https://drive.google.com/open?id=0B4VgIIlwe1ARN2VEN2RCZjdpblU
Introduction
You've probably used computers to play games, and to write reports for school. It's a lot more fun to create your own games to play on the computer. This book will help you get started by using QBASIC. QBASIC is a programming language. With a programming language you can tell the computer what you want it to do. It's a lot like giving someone directions to your house. The computer follows each step and does exactly what you tell it. By programming the computer you can solve math problems, create art or music, and even make new games. It's all up to you.
The best way to start with this book is to type in some of the small programs you'll find in each of the chapters. You might need to work through Chapter 1 first. An adult can help you get up to speed quickly. Then change the programs to do what you want them to do. Before long, you'll be writing your own programs.
(Parents and teachers, see the introduction I've written for you in Appendix B.)
Chapter 1 - Getting Started
Getting to DOS
To run QBASIC we need to get to DOS.
If you are using Windows 7, you might need to use dosbox. See Appendix C for details.
If you are using Windows XP, it is called the "Command Prompt" and you can get to it by clicking on Start, then All Programs, then Accessories, then Command Prompt.
In Windows 2000, it is called the "Command Prompt" and you can find it by clicking on Start, then Programs, then Accessories, then Command Prompt.
In Windows NT 4, I believe it was called the "Command Prompt". Try the steps described for Windows 2000 above.
In Windows ME, click on "Start", then "Programs", then "Accessories", then "MS-DOS Prompt".
In Windows 98, try the Windows 95 steps above, then try the Windows ME steps below. It is the same as one of those, but I don't have a Windows 98 machine handy to test.
In Windows 95, click on "Start", then "Programs", then "MS-DOS Prompt".
If you are using Windows 3.1, you'll need to find the "Main" program group and look for an icon called "MS-DOS Prompt". Double-click on it to get the "
C:\WINDOWS>" prompt in a black window with white text.
You will get the black window with the "
C:\WINDOWS>" prompt.
Now you're ready for the next step.
Starting QBASIC
QBASIC may not be on your computer. Don't be alarmed, Appendix A shows you how to get it there. At the "
C:\WINDOWS>" prompt, try typing:qbasic
and press the <Enter> key.
If you get a blue screen with something about a "Survival Guide", you made it!
If instead you got something about a "Bad command or file name",
you need to check out Appendix A to get QBASIC installed on your machine. Don't worry, we'll wait right here for you.
Survival Guide?
We won't be using the survival guide right now, so press the <Esc> (escape) key to get the big blank blue screen.
Now we're ready to program.
Getting Out
If you need to leave QBASIC, you can click on "File" on QBASIC's menu and then "Exit" to get back to DOS.
At the "
C:\WINDOWS>" prompt, type:exit
and then press the <Enter> key to get rid of the DOS window.
Chapter 2 - PRINT
Follow Along
I'm assuming you will be following along with QBASIC in front of you. This won't be much fun if you aren't. So power up the computer and get QBASIC's blue screen up in front of you. Go ahead and press the <Esc> key to "clear this dialog box" when it asks you.
QBASIC's Screen
Before we start, let's take a look at QBASIC's screen. At the top is QBASIC's menu. The menu has the following items: File, Edit, View, Search, Run, Debug, Options, and Help.
Clicking on any of these with the mouse will show more selections.
Just under the menu is the word "Untitled". This means we haven't given the current program a name. This will change when we learn to save programs. The big blue area with the blinking cursor (
_ ) is where we can enter our programs.
These are the most important areas to know for right now. We will learn more about the rest of QBASIC's screen as we go along.
QBASIC Editor
If you've used your computer to do school work or write a report, you'll notice that most of the editing keys (<Backspace>, <Delete>, <Insert>...) work just the way they always do. In case you haven't used these keys before, here's a list:
- <Backspace> - Removes the character to the left of the cursor
- <Delete> - Removes the character at the cursor
- <Left Arrow> - Moves the cursor left
- <Right Arrow> - Moves the cursor right
- <Up Arrow> - Moves the cursor up
- <Down Arrow> - Moves the cursor down
- <Home> - Moves the cursor to the start of a line
- <End> - Moves the cursor to the end of a line
Your First Program.
With the blinking cursor (_ ) at the top-left corner of the screen, type this:PRINT "Welcome to QBASIC. Your wish is my command."
") are very important. If you make any mistakes, use the <Backspace> key to correct them.
Running Your Program.
That's great, but what does it do? You have to run it to find out. Look for the word "Run" on QBASIC's menu at the top of the screen. Click on it. Now you will see a menu with a list of selections (Start, Restart, and Continue). Click on Start to run your program. You may have to look around on the screen a bit, but you should find your message:
C:\WINDOWS> qbasic
Welcome to QBASIC. Your wish is my command.
PRINT prints things to the screen. Don't forget to put what you want to say in double-quotation marks (").
Press any key to continue?
At the bottom of the screen you will see a message "Press any key to continue". Press <Enter> to get back to QBASIC. This is QBASIC's way of giving you enough time to see what your program did.
As a cool shortcut, use ? for PRINT. Try it. Press <Enter> to start typing on a new line. Now type this:
? "Programming is fun."
Isn't that nice? The ? becomes the word PRINT. That should save us a lot of typing in the long run.
You can try running the program again (remember? Click on Run, then Start on the menu). You should see something like this:
C:\WINDOWS> qbasic
Welcome to QBASIC. Your wish is my command.
Welcome to QBASIC. Your wish is my command.
Programming is fun.
Learned
- QBASIC screen
- Editing Keys
- Run | Start
- ?
Chapter 3 - CLS
It was probably a bit hard to find your messages on the screen with that last program. Wouldn't it be nice to be able to clear all that stuff off the screen? "CLS", which stands for CLear Screen, is just the ticket. You will need to put CLS at the very top of your program to clear the screen before you print anything.
Inserting a new line at the top.
To make CLS the first line of your program, follow these steps:
- Press the <Home> key to get to the beginning of the current line.
