULL if the user didn't specify a database.
Command
This shows the command state (from MySQL's internal point of view) that the thread is currently in. Table 1-1 lists each command with a description of when you are likely to see it. The commands roughly correspond to various function calls in MySQL's C API. Many commands represent very short-lived actions. Two of those that don't, Sleep and Query, appear frequently in day-to- day usage.
Table 1-1. Commands in SHOW PROCESSLIST output Command
Meaning
Binlog Dump
The slave thread is reading queries from the master's binary log.
Change user
The client is logging in as a different user.
Connect
A new client is connecting.
Connect Out
The slave thread is connecting to the master to read queries from its binary log.
Create DB
A new database is being created.
Debug
The thread is producing debugging output. This is very uncommon.
Delayed_insert
The thread is processing delayed inserts.
Drop DB
A database is being dropped.
Field List
The client has requested a list of fields in a table.
Init DB
The thread is changing to a different database, typically as the result of a USE command.
Kill
The thread is executing a KILL command.
Ping
The client is pinging the server to see if it's still connected.
Processlist
The client is running SHOW PROCESSLIST.
Query
The thread is currently executing a typical SQL query: SELECT, INSERT, UPDATE, DELETE. This is the most common state other than Sleep.
Quit
The thread is being terminated as part of the server shutdown process.
Refresh
The thread is issuing the FLUSH PRIVILEGES command.
Register Slave
A slave has connected and is registering itself with the master.
Shutdown
The server is being shut down.
Sleep
The thread is idle. No query is being run.
Statistics
Table and index statistics are being gathered for the query optimizer.
Time
The number of seconds that the process has been running the current command. A process with a Time of 90 and Command of Sleep has been idle for a minute and a half.
State
Additional human-readable information about the state of this thread. Here's an example:
Slave connection: waiting for binlog update
This appears on the master server when a slave is actively replicating from it.
Info
This is the actual SQL currently being executed, if any. Only the first 100 characters are displayed in the output of SHOW PROCESSLIST. To get the full SQL, use SHOW FULL PROCESSLIST.
1.3.3 SHOW STATUS
In addition to all the variable information we can query, MySQL also keeps track of many useful counters and statistics. These numbers track how often various events occur. The SHOW STATUS command produces a tabular listing of all the statistics and their names.
To confuse matters a bit, MySQL refers to these counters as variables too. In a sense, they are variables, but they're not variables you can set. They change as the server runs and handles traffic; you simply read them and reset them using the FLUSH STATUS command.
The SHOW STATUS command, though, offers a lot of insight into your server's performance. It's covered in much greater depth in Appendix A.
1.3.4 SHOW INNODB STATUS
The SHOW INNODB STATUS status command provides a number of InnoDB-specific statistics. As we said earlier, InnoDB is one of MySQL's storage engines; look for more on storage engines in Chapter 2.
The output of SHOW INNODB STATUS is different from that of SHOW STATUS in that it reads more as a textual report, with section headings and such. There are different sections of the report that provide information on semaphores, transaction statistics, buffer information, transaction logs, and so forth.
SHOW INNODB STATUS is covered in greater detail along with SHOW STATUS in Appendix A. Also, note that in a future version of MySQL, this command will be replaced with a more generic SHOW ENGINE STATUS command.
Several SHOW commands provide a window into what's going on inside MySQL. They provide access to MySQL's configuration variables, ongoing statistics, and counters, as well as a description of what each client is doing.
1.3.1 SHOW VARIABLES
The easiest way to verify that configuration changes have taken effect is to ask MySQL for its current variable settings. The SHOW VARIABLES command does just that. Executing it produces quite a bit of output, which looks something like this:
mysql> SHOW VARIABLES;
+---------------------------------+------------------------------------------+
| Variable_name | Value |
+---------------------------------+------------------------------------------+
| back_log | 20 |
| basedir | mysql |
| binlog_cache_size | 32768 |
| character_set | latin1 |
| concurrent_insert | ON |
| connect_timeout | 5 |
| datadir | /home/mysql/data/ |
The output continues from there, covering over 120 variables in total. The variables are listed in alphabetical order, which is convenient for reading, but sometimes related variables aren't anywhere near each other in the output. The reason for this is because as MySQL evolves, new variables are added with more descriptive names, but the older variable names aren't changed; it would break compatibility for any program that expects them.[3]
[3] In the rare event they do change, MySQL retains the old names as aliases for the new ones.
Many of the variables in the list may be adjusted by a set-variable entry in any of MySQL's configuration files. Some of them are compiled-in values that can not be changed. They're really constants (not variables), but they still show up in the output of SHOW VARIABLES. Still others are boolean flags.
Notice that the output of SHOW VARIABLES (and all of the SHOW commands, for that matter) looks just like the output of any SQL query. It's tabular data. MySQL returns the output in a structured format, making it easy to write tools that can summarize and act on the output of these commands. We'll put that to good use in later chapters.
1.3.2 SHOW PROCESSLIST
The other SHOW command we'll look at is SHOW PROCESSLIST. It outputs a list of what each thread is doing at the time you execute the command.[4] It's roughly equivalent to the ps or top commands in Unix or the Task Manager in Windows.
[4] Not all threads appear in the SHOW PROCESSLIST output. The thread that handles incoming network connections, for example, is never listed.
Executing it produces a process list in tabular form:
mysql> SHOW PROCESSLIST;
+----+---------+-----------+------+-------------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+---------+-----------+------+-------------+------+-------+------------------+
| 17 | jzawodn | localhost | NULL | Query | 0 | NULL | show processlist |
+----+---------+-----------+------+-------------+------+-------+------------------+
It's common for the State and Info columns to contain more information that produces lines long enough to wrap onscreen. So it's a good idea to use the \G escape in the mysql command interpreter to produce vertical output rather than horizontal output:
mysql> SHOW PROCESSLIST \G
*************************** 1. row ***************************
Id: 17
User: jzawodn
Host: localhost
db: NULL
Command: Query
Time: 0
State: NULL
Info: show processlist
No matter which way you look at it, the same fields are included:
Id
The number that uniquely identifies this process. Since MySQL is a multi-threaded server, it really identifies the thread (or connection) and is unrelated to process IDs the operating system may use. As the operating system does with processes, MySQL starts numbering the threads at 1 and gives each new thread an ID one higher than the previous thread.
User
The name of the MySQL user connected to this thread.
Host
The name of the host or IP address from which the user is connected.
db
The database currently selected. This may be NULL if the user didn't specify a database.
Command
This shows the command state (from MySQL's internal point of view) that the thread is currently in. Table 1-1 lists each command with a description of when you are likely to see it. The commands roughly correspond to various function calls in MySQL's C API. Many commands represent very short-lived actions. Two of those that don't, Sleep and Query, appear frequently in day-to- day usage.
Table 1-1. Commands in SHOW PROCESSLIST output Command
Meaning
Binlog Dump
The slave thread is reading queries from the master's binary log.
Change user
The client is logging in as a different user.
Connect
A new client is connecting.
Connect Out
The slave thread is connecting to the master to read queries from its binary log.
Create DB
A new database is being created.
Debug
The thread is producing debugging output. This is very uncommon.
Delayed_insert
The thread is processing delayed inserts.
Drop DB
A database is being dropped.
Field List
The client has requested a list of fields in a table.
Init DB
The thread is changing to a different database, typically as the result of a USE command.
Kill
The thread is executing a KILL command.
Ping
The client is pinging the server to see if it's still connected.
Processlist
The client is running SHOW PROCESSLIST.
Query
The thread is currently executing a typical SQL query: SELECT, INSERT, UPDATE, DELETE. This is the most common state other than Sleep.
Quit
The thread is being terminated as part of the server shutdown process.
Refresh
The thread is issuing the FLUSH PRIVILEGES command.
Register Slave
A slave has connected and is registering itself with the master.
Shutdown
The server is being shut down.
Sleep
The thread is idle. No query is being run.
Statistics
Table and index statistics are being gathered for the query optimizer.
Time
The number of seconds that the process has been running the current command. A process with a Time of 90 and Command of Sleep has been idle for a minute and a half.
State
Additional human-readable information about the state of this thread. Here's an example:
Slave connection: waiting for binlog update
This appears on the master server when a slave is actively replicating from it.
Info
This is the actual SQL currently being executed, if any. Only the first 100 characters are displayed in the output of SHOW PROCESSLIST. To get the full SQL, use SHOW FULL PROCESSLIST.
1.3.3 SHOW STATUS
In addition to all the variable information we can query, MySQL also keeps track of many useful counters and statistics. These numbers track how often various events occur. The SHOW STATUS command produces a tabular listing of all the statistics and their names.
To confuse matters a bit, MySQL refers to these counters as variables too. In a sense, they are variables, but they're not variables you can set. They change as the server runs and handles traffic; you simply read them and reset them using the FLUSH STATUS command.
The SHOW STATUS command, though, offers a lot of insight into your server's performance. It's covered in much greater depth in Appendix A.