- Press the <Up Arrow> key once or twice to get the cursor to the top of the program.
- Press the <Enter> key to get a new line.
- Press the <Up Arrow> key once to get the cursor on the new blank line.
- Type CLS
Now your program should look like this:
CLS
PRINT "Welcome to QBASIC. Your wish is my command."
PRINT "Programming is fun."
Run it. Remember, click on Run and then Start in QBASIC's menu. You can also run the program by pressing the <Shift> key and holding it down while pressing <F5> (that's what Shift+F5 means on the menu). Another handy shortcut.
That's much better. Only your message is on the screen, which is the way it should be.
Learned
- CLS
- Shift+F5
Chapter 4 - DO...LOOP
Let's start a new program. To get rid of the old program, click on "File" on QBASIC's menu and you will see the File menu with New, Open..., Save, and others. Click on "New". QBASIC will now tell you that your current program is not saved, and it will ask if you want to "Save it now?". Let's not save it. You can always type it in again. Click on < No > with the mouse. We'll learn how to save programs in Chapter 8.
Now you should have a clean blue screen. Type in this program:
DO
PRINT "Ted was here ";
LOOP
Make sure you get the semi-colon (;) at the end of the PRINT line, and the space between the word "here" and the last quotation mark. You don't need to use my name, put yours in there instead ("Jack was here ", or whatever). Now run it.
DO...LOOP
DO and LOOP will do whatever is between the DO and the LOOP over and over again.
Give me a Break!
Ok, that's great. HOW DO I STOP IT!? Press the <Ctrl> (Control) key and hold it down while pressing the <Pause> key. The <Pause> key is usually at the top of the keyboard and all the way over to the right. This is called "pressing the <Break> key." Don't worry, it doesn't really break your program, it just stops it and takes you back to QBASIC. Maybe it should have been called the "brake" key.
On laptop keyboards, the break key might be a little harder to figure out. Sometimes you have to press the "Fn" key followed by the key that says break. Sometimes it is color-coded to help you figure it out. You'll know you have it when the program stops and the editor comes back up.
Neatness Counts
Notice that PRINT has two spaces in front of it. This is called "indenting" and it makes the program easier to read. You can see the beginning and the end of the DO...LOOP more clearly. QBASIC doesn't care about indenting, only people do. It's important to make your programs look nice so other people can read them.
Learned
- File | New
- DO...LOOP
- <Break>
Chapter 5 - INPUT
So far our programs have only talked to us. Let's write one that will listen. Get rid of the previous program by clicking on File, then New on QBASIC's menu. Click on < No > when it asks if you want to save the old program now. Try this:
CLS
INPUT "Enter your name: ", Name$
PRINT "Hello, "; Name$; ". How are you today?"
Don't forget the comma (,) between "Enter your name: " and Name$. Run it. When it asks, type your name, then press the <Enter> key.
What's in a "Name$"?
"Name$" is called a "variable". To be variable means that things can change. Try running the program again, but this time type in a friend's name (don't forget the <Enter> key). Sure enough, the message changes.
INPUT
INPUT Name$ takes what you type at the keyboard and puts it into the Name$ variable. PRINT Name$ prints out what is in the Name$ variable.
Variables
Variables hold letters and numbers. The dollar sign ($) means this variable can hold letters. These are called "string variables". Variables without a dollar sign can only hold numbers. We'll be seeing them soon.
You can call your variables anything you want. Try going back through this program and changing every "Name$" to "Fred$". What happens when you run it?
Another way to think of a variable is to imagine a small bucket with a name on it. Put "Name$" on it. This is the bucket's (variable's) name. Now take a piece of paper and write your name on it and drop it into the imaginary bucket. Now the variable Name$ has your name in it. Computer variables can only hold one piece of paper (one value) at a time.
PRINT and Variables
When you want to PRINT what's in a variable, leave off the double-quotation marks ("). This program will show you how this works:
CLS
INPUT "Enter your name: ", Name$
PRINT "Name$"
PRINT Name$
The first PRINT statement prints Name$ on the screen. The second PRINT statement prints whatever name you entered.
Learned
- INPUT
- Variables
Chapter 6 - IF...THEN
Let's make that last program a little smarter. I want to be able to identify intruders playing with my programs. Wouldn't it be great if the computer could recognize someone's name and print a special message for them? How about this:
CLS
INPUT "Enter your name: ", Name$
IF Name$="Mike" THEN
PRINT "Go Away!"
ELSE
PRINT "Hello, "; Name$; ". How are you today?"
END IF
You can change the Name$="Mike" to Name$="Joe", or whoever you want to go away (like a brother or sister). Run the program and type in your name (hopefully it isn't Mike). You should see the same old message as before. Now run it again and try entering "Mike" (or "Joe" or whatever you changed it to).
"Mike" Is Not The Same As "mike"
If it didn't tell the right person to go away, make sure you typed the name correctly. In QBASIC, "Mike" is not the same as "mike", so if you don't type the first letter in upper-case, the program won't work. Make sure you enter the name exactly the way you put it in the program.
IF...THEN
The "IF..THEN..ELSE..END IF" statement in this program checks to see if Name$ has "Mike" in it. If so, then it does the PRINT statement after the "THEN". If Name$ isn't "Mike", it does the PRINT statement after the "ELSE". "END IF" tells QBASIC that the "IF" is over.
Conditions
The Name$="Mike" portion of the IF...THEN is called the "condition". With numbers you can also check for "greater than" and "less than":
CLS
INPUT "Enter a number: ", Number
IF Number < 100 THEN
PRINT "Your number was less than 100"
ELSE
PRINT "Your number was greater than or equal to 100"
END IF
If you want to check for "greater than", use "Number > 100". Equals works just like before, "Number = 100". Another option is "not equal" which can be done like this: "Number <> 100".
IF...THEN is one of the most powerful features of QBASIC. Using IF...THEN can make your programs very interesting. It lets your program make decisions and do something appropriate.
Learned
- IF...THEN...ELSE
Chapter 7 - Numbers
Computers are very good at math. Let's get the computer to do some math for us. Here's a simple multiplication calculator:
CLS
INPUT "Enter the first number: ", A
INPUT "Enter the second number: ", B
PRINT "The answer is: "; A * B
If you have trouble finding the star (or asterisk "*") on the keyboard, it is usually above the number 8. Run it, and enter two numbers. It does an excellent job multiplying for you.
Variables and Math
A and B are variables, just like Name$. Unlike Name$, A and B do not have a dollar-sign after their names. This is because they are only holding numbers, not letters.
Star
"A * B" means "A times B". QBASIC doesn't use "X" for multiplication because you might want to have a variable called "X".
What else?
Try changing the "A * B" to "A - B" for subtraction. "A + B" will do addition, and "A / B" will do division. Why "/" for division? Because there's no division sign key on the keyboard. At least I haven't found one.
Expressions
"A * B", "A + B", "A - B", and "A / B" are called mathematical expressions, or simply "expressions".
Learned
- Variables with numbers
- INPUT with numbers
- Multiplication, Division, Addition, Subtraction
- Expressions
Chapter 8 - Saving
Before we get into some fairly big programs, let's look at how to save our masterpieces.
Location Is Everything
The first thing we need to think about is where we are going to save our programs. The two main places we can save things are the hard disk, and onto a floppy disk. Let's look at both places.
Saving To Floppy
Using a floppy is a good idea if you don't want other people looking at your programs. The floppy can be removed from the computer and kept in a safe place. Floppies are also good when you want to move programs from one computer to another.
Let's try it. Find a blank floppy and place it into the computer's floppy drive. The floppy can have stuff on it, as long as there is some free space left. You'll need to have a program up on the screen for this to work properly. Go back to the previous chapter and type in the example if you need to get something up on the screen.
Click on "File" on QBASIC's menu, then "Save".
The "Save" dialog will appear on the screen. Notice that the cursor is in a box (field) called "File Name:". Type this:
a:\testsave
Be careful with that backslash "\". It isn't the same as the front-slash "/" which is on the same key as the question mark "?". QBASIC is very picky about this.
Press the <Enter> key. What happened? Did you hear something? Where'd the "Save" dialog go? Hmmmm, ok, hopefully that worked. How can we check? Let's try loading the program back into QBASIC. Wait, it's already there. Ok, then let's clear it out and try to get it back from the floppy. Remember how to get rid of a program? Hint: "File" then "New" on the menu.
Loading From Floppy
Let's do it. Click on "File" then "Open..." on QBASIC's menu. Now you will see the "Open" dialog. Type this:
a:
and press the <Enter> key. Now you should see "TESTSAVE.BAS" in the box called "Files". Two clicks oughta do it. Double-click on "TESTSAVE.BAS" and the program is back from the floppy.
Saving To Hard Disk
The hard disk is the most convenient place to save programs. Since it is always in the computer, your programs are always there for you to load when you need them.
Neatness Counts, Again
Hard Disks usually have a lot more information on them than a floppy. Because of this, you need to be concerned about keeping things neat. You'll never be able to find what you need otherwise. The best way to organize your collection of programs is to put them into a "subdirectory" or "folder" on the hard disk.
Making Your Own Subdirectory
Let's make a folder on the hard disk for our programs. In Windows 95 we will need to use Windows Explorer. In Windows 3.1 we will use File Manager. First, the Windows 95 way.
Making a Folder With Windows 95
Click on the "Start" button, then "Programs", then "Windows Explorer".
Now click on "File", then "New >", then "Folder" on the Windows Explorer menu.
Notice the words "New Folder" on the right-hand side of Windows Explorer.
Go ahead and type in a name for the new folder, but keep it short! QBASIC is an older program that can't handle names bigger than 8 letters or numbers. I called mine "TedsPrgs" meaning Ted's Programs. Call yours whatever you want, but no more than 8 characters, or it will look real funny to QBASIC.
Press the <Enter> key to create the folder. Good, that's done. We won't need to do that again, unless you'd like to make another directory at a later date. Go ahead and close Windows Explorer and get back to QBASIC.
Making a Folder With Windows 3.1
...
Saving In The Directory
Saving to the hard disk is only a little different from saving to a floppy. You'll need to have a program up on the screen to save. Go back to the previous chapter and type in the example if you need to get something up on the screen.
Click on "File" on QBASIC's menu, then "Save".
The "Save" dialog will appear on the screen. The cursor is in a box (field) called "File Name:". Type this:
c:\TedsPrgs
Once again, watch the backslash "\". Change "TedsPrgs" to whatever you named your directory.
Press the <Enter> key. Now you will see a list of programs in that directory. Since you haven't saved anything yet, there shouldn't be any programs there. Go ahead and type this:
testsave
and press the <Enter> key. Hmmm, no sound at all that time. Hard disks are much more quiet than floppies. So, let's make sure it worked properly. First, get rid of what's on the screen with a "File" then a "New".
Loading From Hard Disk
This is also very similar to the way we loaded a program from floppy. Click on "File" then "Open..." on QBASIC's menu. This will bring up the "Open" dialog. Type this:
c:\TedsPrgs
Like before, watch the backslash "\" and change "TedsPrgs" to whatever your folder was called.
Now you should see "TESTSAVE.BAS" in the "Files" box. Double-click on "TESTSAVE.BAS" to load it.
That wasn't too bad, was it? Once you figure out whether you want to save to floppy or hard disk, you only need to remember one way to save and load.
Learned
- Saving to floppy
- Loading from floppy
- Creating a folder
- Saving to hard disk
- Loading from hard disk
Chapter 9 - SELECT CASE
IF...THEN...ELSE is fine if you only have two things you want to check. What if you have 5 or 6 friends that might use your computer and you want the computer to say something different to each of them? Try this:
CLS
INPUT "Enter your name: ", Name$
SELECT CASE Name$
CASE "Ted"
PRINT "Greetings, oh powerful master"
CASE "Mike"
PRINT "Go away!"