1.3.4 SHOW INNODB STATUS
The SHOW INNODB STATUS status command provides a number of InnoDB-specific statistics. As we said earlier, InnoDB is one of MySQL's storage engines; look for more on storage engines in Chapter 2.
The output of SHOW INNODB STATUS is different from that of SHOW STATUS in that it reads more as a textual report, with section headings and such. There are different sections of the report that provide information on semaphores, transaction statistics, buffer information, transaction logs, and so forth.
SHOW INNODB STATUS is covered in greater detail along with SHOW STATUS in Appendix A. Also, note that in a future version of MySQL, this command will be replaced with a more generic SHOW ENGINE STATUS command.
1.2 Configuration Files
Configuring a MySQL server is often just a matter of editing the configuration file to make any changes you need and then restarting the server. While that sounds rather simple, adjusting the server's configuration is something you're not likely to do on a daily basis. More likely, you've installed MySQL, configured it minimally or with the defaults, and then let it run. Most users never go back and adjust the server configuration until a problem arises. As a result, it's easy to forget how to configure MySQL.
Another possibility is that you didn't even know there was a configuration file for MySQL. For the majority of projects, MySQL's default configuration is more than sufficient on modern hardware. It may not be as fast as it can be (because you haven't optimized it), but it will certainly meet your basic needs.
1.2.1 File Locations
When MySQL starts, it reads its configuration files in a particular order, unless told otherwise. On Unix, the order is:
/etc/my.cnf
datadir/my.cnf
~/.my.cnf
On Windows, the order:
%SystemRoot%/my.ini
C:\my.cnf
Three command-line arguments affect how MySQL reads its configuration files:
--no-defaults
Tells MySQL not to read any configuration files.
--defaults-file=/path/to/file
Tells MySQL to read this file only, and any other files explicitly declared with --defaults-extra-file.
--defaults-extra-file=/path/to/file
Tells MySQL to read this file after reading the /etc/my.cnf global configuration file .
Files read later in the process override those set in previously read files. If both /etc/my.cnf and datadir/my.cnf specify a value for the TCP port that MySQL should listen to, the latter takes precedence.
This behavior can be quite helpful when you need to run multiple servers either on the same host or on several different hosts. You can give all servers an identical copy of /etc/my.cnf that specifies all the values that aren't specific to a single host. With that out of the way, the few host-specific settings can be maintained in a small supplemental file such as datadir/my.cnf.
A similar strategy works if you'd like to run multiple servers on a single host. By putting all the common settings in /etc/my.cnf and the server-specific settings in each datadir/my.cnf, it's easy to keep several servers running with a minimum of effort.
For example, perhaps you want to run a couple different instances of the MySQL server, one for each character set you plan to use (to make your life easier). You might put all your "common" settings in /etc/my.cnf and the following in /etc/my.english.cnf:
default-character-set=latin1
port=3306
socket=/var/lib/mysql/english.sock
Your /etc/my.german.cnf file has:
default-character-set=latin1_de
port=3307
socket=/var/lib/mysql/german.sock
You might even have /etc/my.korean.cnf with:
default-character-set=euc_kr
port=3308
socket=/var/lib/mysql/korean.sock
Now, when you start up the three servers, you want each to load all the settings from the shared /etc/my.cnf file, and then get settings from one of each of the previous language-based configuration files. You can use a command like the following:
$ mysqld_safe --defaults-extra-file=/etc/my.german.cnf
$ mysqld_safe --defaults-extra-file=/etc/my.english.cnf
$ mysqld_safe --defaults-extra-file=/etc/my.korean.cnf
This command yields three different mysqld instances, running on ports 3306 through 3308, each using the language-specific configuration options mentioned in the file indicated by the defaults-extra-file switch.
MySQL is usually installed as a service on Windows. As a result, Windows users must call c:\mysql\bin\mysqld directly to pass command-line arguments.
1.2.2 File Format
The configuration file format consists of one or more sections, each of which may contain one or more lines. Sections begin with a name in square brackets, such as [mysqld]; this identifies the program to which the options should be applied. Each line contains a comment, a key/value pair, a set-variable directive, or a Boolean directive. Blank lines are ignored.
Two special section names can occur in each configuration file: [server] and [client]. Items listed in the [server] block apply to the MySQL server process. Those in the [client] section apply to all client programs that use the MySQL C client library, including mysql, mysqlhotcopy, and mysqldump.
Comments begin with # or ; and continue to the end of the line:
# this is a comment
; so is this
There is no multiline comment format. You can't place a comment at the end of an otherwise non-empty line:
key_buffer=128M # a comment can't go here
The key/value pairs are settings such as:
user = mysql
port = 3306
The set-variable statements look like key/value pairs in which the value is a key/value pair itself:
set-variable = key_buffer=384M
set-variable = tmp_table_size=32M
Spaces aren't important in set-variable lines. You can also write the two previous lines as follows:
set-variable = key_buffer = 384M
set-variable=tmp_table_size=32M
Either way, MySQL will understand you. However, consider using some space to enhance readability.
As of Version 4.1, the set-variable= portion of the variable definition is no longer needed and is deprecated. In current versions:
set-variable = key_buffer=384M
and:
key_buffer=384M
are both interpreted in an identical manner by the server at startup time. If you are running a version that supports leaving out the set-variable clause, it probably is best to do so because it won't be supported forever. We've chosen to use the older format here because it's what you're likely to have already, and the sample configuration files in the standard MySQL distribution continue to use it.
The few boolean directives are just stated plainly:
skip-bdb
Individual lines in the configuration file are limited to 2 KB in length. While it's rare that you'll ever need to use a line that long, it can occasionally be a problem.
1.2.3 Sample Files
The support-files directory of the MySQL distribution[2] contains four sample configuration files:
[2] These files aren't included in the Windows distribution of older MySQL releases.
my-small.cnf
my-medium.cnf
my-large.cnf
my-huge.cnf
The names of the files are meant to signify the size of the machine on which the MySQL server will run. Each contains comments describing where the size comes from. For example, my-medium.cnf says:
# This is for a system with little memory (32M - 64M) where MySQL plays
# a important part and systems up to 128M very MySQL is used together with
# other programs (like a web server)
To use a sample file, simply copy it to /etc/my.cnf (or systemdir\win.ini on Windows) and making changes as necessary. While none is likely to be ideal for any particular setup, each file is a good starting point for setting up a new system. Failure to make adjustments to the sample configuration can lead to worse performance in some cases.
Let's look at the sample my-medium.cnf file from a newly installed system. Some of the information may not make sense right away (depending on how much experience you have), but the more examples you see, the more you'll begin to understand them.
The file starts with some helpful comments about the type of system this configuration is appropriate for and information needed to install it:
# Example mysql config file for medium systems.
#
# This is for a system with little memory (32M - 64M) where MySQL plays
# a important part and systems up to 128M very MySQL is used together with
# other programs (like a web server)
#
# You can copy this file to
# /etc/mf.cnf to set global options,
# mysql-data-dir/my.cnf to set server-specific options (in this
# installation this directory is /usr/local/mysq/var) or
# ~/.my.cnf to set user-specific options.
#
# One can in this file use all long options that the program supports.
# If you want to know which options a program support, run the program
# with --help option.
Next are the options that apply to all the client tools you might run on this host:
# The following options will be passed to all MySQL clients
[client]
#password = your_password
port = 3306
socket = /tmp/mysql.sock
What follows next are the parameters specific to the server. The port and socket options, of course, should agree with what the clients were just told. The remaining settings allow MySQL to allocate more RAM for various caches and buffers as well as enable some basic replication options:
# Here follows entries for some specific programs
# The MySQL server
[mysqld]
port = 3306
socket = /tmp/mysql.sock
skip-locking
set-variable = key_buffer=16M
set-variable = max_allowed_packet=1M
set-variable = table_cache=64
set-variable = sort_buffer=512K
set-variable = net_buffer_length=8K
set-variable = myisam_sort_buffer_size=8M
log-bin
server-id = 1
Next are a few options you probably don't need to change if you have sufficient disk space:
# Point the following paths to different dedicated disks
#tmpdir = /tmp/
#log-update = /path-to-dedicated-directory/hostname
The BDB options refer to the BDB storage engine, which provide MySQL's first transaction-safe storage. You'll learn more about storage engines in Chapter 2.
# Uncomment the following if you are using BDB tables
#set-variable = bdb_cache_size=4M
#set-variable = bdb_max_lock=10000
InnoDB, another of MySQL's storage engines, has numerous options that must be configured before you can use them. Because it provides transaction-safe tables with its own memory management and storage system, you need to specify where the data files will live, as well as how much RAM should be used. (InnoDB was briefly known as Innobase, so you may see that name in configuration files.)