CASE ELSE
PRINT "Hello, "; Name$; ". How are you?"
END SELECT
Whew, that was a big one. Fortunately we learned how to save in Chapter 8. Save it if you want before running it. Feel free to change "Ted" and "Mike" to "Laura" and "Robin" or whoever.
SELECT CASE
SELECT CASE first checks Name$ for the value "Ted". If it finds it, it does the PRINT after the CASE "Ted". When the PRINT is done, it skips over the rest of the CASEs. It keeps checking against each CASE until it gets to CASE ELSE. If it hasn't found anything, it will do whatever is after the CASE ELSE.
Just In CASE
SELECT CASE can also be used with numbers as well as strings. Here's a quick example:
CLS
INPUT "Enter a number: ", Number
SELECT CASE Number
CASE 1234
PRINT "Thank you for entering the secret number 1234"
CASE 22
PRINT "Well, 22 is an interesting number"
CASE ELSE
PRINT "You must not know the secret number"
END SELECT
Learned
- SELECT CASE
Chapter 10 - Equals
So far, we've only let the user fill in our variables. We can fill in variables on our own inside our programs too. Like this:
CLS
A = 1
B = 2
A$ = "Hello"
PRINT A
PRINT B
PRINT A$
"A = 1" places the value 1 in the variable A. "B = 2" places the value 2 in the variable B. A$ = "Hello" places the string "Hello" in the variable A$. You get the picture. Then the program prints them out to prove to you that they are there.
See if you can figure out what this rather clever program will do. Then type it in and run it to see if you were right.
CLS
Count = 1
DO
PRINT Count
Count = Count + 1
LOOP
Did you get it right? Did the output go by way too fast? You'll have to press Break to stop it. Then take a closer look and see if you can see what's going on.
That program is what is called a "counter". It counts 1, 2, 3, 4... until it is stopped.
Chapter 11 - Random Numbers
Random numbers are numbers that you can't predict. Flipping a coin or rolling dice will give you a random number. Random numbers are very important in games and in some kinds of Math. Computers can generate random numbers pretty well. QBASIC's RND function provides random numbers that we can use.
RND
RND is a special function that gives us a random number between 0 and 1. We can use this in games to make things interesting. RND is perfect for rolling dice or flipping a coin. First let's see RND in action:
CLS
PRINT RND
PRINT RND
This program will print RND twice. Notice that you'll get two numbers that appear to be unpredictable and random. But, try running the program again. You'll get the same "random" numbers. This means your games would always be the same each time the user runs them. Fortunately, there's a way to fix this.
RANDOMIZE TIMER
Using RANDOMIZE TIMER will make sure the random numbers you get are different each time you run. Try this:
CLS
RANDOMIZE TIMER
PRINT RND
PRINT RND
Useful Random Numbers
Random numbers between 0 and 1 aren't really very useful. What you will need for a game might be a random number between 1 and 6, like when you roll dice. To get something more useful, we'll use math. Fortunately, computers are very good at math.
There are two problems we must solve to get the results we want. First, the range of random numbers has to be expanded from 0 through 1 to 1 through 6. That's easily done like this:
CLS
RANDOMIZE TIMER
PRINT RND * 6 + 1
PRINT RND * 6 + 1
By multiplying by 6, we increase the range to 0 through 5. By adding 1 we shift the range up to 1 through 6. However, there's still a problem. All that decimal stuff. QBASIC's INT function can be used to convert a decimal number to an integer (a number without a decimal).
CLS
RANDOMIZE TIMER
PRINT INT(RND * 6 + 1)
PRINT INT(RND * 6 + 1)
Roll the Dice
Here's a program that rolls two dice and prints the value of each. The variables Die1 and Die2 are used to hold the values of each die before printing. In a real game, Die1 and Die2 would be used in some clever way to change the outcome of the game.
CLS
RANDOMIZE TIMER
INPUT "Press ENTER to roll dice...", A$
PRINT
Die1 = INT(RND * 6 + 1)
Die2 = INT(RND * 6 + 1)
PRINT "Die 1: "; Die1
PRINT "Die 2: "; Die2
PRINT By Itself
Note that in the last program there was a PRINT on a line by itself. Did you see what it did? It simply printed a blank line on the screen. This can be useful for making the output from your program look nicer.
Chapter 12 - The Fortune Teller
Here's a fun program that uses most of what we've learned so far to make a "Magic 8 Ball".
CLS
RANDOMIZE TIMER
PRINT "I am the magical Fortune Teller."
INPUT "Think of a question and press enter for your answer...", A$
PRINT
Answer = INT(RND * 5 + 1)
SELECT CASE Answer
CASE 1
PRINT "Yes, definitely."
CASE 2
PRINT "Ask again, later."
CASE 3
PRINT "No way!"
CASE 4
PRINT "It is certain."
CASE 5
PRINT "Yes."
END SELECT
As always, go ahead and customize it. Change "No way!" to "You bet!" to get a Fortune Teller that never says "No".
Adding CASEs
Go ahead and try adding a new fortune. You'll need to change
Answer = INT(RND * 5 + 1)
to
Answer = INT(RND * 6 + 1)
since there will be 6 fortunes now. Then you will need to add a "
CASE 6" and a PRINT to print the new fortune.Chapter 13 - DO...WHILE
Back in Chapter 4 we saw a DO...LOOP that went forever. There are a number of ways to make a loop stop. One way is to use WHILE. This next program uses WHILE to make sure the program will only go as long as Answer$ has the letter "y" in it.
CLS
DO
INPUT "Enter the first number: ", A
INPUT "Enter the second number: ", B
PRINT "The answer is: "; A * B
INPUT "Would you like to do it again (y/n)? ", Answer$
LOOP WHILE Answer$="y"
The condition on the LOOP WHILE line is the same as a condition we might use in an IF...THEN. In this case, we check to see if Answer$="y", and if it does, we continue looping. If it doesn't, we fall out of the loop and our program ends.