# Uncomment the following if you are using Innobase tables
#innodb_data_file_path = ibdata1:400M
#innodb_data_home_dir = /usr/local/mysql/var/
#innodb_log_group_home_dir = /usr/local/mysql/var/
#innodb_log_arch_dir = /usr/local/mysql/var/
#set-variable = innodb_mirrored_log_groups=1
#set-variable = innodb_log_files_in_group=3
#set-variable = innodb_log_file_size=5M
#set-variable = innodb_log_buffer_size=8M
#innodb_flush_log_at_trx_commit=1
#innodb_log_archive=0
#set-variable = innodb_buffer_pool_size=16M
#set-variable = innodb_additional_mem_pool_size=2M
#set-variable = innodb_file_io_threads=4
#set-variable = innodb_lock_wait_timeout=50
The final option groups are for specific MySQL command-line utilities, including the mysql shell:
[mysqldump]
quick
set-variable = max_allowed_packet=16M
[mysql]
no-auto-rehash
# Remove the next comment character if you are not familiar with SQL
#safe-updates
[isamchk]
set-variable = key_buffer=20M
set-variable = sort_buffer=20M
set-variable = read_buffer=2M
set-variable = write_buffer=2M
[myisamchk]
set-variable = key_buffer=20M
set-variable = sort_buffer=20M
set-variable = read_buffer=2M
set-variable = write_buffer=2M
[mysqlhotcopy]
interactive-timeout
That file would be considerably larger and certainly more confusing if all the possible settings were listed. For 90% (or more) of MySQL users, there is simply never a need to adjust more than a few of the settings listed in the sample files.
1.2.4 Reconfiguration
When an administrator adjusts the server parameters, it's common to go through an iterative process that involves making changes, restarting the server, performing some tests, and repeating the process. In fact, we'll look at doing just that in Chapter 3. In the meantime, it's worth mentioning that you should strongly consider putting your MySQL configuration files into some sort of revision control system (RCS, CVS, Subversion, etc.). Doing so gives you an easy way to track changes and back out of a bad configuration change.
As of MySQL 4.0, it's possible to change server variables on the fly at runtime. For example, if you wanted to increase the size of the key buffer from what it was set to at startup, you might do the following:
mysql> SET GLOBAL key_buffer=50M;
This sets the global value for key_buffer to 50 MB.
Some variables, such as sort_buffer_size, can be set globally so that they affect all new threads on the server, or they can be defined so that they apply only to the current MySQL client session. For example, if you wish to make a series of queries that might better use a large sort buffer, you can type:
mysql> SET SESSION sort_buffer_size=50M;
Variables set using the SESSION syntax are thread-specific and don't alter the values other threads use.
It's important to note that any change you make here, using either GLOBAL or SESSION syntax, will not survive a restart of the MySQL server; it's completely transient in that regard. Runtime changes like this are excellent for testing scenarios such as, "If I increase my key_buffer value, will it improve my query performance?" Once you've found a value that works for you, though, remember to go back to your /etc/my.cnf file and put that value into your configuration file, or you may find yourself wondering weeks or months later why performance was so horrible after that reboot, completely forgetting the variable change you made on the fly months prior.
It's also possible to use arguments on the mysqld_safe command line to override values defined in the configuration files. For example, you might do something like the following:
$ mysqld_safe -O key_buffer=50M
Like the earlier set-variable syntax, the -O syntax is deprecated as of Version 4.0. Here is a better way to issue that command:
$ mysqld_safe --key_buffer=50M
Command-line argument changes made in the mysql.server startup script will, obviously, survive from server restart to server restart, as long as that startup script is used to disable and reenable the server. It's important to point out, though, that it's usually better to have all your configuration declarations in a single place, so that maintenance doesn't become a game of hide-and-seek with the configuration options, trying to remember where you set which values.
Another possibility is that you didn't even know there was a configuration file for MySQL. For the majority of projects, MySQL's default configuration is more than sufficient on modern hardware. It may not be as fast as it can be (because you haven't optimized it), but it will certainly meet your basic needs.
1.2.1 File Locations
When MySQL starts, it reads its configuration files in a particular order, unless told otherwise. On Unix, the order is:
/etc/my.cnf
datadir/my.cnf
~/.my.cnf
On Windows, the order:
%SystemRoot%/my.ini
C:\my.cnf
Three command-line arguments affect how MySQL reads its configuration files:
--no-defaults
Tells MySQL not to read any configuration files.
--defaults-file=/path/to/file
Tells MySQL to read this file only, and any other files explicitly declared with --defaults-extra-file.
--defaults-extra-file=/path/to/file
Tells MySQL to read this file after reading the /etc/my.cnf global configuration file .
Files read later in the process override those set in previously read files. If both /etc/my.cnf and datadir/my.cnf specify a value for the TCP port that MySQL should listen to, the latter takes precedence.
This behavior can be quite helpful when you need to run multiple servers either on the same host or on several different hosts. You can give all servers an identical copy of /etc/my.cnf that specifies all the values that aren't specific to a single host. With that out of the way, the few host-specific settings can be maintained in a small supplemental file such as datadir/my.cnf.
A similar strategy works if you'd like to run multiple servers on a single host. By putting all the common settings in /etc/my.cnf and the server-specific settings in each datadir/my.cnf, it's easy to keep several servers running with a minimum of effort.
For example, perhaps you want to run a couple different instances of the MySQL server, one for each character set you plan to use (to make your life easier). You might put all your "common" settings in /etc/my.cnf and the following in /etc/my.english.cnf:
default-character-set=latin1
port=3306
socket=/var/lib/mysql/english.sock
Your /etc/my.german.cnf file has:
default-character-set=latin1_de
port=3307
socket=/var/lib/mysql/german.sock
You might even have /etc/my.korean.cnf with:
default-character-set=euc_kr
port=3308
socket=/var/lib/mysql/korean.sock
Now, when you start up the three servers, you want each to load all the settings from the shared /etc/my.cnf file, and then get settings from one of each of the previous language-based configuration files. You can use a command like the following:
$ mysqld_safe --defaults-extra-file=/etc/my.german.cnf
$ mysqld_safe --defaults-extra-file=/etc/my.english.cnf
$ mysqld_safe --defaults-extra-file=/etc/my.korean.cnf
This command yields three different mysqld instances, running on ports 3306 through 3308, each using the language-specific configuration options mentioned in the file indicated by the defaults-extra-file switch.
MySQL is usually installed as a service on Windows. As a result, Windows users must call c:\mysql\bin\mysqld directly to pass command-line arguments.
1.2.2 File Format
The configuration file format consists of one or more sections, each of which may contain one or more lines. Sections begin with a name in square brackets, such as [mysqld]; this identifies the program to which the options should be applied. Each line contains a comment, a key/value pair, a set-variable directive, or a Boolean directive. Blank lines are ignored.
Two special section names can occur in each configuration file: [server] and [client]. Items listed in the [server] block apply to the MySQL server process. Those in the [client] section apply to all client programs that use the MySQL C client library, including mysql, mysqlhotcopy, and mysqldump.
Comments begin with # or ; and continue to the end of the line:
# this is a comment
; so is this
There is no multiline comment format. You can't place a comment at the end of an otherwise non-empty line:
key_buffer=128M # a comment can't go here
The key/value pairs are settings such as:
user = mysql
port = 3306
The set-variable statements look like key/value pairs in which the value is a key/value pair itself:
set-variable = key_buffer=384M
set-variable = tmp_table_size=32M
Spaces aren't important in set-variable lines. You can also write the two previous lines as follows:
set-variable = key_buffer = 384M
set-variable=tmp_table_size=32M
Either way, MySQL will understand you. However, consider using some space to enhance readability.
As of Version 4.1, the set-variable= portion of the variable definition is no longer needed and is deprecated. In current versions:
set-variable = key_buffer=384M
and:
key_buffer=384M
are both interpreted in an identical manner by the server at startup time. If you are running a version that supports leaving out the set-variable clause, it probably is best to do so because it won't be supported forever. We've chosen to use the older format here because it's what you're likely to have already, and the sample configuration files in the standard MySQL distribution continue to use it.
The few boolean directives are just stated plainly:
skip-bdb
Individual lines in the configuration file are limited to 2 KB in length. While it's rare that you'll ever need to use a line that long, it can occasionally be a problem.
1.2.3 Sample Files
The support-files directory of the MySQL distribution[2] contains four sample configuration files:
[2] These files aren't included in the Windows distribution of older MySQL releases.
my-small.cnf
my-medium.cnf
my-large.cnf
my-huge.cnf
The names of the files are meant to signify the size of the machine on which the MySQL server will run. Each contains comments describing where the size comes from. For example, my-medium.cnf says:
# This is for a system with little memory (32M - 64M) where MySQL plays
# a important part and systems up to 128M very MySQL is used together with
# other programs (like a web server)
To use a sample file, simply copy it to /etc/my.cnf (or systemdir\win.ini on Windows) and making changes as necessary. While none is likely to be ideal for any particular setup, each file is a good starting point for setting up a new system. Failure to make adjustments to the sample configuration can lead to worse performance in some cases.