You can add this feature to any program. Try adding it to the fortune teller.
Chapter 14 - OR and LEFT$
That last program works great, as long as the user always types in a lowercase "y". What happens if the user types in "yes"? Since "yes" is not the same as "y" to the computer, the test for Answer$="y" will fail, and the program will end. Probably not a good idea. We have the same problem if our user enters a capital "Y". Try a few of these to see what I mean.
There are several ways to make this program smarter and easier to use for our users. We could have it check for a few different ways of saying yes by using "OR", like this:
CLS
DO
INPUT "Enter the first number: ", A
INPUT "Enter the second number: ", B
PRINT "The answer is: "; A * B
INPUT "Would you like to do it again (y/n)? ", Answer$
LOOP WHILE Answer$="y" OR Answer$="Y"
This version will allow the user to enter "y" or "Y" and the program will run again. We can get even more clever by using LEFT$ like this:
CLS
DO
INPUT "Enter the first number: ", A
INPUT "Enter the second number: ", B
PRINT "The answer is: "; A * B
INPUT "Would you like to do it again? ", Answer$
FirstLetter$ = LEFT$(Answer$, 1)
LOOP WHILE FirstLetter$="y" OR FirstLetter$="Y"
This version will let the user enter "Yes", "yes", or just about anything that starts with a "y" because LEFT$ is used to only look at the first character in their answer. You could even enter "yep" or "YEAH!" and the program will begin again.
This may seem to make the computer smarter, but we know what's really going on. To prove the computer really isn't very smart, try entering "sure" or "yellow". It thinks "sure" is "no", and "yellow" is "yes".
LEFT$
LEFT$ can be used to take a certain number of letters from the left side of a string variable. As an example, if we have:
A$="TEST"
Then LEFT$(A$,2) will give us "TE". LEFT$(A$,3) will give us "TES". The first "parameter" you pass to LEFT$ is the string you want to work with. The second parameter you pass to LEFT$ is the number of characters (letters) you want. Let's try a program that uses LEFT$ in a different way:
INPUT "Enter something:", A$
PRINT A$
PRINT LEFT$(A$,1)
PRINT LEFT$(A$,2)
PRINT LEFT$(A$,3)
This program will print the first character of whatever you enter, followed by the first two characters, followed by the first three characters:
Enter something: Jack
Jack
J
Ja
Jac
QBASIC also provides a RIGHT$() in case you were curious, and it works just like LEFT$(). Try this:
INPUT "Enter something:", A$
PRINT A$
PRINT RIGHT$(A$,1)
PRINT RIGHT$(A$,2)
PRINT RIGHT$(A$,3)
Here's an example of what that program will do:
Enter something: Jack
Jack
k
ck
ack
Chapter 15 - COLOR
Tired of all this black and white? Then the COLOR statement is for you. Try this program for size:
CLS
COLOR 2, 0
PRINT "That's ";
COLOR 3, 0
PRINT "more ";
COLOR 5, 0
PRINT "like it!"
Color takes two numbers. The first number is the foreground color. The second number is the background color. For example, if you want to print black on white instead of white on black, use "COLOR 0,7". The colors each have their own number:
- 0 - Black
- 1 - Blue
- 2 - Green
- 3 - Cyan
- 4 - Red
- 5 - Magenta
- 6 - Yellow
- 7 - White
There are plenty of other colors too. Try the numbers from 8 through 15 to see what colors you get. Basically, if you add 8 to any of the above colors, you get brighter versions of the same color. Take blue which is 1 and add 8 and you get 9 which is bright blue.
Blinking
Adding 16 to a color number gives you a blinking version. This doesn't work in a DOS window, though. Press <ALT><ENTER> to switch to full-screen mode which will show the blinking. Try this program:
CLS
COLOR 28, 0
PRINT "*** WARNING ***"
COLOR 15, 0
PRINT "Programming can be too much fun!"
Color can be used in many ways to make your programs more interesting.
Chapter 16 - FOR...NEXT
A New Counter
FOR...NEXT is a loop like DO...LOOP, but a FOR...NEXT loop has its own counter built in. Try this:
CLS
FOR I = 1 TO 10
PRINT I
NEXT I
Much better than our last counter. This one stops on its own after counting to 10.
A Color Chart
Here's a color chart program using a FOR...NEXT loop.
CLS
FOR I = 1 TO 15
COLOR I, 0
PRINT I; "*** COLOR ***"
NEXT I
STEP
FOR...NEXT can also do "step counting". Try this:
CLS
FOR I = 2 TO 20 STEP 2
PRINT I
NEXT I
That will count by 2's from 2 to 20. STEP tells QBASIC what to count by. Try changing it to count by 10's from 10 to 100.
FOR...NEXT can also count backwards if you use a negative STEP value:
CLS
FOR I = 10 TO 1 STEP -1
PRINT I
SLEEP 1
NEXT I
PRINT "BLAST OFF!"
"SLEEP 1" tells QBASIC to wait for one second, then continue.
Chapter 17 - Sound
If you just need a beep in your program, you can use BEEP:
CLS
INPUT "Press Enter to hear a beep", A$
BEEP
SOUND lets you play a beep and tell it how high or low the beep will be, and how long it will last. This program makes a 1000Hz beep for about 1 second:
SOUND 1000, 18
SOUND is good for making sound effects. Here's a bomb dropping:
FOR I = 4000 TO 1000 STEP -5
SOUND I, .1
NEXT I
If you want to play a song, PLAY is exactly what you need. Try this:
PLAY "e8 d8 c8 d8 e8 e8 e4"
PLAY is like a little programming language inside of QBASIC. "e8" means play an eighth note "e". If you are familiar with sheet music, this will make sense. Here's a scale:
PLAY "c8 d8 e8 f8 g8 a8 b8 > c4"
The ">" greater than sign means "go up one octave". There are many more special commands in PLAY. Check the QBASIC help for a list of all of them.