Let's look at the sample my-medium.cnf file from a newly installed system. Some of the information may not make sense right away (depending on how much experience you have), but the more examples you see, the more you'll begin to understand them.
The file starts with some helpful comments about the type of system this configuration is appropriate for and information needed to install it:
# Example mysql config file for medium systems.
#
# This is for a system with little memory (32M - 64M) where MySQL plays
# a important part and systems up to 128M very MySQL is used together with
# other programs (like a web server)
#
# You can copy this file to
# /etc/mf.cnf to set global options,
# mysql-data-dir/my.cnf to set server-specific options (in this
# installation this directory is /usr/local/mysq/var) or
# ~/.my.cnf to set user-specific options.
#
# One can in this file use all long options that the program supports.
# If you want to know which options a program support, run the program
# with --help option.
Next are the options that apply to all the client tools you might run on this host:
# The following options will be passed to all MySQL clients
[client]
#password = your_password
port = 3306
socket = /tmp/mysql.sock
What follows next are the parameters specific to the server. The port and socket options, of course, should agree with what the clients were just told. The remaining settings allow MySQL to allocate more RAM for various caches and buffers as well as enable some basic replication options:
# Here follows entries for some specific programs
# The MySQL server
[mysqld]
port = 3306
socket = /tmp/mysql.sock
skip-locking
set-variable = key_buffer=16M
set-variable = max_allowed_packet=1M
set-variable = table_cache=64
set-variable = sort_buffer=512K
set-variable = net_buffer_length=8K
set-variable = myisam_sort_buffer_size=8M
log-bin
server-id = 1
Next are a few options you probably don't need to change if you have sufficient disk space:
# Point the following paths to different dedicated disks
#tmpdir = /tmp/
#log-update = /path-to-dedicated-directory/hostname
The BDB options refer to the BDB storage engine, which provide MySQL's first transaction-safe storage. You'll learn more about storage engines in Chapter 2.
# Uncomment the following if you are using BDB tables
#set-variable = bdb_cache_size=4M
#set-variable = bdb_max_lock=10000
InnoDB, another of MySQL's storage engines, has numerous options that must be configured before you can use them. Because it provides transaction-safe tables with its own memory management and storage system, you need to specify where the data files will live, as well as how much RAM should be used. (InnoDB was briefly known as Innobase, so you may see that name in configuration files.)
# Uncomment the following if you are using Innobase tables
#innodb_data_file_path = ibdata1:400M
#innodb_data_home_dir = /usr/local/mysql/var/
#innodb_log_group_home_dir = /usr/local/mysql/var/
#innodb_log_arch_dir = /usr/local/mysql/var/
#set-variable = innodb_mirrored_log_groups=1
#set-variable = innodb_log_files_in_group=3
#set-variable = innodb_log_file_size=5M
#set-variable = innodb_log_buffer_size=8M
#innodb_flush_log_at_trx_commit=1
#innodb_log_archive=0
#set-variable = innodb_buffer_pool_size=16M
#set-variable = innodb_additional_mem_pool_size=2M
#set-variable = innodb_file_io_threads=4
#set-variable = innodb_lock_wait_timeout=50
The final option groups are for specific MySQL command-line utilities, including the mysql shell:
[mysqldump]
quick
set-variable = max_allowed_packet=16M
[mysql]
no-auto-rehash
# Remove the next comment character if you are not familiar with SQL
#safe-updates
[isamchk]
set-variable = key_buffer=20M
set-variable = sort_buffer=20M
set-variable = read_buffer=2M
set-variable = write_buffer=2M
[myisamchk]
set-variable = key_buffer=20M
set-variable = sort_buffer=20M
set-variable = read_buffer=2M
set-variable = write_buffer=2M
[mysqlhotcopy]
interactive-timeout
That file would be considerably larger and certainly more confusing if all the possible settings were listed. For 90% (or more) of MySQL users, there is simply never a need to adjust more than a few of the settings listed in the sample files.
1.2.4 Reconfiguration
When an administrator adjusts the server parameters, it's common to go through an iterative process that involves making changes, restarting the server, performing some tests, and repeating the process. In fact, we'll look at doing just that in Chapter 3. In the meantime, it's worth mentioning that you should strongly consider putting your MySQL configuration files into some sort of revision control system (RCS, CVS, Subversion, etc.). Doing so gives you an easy way to track changes and back out of a bad configuration change.
As of MySQL 4.0, it's possible to change server variables on the fly at runtime. For example, if you wanted to increase the size of the key buffer from what it was set to at startup, you might do the following:
mysql> SET GLOBAL key_buffer=50M;
This sets the global value for key_buffer to 50 MB.
Some variables, such as sort_buffer_size, can be set globally so that they affect all new threads on the server, or they can be defined so that they apply only to the current MySQL client session. For example, if you wish to make a series of queries that might better use a large sort buffer, you can type:
mysql> SET SESSION sort_buffer_size=50M;
Variables set using the SESSION syntax are thread-specific and don't alter the values other threads use.
It's important to note that any change you make here, using either GLOBAL or SESSION syntax, will not survive a restart of the MySQL server; it's completely transient in that regard. Runtime changes like this are excellent for testing scenarios such as, "If I increase my key_buffer value, will it improve my query performance?" Once you've found a value that works for you, though, remember to go back to your /etc/my.cnf file and put that value into your configuration file, or you may find yourself wondering weeks or months later why performance was so horrible after that reboot, completely forgetting the variable change you made on the fly months prior.
It's also possible to use arguments on the mysqld_safe command line to override values defined in the configuration files. For example, you might do something like the following:
$ mysqld_safe -O key_buffer=50M
Like the earlier set-variable syntax, the -O syntax is deprecated as of Version 4.0. Here is a better way to issue that command:
$ mysqld_safe --key_buffer=50M
Command-line argument changes made in the mysql.server startup script will, obviously, survive from server restart to server restart, as long as that startup script is used to disable and reenable the server. It's important to point out, though, that it's usually better to have all your configuration declarations in a single place, so that maintenance doesn't become a game of hide-and-seek with the configuration options, trying to remember where you set which values.
1.1 Binary Versus Compiled-From-Source Installations
There are two ways you can install MySQL. As a novice administrator, you may have simply installed a binary package that had precompiled executables, libraries, and configuration files, and placed those files wherever the maker of the binary package decided they should go.
It's exceedingly rare for a Windows user to compile his own copy of MySQL. If you're running MySQL on Windows, feel free to download your copy from the MySQL web site and skip this discussion.
Alternatively, for any number of reasons, you might have decided to compile the MySQL binaries on your own, by downloading a source tarball and configuring the installation to best meet your needs. However, don't do so lightly. Compiling from source has led to countless hours of pain for some users, mostly due to subtle bugs in their compilers or thread libraries. For this very reason, the standard binaries provided by MySQL AB are statically linked. That means they are immune to any bugs in your locally installed libraries.
There aren't too many places where the issue of "binary versus compiled-from-source" will come into play in the average MySQL tuning regimen, but they do happen. For example, in Chapter 10, our advice on chrooting your installation can be used only if every file MySQL needs is brought into a single directory tree, which might not be the case in a binary installation.
For a novice administrator on a simple installation, we recommend using a binary package (such as an RPM) to set up your system. However, once you progress to the point of really needing to tinker with the "guts" of MySQL, you will probably want to quickly go back, change a configure flag, and recompile.
1.1.1 MySQL.com Binary Versus Distribution Binary
One thing to keep in mind is that there are a number of sources for binary packages, and nearly all of them set up the system differently.
For example, you can download the binary installation from the MySQL.com web site. You can also nstall the binary distribution included by your Linux distribution vendor, or the one you grabbed from the FreeBSD ports collection. Finally, you can downloaded a binary for a platform that isn't officially supported, but on which someone is keeping a MySQL version current, such as the Amiga architecture.[1] In any of these cases, you will end up with different directory layouts, compilation options, etc.
[1] At the time that sentence was written, it was entirely theoretical: the thinking was "I'm not aware of anything, but surely someone will do that!" In researching it, we found that MySQL for Amiga was, indeed, happening. For those who read German, there's an article from Amiga Magazine at http://www.amiga-magazin.de/magazin/a08-01/mysql/ that describes how to do it, and a mailing list at http://groups.yahoo.com/group/Amiga_MySql/ for people working on it as well.
If you use the binary distributions from anyone other than MySQL AB, your support options may be significantly decreased, simply by virtue of having limited yourself to seeking help from those who use that particular distribution. Even a question as simple as, "Where is the my.cnf file located on the FreeBSD port of MySQL?" is going to limit those who can respond to two groups: those who have run MySQL using the FreeBSD port, and those on the mailing list or newsgroup, etc. who have encountered that question before. On the plus side, if your distribution has automated security announcements and updates, you probably never need to worry about patching MySQL if a security flaw is discovered.