PLAY and Triplets
Here's a familiar tune that uses a trick to do triplets.
PLAY "T180 <d8d8d8 T120 g2>d2"
PLAY "T180 c8<b8a8 T120 >g2d4"
PLAY "T180 c8<b8a8 T120 >g2d4"
PLAY "T180 c8<b8>c8 T120 <a2>"
Since PLAY doesn't do triplets, you have to modify the tempo to get the right rhythm. PLAY begins with a default tempo of "T120" which means 120 quarter notes per minute. In the above song, we switch to T180 which is the triplet tempo for T120. By multiplying our tempo by 1.5, we get the triplet tempo. When the triplets are done, we switch back to the regular tempo. You can see in the above example that we switch back and forth between the main tempo (T120) and the triplet tempo (T180) several times as needed.
As with everything, there's more than one way to do triplets. 8th note triplets can also be called 12th notes, like this:
PLAY "<d12d12d12g2>d2"
PLAY "c12<b12a12>g2d4"
PLAY "c12<b12a12>g2d4"
PLAY "c12<b12>c12<a2>"
Using this technique, 16th note triplets are 24th notes, etc.... You just multiply the note value by 1.5 instead of changing the tempo.
Each of these techniques has its advantages and disadvantages. The tempo-changing technique uses more space, but the notes retain their values. The 12th note technique is more compact, but not as easy to understand. Which one you use is up to you. Just make sure the next person to read your code understands what you are doing. Comments are a good idea.
Chapter 18 - LOCATE
LOCATE allows you to print in a specific place on the screen.
CLS
LOCATE 5, 10
PRINT "Here"
LOCATE 20, 70
PRINT "There"
The two numbers after LOCATE are the coordinates where the print will be. Just like coordinates in math class, these numbers give the row and the column. The first number in LOCATE is the row, or how far down the screen the print will start. The second number is the column, or how far over the print will start.
Let's use some random numbers, COLOR and LOCATE to make a more interesting version of our first looping program:
CLS
DO
Row = INT(RND * 23 + 1)
Column = INT(RND * 79 + 1)
LOCATE Row, Column
Color1 = INT(RND * 15 + 1)
COLOR Color1, 0
PRINT "Ted was here!";
LOOP
Kind of messy, but interesting.
How about a clock?
CLS
DO
LOCATE 1, 1
PRINT TIME$
SLEEP 1
LOOP
TIME$ is a special variable that contains the current time. Press Break to stop.
Chapter 19 - WIDTH
Use WIDTH 40 for big text. It changes the entire screen to wide text mode. Use WIDTH 80 to go back to normal size text.
SCREEN 0
WIDTH 40
CLS
PRINT "Wow! This is big!"
INPUT "Press <Enter> to go back", A$
WIDTH 80
PRINT "That's more like it."
Unfortunately, you won't see the big text in a window. You'll have to press <Alt>-<Enter> to switch to full-screen mode. Be sure to press <Alt>-<Enter> again to switch back to window mode.
Chapter 20 - CHR$
There are many special symbols and other characters you can display that aren't on the keyboard. Try this:
CLS
PRINT CHR$(1); CHR$(2)
That prints a couple of smiley faces. There are plenty of other characters too. This program will show you many, but not all of them:
CLS
FOR I = 32 to 255
PRINT I; CHR$(I); " ";
NEXT I
Chapter 21 - Graphics
So far, we've only been dealing with text (words and numbers). How do we do pictures in QBASIC? First, we need to use SCREEN to change from text mode to graphics mode.
SCREEN
SCREEN lets you select a "graphics" screen instead of the "text" screen we've been using. This will let you draw pictures. In the next program, we'll use DRAW to draw a square on the screen in SCREEN 12 graphics mode.
SCREEN 12
CLS
DRAW "D100 R100 U100 L100"
There are many other SCREEN numbers you can use, but 12 is probably the easiest to work with. It gives you a lot of space and the color numbers are familiar. QBASIC Help explains all the possible values of SCREEN. You can always try them and see what happens.
DRAW
DRAW is kind of like the turtle in the programming language Logo. With DRAW, you can move around the screen and draw lines along the way. In the above example we used the following DRAW commands:
- D100 - Go down 100 units
- R100 - Go right 100 units
- U100 - Go up 100 units
- L100 - Go left 100 units
DRAW can do a lot more than that. It is like PLAY. It's a small programming language inside of QBASIC. Look at QBASIC Help for a complete description of everything it can do. Here's a filled in box:
SCREEN 12
CLS
DRAW "C15 D100 R100 U100 L100 BF1 P15,15"
"C15" sets the color to bright white. "BF1" moves into the square, then "P15,15" fills it with bright white. Finally, here's something very Logo-like:
SCREEN 12
CLS
FOR I = 0 TO 360 STEP 10
DRAW "D100 R100 U100 L100 TA" + STR$(I)
NEXT I
"TA" means to turn to a specific angle. STR$ converts the value in I to a string. This lets DRAW turn to the angle in the variable I. It's not quite as easy as Logo, but it's still pretty impressive.
LINE
QBASIC also lets you draw using a coordinate system. It's like drawing graphs on graph paper. Try this:
SCREEN 12
CLS
LINE (0, 0)-(320, 240), 15
LINE lets you draw a line between two points. The points are specified in (x, y) coordinates. You may have seen this when learning about graphs in math class. In QBASIC, the coordinates are almost the same. The only thing that is different is the Y coordinate. In QBASIC, the Y coordinate is upside down. "0" is at the top, and bigger numbers go toward the bottom of the screen.
"LINE (0, 0)-(320, 240), 15" draws a line starting at coordinate (0, 0) which is the upper left corner of the screen. The line ends at (320, 240) which is the center of the screen. The last number is the color (15 which is bright white).