Many binary distributors of MySQL mold it to fit "their" layout. For example, the Debian distribution places the config files in /etc/mysql/, some language-specific files in /usr/share/mysql/, the executables directly into /usr/bin/, etc. It's not "the Debian way" to segregate an application's binaries; it incorporates them into the system as a whole. Likewise, in those places it does incorporate them, it does so in what may seem like an odd manner. For instance, you might expect config files to go directly into /etc/, but instead they get put in /etc/mysql/. It can be confusing if you're trying to find everything you need to modify, or if you're trying to later convert from one type of installation to the other.
The MySQL.com-supplied tarball binary packages, however, behave more like the source-compilation process. All the files—configuration files, libraries, executables, and the database files themselves—end up in a single directory tree, created specifically for the MySQL install. This is typically /usr/local/mysql, but it can be altered as needed at installation time. Because this behavior is much the same as a source-compiled installation, the available support from the MySQL community is much greater. It also makes things easier if you decide later to instead use a MySQL installation you compile from source.
On the other hand, the MySQL-supplied binary packages that are distributed using package-management formats such as RPM are laid out similarly to the format of the system they are designed for. For example, the RPM installation you get from MySQL.com will have its files laid out similarly to the Red Hat-supplied RPM. This is so because it's not uncommon for a Linux distribution to ship an RPM that hasn't been thoroughly tested and is broken in fairly serious ways. The RPM files MySQL.com distributes are intended as upgrade paths for users with such a problem so they can have "just what they have now, but it works."
Because of that, if you're going to install a binary you download from MySQL.com, we highly recommend using the tarball formatted files. They will yield the familiar directory structure the average MySQL administrator is used to seeing.
It's exceedingly rare for a Windows user to compile his own copy of MySQL. If you're running MySQL on Windows, feel free to download your copy from the MySQL web site and skip this discussion.
Alternatively, for any number of reasons, you might have decided to compile the MySQL binaries on your own, by downloading a source tarball and configuring the installation to best meet your needs. However, don't do so lightly. Compiling from source has led to countless hours of pain for some users, mostly due to subtle bugs in their compilers or thread libraries. For this very reason, the standard binaries provided by MySQL AB are statically linked. That means they are immune to any bugs in your locally installed libraries.
There aren't too many places where the issue of "binary versus compiled-from-source" will come into play in the average MySQL tuning regimen, but they do happen. For example, in Chapter 10, our advice on chrooting your installation can be used only if every file MySQL needs is brought into a single directory tree, which might not be the case in a binary installation.
For a novice administrator on a simple installation, we recommend using a binary package (such as an RPM) to set up your system. However, once you progress to the point of really needing to tinker with the "guts" of MySQL, you will probably want to quickly go back, change a configure flag, and recompile.
1.1.1 MySQL.com Binary Versus Distribution Binary
One thing to keep in mind is that there are a number of sources for binary packages, and nearly all of them set up the system differently.
For example, you can download the binary installation from the MySQL.com web site. You can also nstall the binary distribution included by your Linux distribution vendor, or the one you grabbed from the FreeBSD ports collection. Finally, you can downloaded a binary for a platform that isn't officially supported, but on which someone is keeping a MySQL version current, such as the Amiga architecture.[1] In any of these cases, you will end up with different directory layouts, compilation options, etc.
[1] At the time that sentence was written, it was entirely theoretical: the thinking was "I'm not aware of anything, but surely someone will do that!" In researching it, we found that MySQL for Amiga was, indeed, happening. For those who read German, there's an article from Amiga Magazine at http://www.amiga-magazin.de/magazin/a08-01/mysql/ that describes how to do it, and a mailing list at http://groups.yahoo.com/group/Amiga_MySql/ for people working on it as well.
If you use the binary distributions from anyone other than MySQL AB, your support options may be significantly decreased, simply by virtue of having limited yourself to seeking help from those who use that particular distribution. Even a question as simple as, "Where is the my.cnf file located on the FreeBSD port of MySQL?" is going to limit those who can respond to two groups: those who have run MySQL using the FreeBSD port, and those on the mailing list or newsgroup, etc. who have encountered that question before. On the plus side, if your distribution has automated security announcements and updates, you probably never need to worry about patching MySQL if a security flaw is discovered.
Many binary distributors of MySQL mold it to fit "their" layout. For example, the Debian distribution places the config files in /etc/mysql/, some language-specific files in /usr/share/mysql/, the executables directly into /usr/bin/, etc. It's not "the Debian way" to segregate an application's binaries; it incorporates them into the system as a whole. Likewise, in those places it does incorporate them, it does so in what may seem like an odd manner. For instance, you might expect config files to go directly into /etc/, but instead they get put in /etc/mysql/. It can be confusing if you're trying to find everything you need to modify, or if you're trying to later convert from one type of installation to the other.
The MySQL.com-supplied tarball binary packages, however, behave more like the source-compilation process. All the files—configuration files, libraries, executables, and the database files themselves—end up in a single directory tree, created specifically for the MySQL install. This is typically /usr/local/mysql, but it can be altered as needed at installation time. Because this behavior is much the same as a source-compiled installation, the available support from the MySQL community is much greater. It also makes things easier if you decide later to instead use a MySQL installation you compile from source.
On the other hand, the MySQL-supplied binary packages that are distributed using package-management formats such as RPM are laid out similarly to the format of the system they are designed for. For example, the RPM installation you get from MySQL.com will have its files laid out similarly to the Red Hat-supplied RPM. This is so because it's not uncommon for a Linux distribution to ship an RPM that hasn't been thoroughly tested and is broken in fairly serious ways. The RPM files MySQL.com distributes are intended as upgrade paths for users with such a problem so they can have "just what they have now, but it works."
Because of that, if you're going to install a binary you download from MySQL.com, we highly recommend using the tarball formatted files. They will yield the familiar directory structure the average MySQL administrator is used to seeing.
Chapter 1. Back To Basics
Many MySQL users and administrators slide into using MySQL. They hear its benefits, find that it's easy to install on their systems (or better yet, comes pre-installed), and read a quick book on how to attach simple SQL operations to web sites or other applications.
It may take several months for the dragons to raise their heads. Perhaps one particular web page seems to take forever, or a system failure corrupts a database and makes recovery difficult.
Real-life use of MySQL requires forethought and care—and a little benchmarking and testing. This book is for the MySQL administrator who has the basics down but realizes the need to go further. It's a good book to read after you've installed and learned how to use MySQL but before your site starts to get a lot of traffic, and the dragons are breathing down your neck. (When problems occur during a critical service, your fellow workers and friendly manager start to take on decidedly dragon-like appearances.)
The techniques we teach are valuable in many different situations, and sometimes to solve different problems. Replication, for instance, may be a matter of reliability for you—an essential guarantee that your site will still be up if one or two systems fail. But replication can also improve performance; we show you architectures and techniques that solve multiple problems.
We also take optimization far beyond the simple use of indexes and diagnostic (EXPLAIN) statements: this book tells you what the factors in good performance are, where bottlenecks occur, how to benchmark MySQL, and other advanced performance topics.
We ask for a little more patience and time commitment than the average introductory computer book. Our approach involves a learning cycle, and experience convinces us that it's ultimately the fastest and most efficient way to get where you want.
After describing the problems we're trying to solve in a given chapter, we start with some background explanation. In other words, we give you a mental model for understanding what MySQL is doing. Then we describe the options you have to solve the problem, and only after all that do we describe particular tools and techniques.
This book is clearly not the end of the line in terms of information. Knowing that, we've started a web site, http://www.highperformancemysql.com, where we put useful scripts and new topics. See the Preface for more information.
Before we dig into how to tune your MySQL system to optimum performance, it's best if we go over a couple of ground rules and make sure everyone is on the same page.
It may take several months for the dragons to raise their heads. Perhaps one particular web page seems to take forever, or a system failure corrupts a database and makes recovery difficult.
Real-life use of MySQL requires forethought and care—and a little benchmarking and testing. This book is for the MySQL administrator who has the basics down but realizes the need to go further. It's a good book to read after you've installed and learned how to use MySQL but before your site starts to get a lot of traffic, and the dragons are breathing down your neck. (When problems occur during a critical service, your fellow workers and friendly manager start to take on decidedly dragon-like appearances.)
The techniques we teach are valuable in many different situations, and sometimes to solve different problems. Replication, for instance, may be a matter of reliability for you—an essential guarantee that your site will still be up if one or two systems fail. But replication can also improve performance; we show you architectures and techniques that solve multiple problems.
We also take optimization far beyond the simple use of indexes and diagnostic (EXPLAIN) statements: this book tells you what the factors in good performance are, where bottlenecks occur, how to benchmark MySQL, and other advanced performance topics.
We ask for a little more patience and time commitment than the average introductory computer book. Our approach involves a learning cycle, and experience convinces us that it's ultimately the fastest and most efficient way to get where you want.
After describing the problems we're trying to solve in a given chapter, we start with some background explanation. In other words, we give you a mental model for understanding what MySQL is doing. Then we describe the options you have to solve the problem, and only after all that do we describe particular tools and techniques.