Box
By adding a "B" to the end of a LINE statement, you can draw a box. Try this:
SCREEN 12
CLS
LINE (0, 0)-(320, 240), 15, B
The first coordinate is the upper left corner while the second coordinate is the lower right.
Try "BF" instead of "B". Interesting?
CIRCLE
QBASIC can also draw circles using the CIRCLE statement:
SCREEN 12
CLS
CIRCLE (320, 240), 100, 15
The coordinate (320, 240) tells the computer where to put the center of the circle. "100" is the radius, or how big the circle will be. "15" is the color number (bright white again).
PAINT
Notice how that circle was not filled in. LINE has a "BF" option that will let us fill in the boxes it draws. CIRCLE has no such option, so we have to use PAINT:
SCREEN 12
CLS
CIRCLE (320, 240), 100, 15
PAINT (320, 240), 15, 15
PAINT fills an area with a color. It stops painting when it runs into a certain color on the screen. The coordinate (320, 240) tells PAINT where to start filling in, and the first "15" tells PAINT to use bright white as the paint color. The second "15" tells PAINT to stop painting when it runs into anything that is bright white.
Circle Art
Concentric circles are very easy to draw:
SCREEN 12
CLS
FOR I = 5 TO 200 STEP 5
CIRCLE (320, 240), I, 15
NEXT I
With CIRCLE, PAINT and some random numbers, we can make some interesting pictures:
SCREEN 12
CLS
FOR I = 1 TO 50
X = INT(RND * 640)
Y = INT(RND * 480)
R = INT(RND * 100)
Color1 = INT(RND * 16)
CIRCLE (X, Y), R, Color1
PAINT (X, Y), Color1, Color1
NEXT I
More Graphics
There's a lot more about graphics in my Sprites in QBASIC tutorial.
Chapter 22 - INKEY$
Up to now, we've been using INPUT to get things from the keyboard. The problem with INPUT is that our program stops until the user presses the enter key. Wouldn't it be nice to keep the program running and still be able to get input from the keyboard? INKEY$ will let you do this. Using INKEY$ is very important if you want to make "real-time" game programs.
Let's fix the clock program to let the user press any key to stop the program. This way the user doesn't have to know about the Break key.
CLS
LOCATE 3, 1
PRINT "Press any key to exit"
DO
LOCATE 1, 1
PRINT TIME$
SLEEP 1
LOOP WHILE INKEY$ = ""
Not bad at all. Now we don't need to teach the user about the Break key. We can do the same thing in any of our other programs that need the Break key. If the user does not press a key, INKEY$ returns nothing or "".
This next program will sit in a loop getting keys from the keyboard with INKEY$ and printing them to the screen:
CLS
DO
Key$ = INKEY$
IF Key$ <> "" THEN
PRINT Key$;
END IF
LOOP
That little program can be used to find the various secret codes used by INKEY$ to let you know the arrow keys have been pressed. This is very useful in game programming where the arrow keys might control a player in a game. If you press an arrow key, you'll see that a space and a letter are generated.
Chapter 23 - String Functions
Concatenation
Concat-uh-what?! It's just a fancy word for putting things together, one after another. It's much easier done than said. When you use the plus-sign "+" with strings, it doesn't add them up. Instead, it puts them together.
A$ = "Hello "
B$ = "there!"
C$ = A$ + B$
PRINT C$
That will print "Hello there!" to the screen.
LEFT$() and RIGHT$()
LEFT$() and RIGHT$() let you work with parts of strings. Try this example:
A$ = "Ted Felix"
B$ = LEFT$(A$, 3)
PRINT B$
LEFT$(A$, 3) means "take 3 characters from the left of A$". Since the 3 characters on the left happen to be "Ted", this program prints "Ted" as expected. Try changing the number to 2 or 5 and see what happens.
Once you understand LEFT$(), RIGHT$() is easy. Let's try it:
A$ = "QBASIC is cool"
B$ = RIGHT$(A$, 4)
PRINT B$
RIGHT$(A$, 4) means "take 4 characters from the right of A$". This gives us "cool".
MID$()
LEFT$() gives us something from the left side of a string. RIGHT$() gives us something from the right side of the string. MID$() gives us something from the middle of a string. Try this:
A$ = "one two three"
B$ = MID$(A$, 5, 3)
PRINT B$
MID$(A$, 5, 3) means "take 3 characters from the middle of A$, starting at the fifth character". This gives us the word in the middle: "two".
You can also use MID$() to change a portion of what is in a string variable. Try this:
A$ = "cabinet"
PRINT A$
MID$(A$, 4, 2) = "ar"
PRINT A$
Here, we replaced the "in" in cabinet with "ar". This gives us a completely different word.
This would be a pretty sneaky way to hide something like a password in a program. Someone who didn't know how to program in QBASIC might not be able to figure it out.
LCASE$() and UCASE$()
If you need to convert a string to all uppercase or all lowercase, UCASE$() and LCASE$() are exactly what you need.
A$ = "Fly Away With Me"
PRINT A$
PRINT UCASE$(A$)
PRINT LCASE$(A$)
You can use UCASE$() and LCASE$() to do "case-insensitive" tests. In other words, upper and lower case are ignored. Here's an improvement to a previous program.
CLS
INPUT "Enter your name: ", Name$
IF LCASE$(Name$) = "mike" THEN
PRINT "Go Away!"
ELSE
PRINT "Hello, "; Name$; ". How are you today?"
END IF
In this new version, the user can type "mike", "Mike" or even "MIKE" and the name will be recognized.
STRING$() and SPACE$()
Let's say you need to print 20 stars on the screen in a line. You could do it like this:
PRINT "********************"
But, there has got to be a better way. How about with a FOR loop?
FOR I = 1 to 20
PRINT "*";
NEXT I
PRINT
That works well, but QBASIC provides an even easier way to do this with STRING$().
PRINT STRING$(20, "*")
The first argument to STRING$() is the number of times you want a character repeated. The second argument is the character you want to repeat. So, STRING$(20, "*") means "give me 20 stars".