This book is clearly not the end of the line in terms of information. Knowing that, we've started a web site, http://www.highperformancemysql.com, where we put useful scripts and new topics. See the Preface for more information.
Before we dig into how to tune your MySQL system to optimum performance, it's best if we go over a couple of ground rules and make sure everyone is on the same page.
Sabtu, 2008 September 27
AutoCad Modifying Object
IntroductionAutoCAD drawings are rarely completed simply by drawing lines, circles etc. Most likely you will need to Modify these basic drawing objects in some way in order to create the image you need. AutoCAD provides a whole range of modify tools such as Move, Copy, Rotate and Mirror. As you can see, the command names are easily understandable. However, the way these commands work is not always obvious. This tutorial is designed to show you how all of the Modify commands work. If you just need information quickly, use the QuickFind toolbar below to go straight to the information you need or select a topic from the contents list above.
Erase Copy Mirror Offset Array Move Rotate Scale Stretch Lengthen Trim Extend Break Chamfer Fillet Explode
As is usual with AutoCAD, the Modify tools can be accessed in one of three ways, from the keyboard, from the pull-down menu and from the toolbar. All of the Modify tools are available from the Modify pull-down and the Modify toolbar. In each section below, the toolbar, pull-down and keyboard options are given. The method you choose is entirely up to you. Ultimately you will use the method that you feel most comfortable with or the one you find most efficient. AutoCAD allows great flexibility and there aren't any right or wrong ways of working. That said, it should be pointed out that the use of toolbars in AutoCAD is almost always quicker than any other method.
Toolbar dialogue box
The Modify toolbar is usually displayed by default but if it is not already displayed, you can display it using the TOOLBAR command, ViewToolbars… from the pull-down menu. When the Toolbar dialogue box (shown above) appears, simply check the box next to "Modify" in the toolbars list. Many AutoCAD users work with the Modify toolbar permanently docked on their screen because it gives one-click access to all of the commands, making the drawing process much more efficient.
The Erase Command
Toolbar Modify Erase
Pull-down ModifyErase
Keyboard ERASE short-cut E
The Erase command is one of the simplest AutoCAD commands and is one of the most used. The command erases (deletes) any selected object(s) from the drawing. Remember you can always get deleted objects back by typing U to undo, Undo from the Standard toolbar or by using the OOPS command.
Command Sequence
Command: ERASE
Select objects: (pick an object to erase)
Select objects: Return (to end the selection and erase the object)
If you simply want to erase the last object you created you can type L at the "Select objects" prompt. The last object will be highlighted and you can then select more objects or Return to end the command. See the "Object Selection" tutorial for more information on selecting objects.
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The Copy Command
Toolbar Modify Copy Object
Pull-down ModifyCopy
Keyboard COPY short-cut CP
The Copy command can be used to create one or more duplicates of any drawing object or objects which you have previously created. Copy is a very useful and time-saving command because you can create very complex drawing elements and then simply copy them as many times as you like.
Copying an object
Command Sequence
Command: COPY
Select objects: (pick object to copy, P1)
Select objects: Return (to end selection)
Specify base point or displacement, or [Multiple]: (pick P2 or M for multiple copies)
Specify second point of displacement or : (pick P3)
The multiple option allows you to create additional copies of the selected object(s) by picking as many new points as you like. To end a multiple copy, just hit the Return key.
Notice that the "Base point", P2 and the "Second point", P3 do not have to be picked on or near the object. The two points are simply used to indicate the distance and direction of the copied object from the original object.
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The Mirror Command
Toolbar Modify Mirror
Pull-down ModifyMirror
Keyboard MIRROR short-cut MI
The Mirror command allows you to mirror selected objects in your drawing by picking them and then defining the position of an imaginary mirror line using two points.
Mirroring an object
Command Sequence
Command: MIRROR
Select objects: (pick object to mirror, P1)
Select objects: Return (to end selection)
Specify first point of mirror line: (pick P2)
Specify second point of mirror line: (pick P3)
Delete source objects? [Yes/No] : Return (for No to keep the original object)
Notice that in the command sequence above, pressing the Return key when asked whether to "Delete source objects?" resulted in a "No" response. This is because "No" is the default option. AutoCAD always shows the default option within triangular brackets, in this case "". Many AutoCAD commands offer default options. They are usually the most commonly used and you need only right-click and select "Enter" from the menu or press Return on the keyboard to accept them.
In order to create perfectly horizontal or vertical mirror lines use Ortho. For more information on the use of Ortho and other drawing aids, see the "Drawing Aids" tutorial.
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The Offset Command
Toolbar Modify Offset
Pull-down ModifyOffset
Keyboard OFFSET short-cut O
Offset is probably one of the most useful commands for constructing drawings. The Offset command creates a new object parallel to or concentric with a selected object. The new object is drawn at a user defined distance (the offset) from the original and in a direction chosen by the user with a pick point. You can offset lines, arcs, circles, ellipses, 2D polylines, xlines, rays and planar splines.
Command Sequence
Command: OFFSET
Specify offset distance or [Through] <1.0000>: 10 (specify distance)
Select object to offset or : (select object, P1)
Specify point on side to offset: (pick direction, P2)
Select object to offset or : Return (to end or select another object to offset)
In the illustration below, a line (cyan) has been offset to the right through a distance "Offset" by picking a point to the right of the original line. The result is a new line (red) to the right of the original.
Offsetting
Circles can be offset inside or outside of themselves to create a new circle which is concentric (has the same centre point) with the original circle. In the illustration, a new circle (red) has been created outside of the original by picking a point outside of the original circle. The radius of the new circle is the offset distance "Offset" plus the radius of the original circle.
In the illustration above, the new objects and original objects are shown in different colours for clarity. In reality, offset objects inherit their object properties from the original object. See the "Object Properties" tutorial for details.
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The Array Command
Toolbar Modify Array
Pull-down ModifyArray
Keyboard ARRAY short-cut AR
The Array command makes multiple copies of selected objects in a rectangular matrix (columns and rows) or a polar (circular) pattern. This command has been completely transformed in AutoCAD 2000i. It is now completely dialogue box driven with the option to see a preview of the array before it is created. You can also now create rectangular arrays at a user specified angle angle. This constitutes a major improvement in usability.
The Rectangular Array
Rectangular Array The illustration on the right shows the results of a rectangular array with two columns an three rows. The distance between rows is indicated with the dimension DR and between columns with DC. When creating rectangular arrays it is important to remember that new rows are created above the original object and new columns are created to the right of the original object. The resulting array is, therefore, always created with the original object in the bottom left hand position with respect to the current co-ordinate system. In the illustration, the original object is shown in cyan and the new objects in red. It is possible to create arrays which do not align with the World Co-ordinate System by setting a User Co-ordinate System first. See the UCS tutorial for details.
Try creating this array yourself. First, draw a rectangle 20 drawing units wide and 10 drawing units tall. To do this, start the rectangle command DrawRectangle from the pull-down menu. When you are prompted to pick a point, pick somewhere in the lower left quarter of the drawing area. Then, when you are prompted for a second point, enter the relative cartesian co-ordinate @20,10. You have now drawn a rectangle at the right size. See the Using Co-ordinates tutorial for more information on drawing with co-ordinates.
Rectangular Array dialogue box
Now start the Array command.First of all, make sure that the Rectangular Array radio button is selected and that you are looking at the Rectangular Array dialogue box and now follow the steps below:
1. Click the Select objects button. The dialogue box will temporarily disappear enabling you to select the rectangle you just drew. Press the Return button on your keyboard to complete the selection. You are now returned to the dialogue box and the message immediately below the Select Objects button should read "1 objects selected".
2. Enter the number of rows required in the Rows edit box. For this example, enter the value "3". Notice that the schematic preview on the right hand side of the dialogue box updates to reflect the values you are entering.
3. Enter the number of columns required in the Columns edit box. Enter the value "2".
4. Enter the row offset in the Row Offset edit box. This is the distance DR in the illustration above. Note that tis is not the distance between rows. In this example, our rectangle is 10 units high and we will enter a row offset of 15. The result will be a 5 unit gap between rectangles.
5. Enter the column offset in the Column Offset edit box. The same parameters apply as for the row offset. Enter a value of 25 to give a 5 unit gap between our rectangles.
6. ArrayClick on the Preview button. Once again, the dialogue box disappears and the specified array is temporarily drawn so that we can preview it. We are now offered 3 choices. If the array isn't quite right, click the Modify button to return to the Array dialogue box. If you are happy with the array, click the Accept button, the array will be permanently drawn and the command is ended.
You should now have an array that looks similar to the one in the illustration above consisting of 6 rectangles arranged in 3 rows and 2 columns. As you can se, this command is very powerful and can save lots of time if used carefully.
The Polar Array
The polar array works in a similar way to the rectangular array. The main difference is that rather than specifying the number and offset for rows and columns, you must specify a center point and the total number of objects in the array (including the original object).
Polar Array dialogue box
You can try this for yourself by drawing another rectangle as described above. This time, though, locate the rectangle in the centre top half of the drawing area. Now, start the Array command, click the Polar Array radio button and follow the steps below:
1. Click the Select objects button. The dialogue box will temporarily disappear enabling you to select the rectangle you just drew. Press the Return button on your keyboard to complete the selection. You are now returned to the dialogue box and the message immediately below the Select Objects button should read "1 objects selected".
2. Specify the center point for the array. This is the point C in the illustration below. You can do this by entering x and y co-ordinates into the appropriate edit boxes if you know what these values should be. However, this is rarely the case and most often you will want to click the Pick Center Point button to pick a point from the drawing area. Pick a point somewhere below the rectangle you have just drawn.
3. Enter a value for the total number of items. For this example, enter the value "6". Notice that once again, the schematic preview updates to reflect the values you have entered.
4. Make sure that the Rotate items as copied checkbox is checked.
5. ArrayClick on the Preview button. Once again, the dialogue box disappears and the specified array is temporarily drawn so that we can preview it. We are now offered 3 choices. If the array isn't quite right, click the Modify button to return to the Array dialogue box. If you are happy with the array, click the Accept button, the array will be permanently drawn and the command is ended.
The array you have just drawn should look something like the one illustrated below, left. Take some time to play around with the other options in the Array dialogue box to see what they can do.
Polar Array
The illustration above shows the results of a polar array using an original rectangle (cyan) copied six times about a centre point C, through an angle of 360 degrees (full circle). The only difference between the array on the left and the one on the right is that the Rotate items as copied checkbox was not checked in the right hand example.
All new objects created with the Array command will inherit the object properties of the original object. See the "Object Properties" tutorial for information on object properties.
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The Move Command
Toolbar Modify Move
Pull-down ModifyMove
Keyboard MOVE short-cut M
The Move command works in a similar way to the Copy command except that no copy is made, the selected object(s) is simply moved from one location to another.
Moving an object
Command Sequence
Command: MOVE
Select objects: (pick object to move, P1)
Select objects: Return (to end selection)
Specify base point or displacement: (pick P2)
Specify second point of displacement or : (pick P3)
Note that as with the Copy command, the two pick points, P2 and P3 are used only to indicate the distance and direction of movement.
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The Rotate Command
Toolbar Modify Rotate
Pull-down ModifyRotate
Keyboard ROTATE short-cut RO
The Rotate command allows an object or objects to be rotated about a point selected by the user. AutoCAD prompts for a second rotation point or an angle which can be typed at the keyboard.
Rotating a rectangle
Command Sequence
Command: ROTATE
Current positive angle in UCS: ANGDIR=counterclockwise ANGBASE=0
Select objects: (pick object to rotate, P1)
Select objects: Return (to end selection)
Specify base point: (pick base point, P2)
Specify rotation angle or [Reference]: (pick second point, P3 or enter angle)
Remember, by default, AutoCAD angles start at 3 o'clock and increase in an anti-clockwise direction. The "ANGDIR" and "ANGBASE" variables remind you of this. If you want to rotate in a clockwise direction you can enter a negative angle by using a minus sign.
Note: You can change the angle direction and the base angle using the Units command, FormatUnits… from the pull-down menu. Click the "Clockwise" check box to change the direction and click the "Direction…" button to set the base angle.
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The Scale Command
Toolbar Modify Scale
Pull-down ModifyScale
Keyboard SCALE short-cut SC
The Scale command can be used to change the size of an object or group of objects. You are prompted for a pick point about which the selection set will be scaled. Scaling can then be completed by picking a second point (not always easy because it can sometimes be difficult to precisely control the scaling) or by entering a scale factor at the keyboard. For example a scale factor of 2 will double the size of the objects in the selection set and a factor of 0.5 will half them.
Scaling an object
Command Sequence
Command: SCALE
Select objects: (pick objects to be scaled, P1)
Select objects: Return (to end selection)
Specify base point: (pick base point, P2)
Specify scale factor or [Reference]: (pick second point, P3 or enter scale factor)
In the example shown above, the original tree symbol has been enlarged by dynamically scaling it using pick points to determine the change in scale. If you want to scale an object precisely, it is much easier to enter a scale factor using the keyboard.
Note that the position of the new sized tree symbol is determined by the location of the base point. The base point, P2 has been picked to the upper right of the centre of the tree which resulted in the centre of the tree shifting to a lower left position. If the base point had been picked in the centre of the tree symbol, the tree would have remained in the same position. In theory the base point can be any point in the drawing area but for ease of control it is best to choose a known point so that the results are obvious.
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The Stretch Command
Toolbar Modify Stretch
Pull-down ModifyStretch
Keyboard STRETCH short-cut S
The Stretch command can be used to move one or more vertices of an object whilst leaving the rest of the object unchanged. In the example below, a rectangle has been stretched by moving one vertex to create an irregular shape.
Stretching a rectangle
Command Sequence
Command: STRETCH
Select objects to stretch by crossing-window or crossing-polygon...
Select objects: (pick first point of crossing window)
Specify opposite corner: (pick second point of window)
Select objects: Return (to end selection)
Specify base point or displacement: (pick base point)
Specify second point of displacement: (pick second point)
To select vertices to stretch, you must use a crossing window or polygon. See the "Object Selection" tutorial for details of these selection methods.
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Stretching with Grips
Although the Stretch command can be very useful, it has largely been superseded by the use of "Grips" which allow this sort of modification to be made much more intuitively.
Stretching with Grips
To stretch an object using grips, simply select the object by clicking on it (you can do this without starting a command). The object becomes highlighted and small square grips appear at each vertex and various snap points, depending upon the object type. Click a grip to activate it and click again to reposition it. When you have completed your modifications, use the Escape key (Esc) at the top left of your keyboard to deselect the object and release grips.
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The Lengthen Command
Toolbar Modify Lengthen
Pull-down ModifyLengthen
Keyboard LENGTHEN short-cut LEN
The Lengthen command can often be used instead of either the Trim or Extend commands. Indeed the end result is exactly the same. The Lengthen command can be used to either lengthen or shorten Lines, Arcs, open Polylines, elliptical Arcs and open Splines without the use of cutting or boundary edges.
Command Sequence
Command: LENGTHEN
Select an object or [DElta/Percent/Total/DYnamic]: DY
Select an object to change or [Undo]: (select a line or arc)
Specify new end point: (pick new end point)
Select an object to change or [Undo]: Return (to end)
The command sequence above demonstrates the use of the Dynamic Lengthen option which is probably the most useful for general purpose drafting. However, the other options are worth getting to know because they can save lots of time and effort.
The Total option allows you to change the total length of a line to any value that you specify. The Percent option allows you to change a line length using a percentage. For example, a value of 50 will result in a line one half the length of the original and a value of 200 will result in a line twice the length of the original. The Delta option can be used to extend or reduce the endpoint of a line by a given distance. The endpoint affected by the change is the one closest to the pick point when the object selection is made.
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The Trim Command
Toolbar Modify Trim
Pull-down ModifyTrim
Keyboard TRIM short-cut TR
Trimming objects The Trim command can be used to trim a part of an object. In order to trim an object you must draw a second object which forms the "cutting edge". Cutting edges can be lines, xlines, rays, polylines, circles, arcs or ellipses. Blocks and text cannot be trimmed or used as cutting edges. The illustration on the right shows the Trim command in action. The red square and circle have been drawn using the Polygon and Circle commands respectively. In order to trim these objects, a line has been drawn (cyan in the illustration), this forms the cutting edge. The Trim command, unlike most other modify commands requires that two separate object selections are made. The cutting edges are selected first (there can be one or more) and then the objects to be trimmed are selected. In the example above, the line is selected first because it forms the cutting edge and then the square and circle are selected.
The Trim command is slightly more complicated than many other modify commands. To get a better understanding of how it works, draw a square, circle and line as illustrated above and then follow the command sequence below. Don't forget to watch the AutoCAD command line at each stage of the process.
Command Sequence
Command: TRIM
Current settings: Projection=UCS Edge=None
Select cutting edges ...
Select objects: (select the cutting edge, P1)
Select objects: Return (to end cutting edge selection)
Select object to trim or shift-select to extend or [Project/Edge/Undo]:(pick the part of the square which you want to trim, P2)
Select object to trim or shift-select to extend or [Project/Edge/Undo]: (pick the circle, P3)
Select object to trim or shift-select to extend or [Project/Edge/Undo]: Return (to end)
Notice that at each trimming step you are given the option to undo the previous trim. This can be very useful if you inadvertently pick the wrong object.
In the above example, when the objects were trimmed, both pick points were made to the right of the cutting edge, resulting in the removal of that part of the objects to the right of the cutting edge. Obviously, the portion of square and circle to the left of the cutting edge could have been removed by picking to the left of the cutting edge. Also, you may not have noticed it, but by trimming a circle you have created an Arc object. This makes no visible difference but the object type has changed.
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The Extend Command
Toolbar Modify Extend
Pull-down ModifyExtend
Keyboard EXTEND short-cut EX
Extending lines This command extends a line, polyline or arc to meet another drawing object (known as the boundary edge). In the illustration on the right, two lines (red) are extended to meet another line (cyan) which forms the boundary edge. This command works in a similar way to the Trim command, described above. Two selections are made, one for the boundary edge(s) and one for the object(s) to extend.
Extending about the midpoint Lines and other objects can be extended in one of two directions. In the illustration on the right, the red line could be extended either to the right or to the left. You can tell AutoCAD in which direction to extend by picking a point to the right or left of the midpoint respectively. AutoCAD does not intuitively know where the boundary edge lies so you must explicitly indicate the direction of extension by picking either one side or other of the midpoint.
Draw the lines as shown in the illustration and follow the command sequence below.
Command Sequence
Command: EXTEND
Current settings: Projection=UCS Edge=None
Select boundary edges ...
Select objects: (select the boundary edge, P1)
Select objects: Return (to end boundary edge selection)
Select object to extend or shift-select to trim or [Project/Edge/Undo]: (pick the object which you want to be extended, P2)
Select object to extend or shift-select to trim or [Project/Edge/Undo]: (pick another object which you want to be extended, P3)
Select object to extend or shift-select to trim or [Project/Edge/Undo]: Return (to end)
Sometimes you may get the message "Object does not intersect an edge" or "No edge in that direction". If this happens it means that you are either picking the wrong end of the object or the object you are trying to extend will not meet the boundary edge. The solution is either to pick near the end you want to extend or to move the boundary edge so that the extended line will intersect with it.
Using Edgemode
If the line you are trying to extend does not intersect with the boundary line, you can use the "Edge" option to toggle Edgemode to "Extend" (the default is "No Extend"). When the Extend command is set to Extend Mode, the objects being extended will extend to an imaginary line through the boundary edge, irrespective of whether the extended object actually intersects with the boundary edge. This is particularly useful and can save lots of time.
Edgemode = Extend The illustration on the left shows the result of extending a line (red) to a boundary edge (cyan) with Edgemode set to "Extend". The same process would have resulted in an error message if Edgemode had been set to "No Extend". To get a better understanding of how this works, draw the two lines as shown in the illustration and try to extend them using the default settings. When you have done that, follow the command sequence below.
Command Sequence (Edgemode)
Command: EXTEND
Select boundary edges: (Projmode = UCS, Edgemode = No extend)
Select objects: (select the boundary edge, P1)
Select objects: Return (to end boundary edge selection)
Select object to extend or shift-select to trim or [Project/Edge/Undo]: E (to use the Edge option)
Enter an implied edge extension mode [Extend/No extend] : E (to set Edgemode to Extend)
Select object to extend or shift-select to trim or [Project/Edge/Undo]: (pick the object to be extended, P2)
Select object to extend or shift-select to trim or [Project/Edge/Undo]: Return (to end)
Notice that the current value of Edgemode is always displayed on the command line when you start the Extend command. The Edge option can also be used with the Trim command to enable trimming to cutting edges which do not actually intersect the object to trim. Edgemode is a system variable, so any change to its value will affect both the Trim and Extend commands.
See the Lengthen command for more ways to extend and trim objects.
Shift Selection with Trim & Extend
You may have noticed during the command sequences for the Trim and Extend commands that you have the option to "shift-select". This feature is new to AutoCAD 2000i and it enables you to extend while using the Trim command and to trim while using the Extend command. These two commands are very closely related and you often need to trim and extend objects at the same time. If you are a beginner with AutoCAD it may be a good idea to avoid this feature initially, the Trim and Extend commands can be tricky to get to grips with in any case. However, do remember this feature because it is a great time saver.
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The Break Command
Toolbar Modify Break
Pull-down ModifyBreak
Keyboard BREAK short-cut BR
The Break command enables you to break (remove part of) an object by defining two break points. In the illustration below, a corner of a rectangle has been removed. The Break command can be used with lines, polylines, circles, arcs ellipses, splines, xlines and rays. When you break an object, you can either select the object using the first break point and then pick the second break point, or you can select the object and then pick the two break points.
Breaking a rectangle
Command Sequence
Command: BREAK
Select objects: (select the object using the first break point, P1)
Specify second break point or [First point]: (pick the second break point, P2)
The section of the object is removed and the command ends.
Sometimes you may want to select the object first and then specify the two break points. If this is the case, use the "First point" option to specify the first break point. By default, AutoCAD assumes that the point used to select the object is the first break point. This is often confusing for new users.
It may sometimes be necessary to break a line into two without removing any part of it. In this case, simply pick the first and second break points in the same position.
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The Chamfer Command
Toolbar Modify Chamfer
Pull-down ModifyChamfer
Keyboard CHAMFER short-cut CHA
The Chamfer command enables you to create a chamfer between any two non-parallel lines as in the illustration below or any two adjacent polyline segments. Usually, the Chamfer command is used to set the chamfer distances before drawing the chamfer. Follow the command sequence below where the chamfer distances are changed to 20 before the chamfer is made.
Chamfering one corner of a rectangle
Command Sequence
Command: CHAMFER
(TRIM mode) Current chamfer Dist1 = 10.0000, Dist2 = 10.0000
Select first line or [Polyline/Distance/Angle/Trim/Method]: D (to set distances)
Specify first chamfer distance <10.0000>: 20 (enter required distance)
Specify second chamfer distance <20.0000>: Return (first distance value or enter a different value)
Select first line or [Polyline/Distance/Angle/Trim/Method]: (pick P1)
Select second line: (pick P2)
The chamfer is made and the command ends.
Notice from the command sequence that there are a number of options which can be used to control the way the Chamfer command behaves. The Polyline option can be used to chamfer all vertexes of a polyline simultaneously. The Distance option allows you to specify the two chamfer distances. Angle allows the angle between the first line and the chamfer to be specified. Trim is used to control whether the original lines are trimmed to the chamfer or remain as they are. Finally, Method is used to toggle the command between Distance and Angle mode. When Angle mode is used, the chamfer is defined using one distance and an angle rather than two distances.
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The Fillet Command
Toolbar Modify Fillet
Pull-down ModifyFillet
Keyboard FILLET short-cut F
The Fillet command is a very useful tool which allows you to draw an arc between two intersecting lines or adjacent polyline segments. You first need to use the command to set the required radius and then a second time to select the two lines.
Command Sequence
Command: FILLET
Current settings: Mode = TRIM, Radius = 10.0000
Select first object or [Polyline/Radius/Trim]: R
Specify fillet radius <10.000>: 25
Select first object or [Polyline/Radius/Trim]: (pick P1)
Select second object : (pick P2)
Filleting two linesThe Fillet command can also be used to fillet arcs and circles. The "Polyline" option also allows you to fillet all vertices of a polyline with a single command. It's worth experimenting with this command, it can save you lots of time and enables you to construct shapes which otherwise would be quite difficult.
Filleting a closed polylineFor example, you can easily create the lozenge shape shown on the right from a simple rectangle. Since AutoCAD rectangles are just closed polylines, you can use the Polyline option of the Fillet command to fillet all polyline vertexes simultaneously. Try this for yourself; draw a rectangle and then follow the command sequence below.
Command Sequence
Command: FILLET
Current settings: Mode = TRIM, Radius = 10.0000
Select first object or [Polyline/Radius/Trim]: P
Select 2D polyline: (pick P1)
4 lines were filleted
Tip: Make sure that the radius you specify will fit the objects you select, otherwise the fillet command will not work.
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The Explode Command
Toolbar Modify Explode
Pull-down ModifyExplode
Keyboard EXPLODE
The Explode command is used to "explode" single objects back to their constituent parts. In other words, the command is used to return blocks, polylines etc. (which may be composed of a number of component objects) back to their individual component parts. The change has no visible effect.
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Tips & TricksHot Tips
* Always look at the command line for guidance when you are learning new AutoCAD commands. The command line will prompt you for information and this is the easiest way to find out how a new command works. This is especially true for the more complicated commands like Trim and Extend.
* The command line window displays 3 lines of text by default. You can change this by clicking and dragging the top of the window frame. When you are starting with AutoCAD you may like to see more than 3 lines. The illustration below shows the command line window increased in size to 6 lines.
AutoCAD command line
* The command line can also be displayed as a floating text window. The AutoCAD text window is displayed by pressing the F2 key on the keyboard. See Function Keys on the Drawing Aids tutorial for more details.
* You can control the way text is mirrored using the MIRRTEXT variable.
* Fillet 0You can use the Fillet command with a radius of zero to trim intersecting lines back to their intersection. Of course, you could also achieve this effect with the Trim command but if you have a number of operations to complete, the Fillet method is much quicker.
* When using the Trim and Extend commands, hitting Return when prompted to "Select cutting edges" and "Select boundary edges" respectively will automatically select all valid cutting or boundary edges on the screen. This can be a real time saver if you have a complicated set of edges to select
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