If you want to print a lot of spaces, you could do it with STRING$():
PRINT "A"; STRING$(20, " "); "B"
Or you can use SPACE$().
PRINT "A"; SPACE$(20); "B"
FOR loops always make things interesting:
FOR I = 0 to 20
PRINT SPACE$(I); "QBASIC!"
NEXT I
LEN()
LEN() gives you the length of a string. Try this:
A$ = "Hello"
PRINT LEN(A$)
As expected, that should print the number 5 since the word "Hello" has 5 characters.
LEN() is handy in FOR loops when you aren't sure how long the string is, and you don't feel like counting it yourself:
A$ = "Hello QBASIC!"
FOR I = 1 to LEN(A$)
PRINT LEFT$(A$, I)
NEXT I
LEN() is very useful when you want the user to provide the string. Try this:
INPUT "Enter a string: ", A$
PRINT "The string you entered was"; LEN(A$); "characters long."
CHR$() and ASC()
Computers really only know about numbers. To make strings of letters, computers convert letters to numbers so they can work with them. CHR$() and ASC() let you do the same kinds of conversions.
CHR$() converts a number to a character. We've already seen this in a previous chapter. Try this:
FOR I = 14 to 255
PRINT I; CHR$(I); " ";
NEXT I
That program displays all the character numbers from 14 through 255 followed by the characters that they represent. CHR$() is very useful for getting characters that aren't on the keyboard. Like this:
PRINT STRING$(40, CHR$(1))
There are some very interesting characters between CHR$(1) and CHR$(31). As an example, CHR$(7) makes a beep. Try it:
PRINT CHR$(7)
There are other interesting things as well. Play around with CHR$() a bit, and you'll find several.
ASC() is the opposite of CHR$(). ASC() takes a character and tells you its number. Try this:
PRINT ASC("A")
That program prints 65 because the character code for a capital "A" is 65. The code that QBASIC uses to convert letters to numbers is called "ASCII" (pronounced "askee").
INSTR()
INSTR() lets you search for a string in another string. If it finds the string, it will tell you where it is. Try this:
A$ = "Hello everyone"
PRINT INSTR(A$, "every")
That program prints "7" because "every" is found at the 7th position in "Hello everyone". If the string isn't found, INSTR() returns zero.
INSTR() can come in very handy when you want to break a string into pieces:
A$ = "two pieces"
SpacePos = INSTR(A$, " ")
PRINT LEFT$(A$, SpacePos)
PRINT RIGHT$(A$, LEN(A$) - SpacePos)
Chapter 24 - Comments and Constants
Most programming languages allow you to add notes to your programs that are ignored by the computer. This lets you explain what you've done to someone else who might read your program later. In QBASIC we use the apostrophe (') to begin a comment. Here's an example:
' A program to draw boxes all over the screen
' This is a comment, QBASIC will ignore it
SCREEN 12
CLS
' Draw 50 boxes
FOR I = 1 TO 50
' Pick the location of the box
X1 = INT(RND * 640)
Y1 = INT(RND * 480)
X2 = INT(RND * 640)
Y2 = INT(RND * 480)
' Pick the color for the box
Color1 = INT(RND * 16)
' Draw the box
LINE (X1, Y1) - (X2, Y2), Color1, BF
NEXT I
The computer will ignore all those comment lines, but us humans can read them and remember how a program works. Good programmers use comments to help others understand what they have done. Comments can also help us remember what we did when we come back to a program after working on something else for a while.
Constants
Another way to make your programs easier to understand is to use constants. Constants look and act like variables, but they cannot be changed. Here's a useful program:
CONST Pi = 3.141593
INPUT "Enter the radius of a circle: ", Radius
PRINT "The circumference is:"; 2 * Pi * Radius
PRINT "The area is:"; Pi * Radius * Radius
If we didn't use the constant Pi, we would have to copy the number 3.141593 two places in the above program. Using a constant makes the program easier to read and understand. It also keeps us from making mistakes when copying.
Chapter 25 - Sub-Procedures (SUBs)
When programs get big, you need to break them into smaller pieces that are easier to work with. QBASIC calls these pieces "sub-procedures" or SUBs. Other programming languages have other names, like procedures, subroutines, or subprograms.
To work with SUBs in QBASIC, we need to look at the "SUBs" dialog box which shows us a list of our SUBs. Select "View | SUBs..." from the menu to bring up the SUBs dialog box. You can also press the F2 key to get there more quickly. In here, you can select a SUB to work with, or you can select the main module. If you are just starting with a clean slate (File | New) you'll see that the main module is called "Untitled", and there are no SUBs.
You can define a new SUB in QBASIC simply by typing it in. This will jump you to the view of the new SUB. Try typing this:
SUB DrawCircle
CIRCLE (320, 240), 100, 15
END SUB
Notice that after you pressed enter on the first line, you were taken to a new screen with just your new SUB in it. Now, if you go to the SUBs dialog box (View | SUBs...), you can see that you have a SUB named "DrawCircle" and a Main Module named "Untitled".
Now we need to go back to the Main Module ("Untitled") to actually use the new SUB. From the menu, select View | SUBs... to get the SUBs dialog box. Now double-click on "Untitled" to get back to the Main Module. The screen will go blank, but don't worry, your SUB is still out there. Now type this in and run it:
SCREEN 12
CLS
DrawCircle
See? DrawCircle did what it was supposed to do.
Let's try adding another SUB. See if you can remember the steps on your own. Refer back to the previous example if you need help.
SUB PlayMary
PLAY "e8 d8 c8 d8 e8 e8 e4"
END SUB
Now we need to change the Main Module to use our new SUB. So, go back to the Main Module, and change it to look like this:
SCREEN 12
CLS
DrawCircle
PlayMary
Now run it and you should see the circle and hear the song.
Dividing programs into smaller pieces like this will help you make sense out of big programs.
Arguments
Sometimes you want to pass numbers or strings to a SUB. QBASIC lets you do this. Here is a new version of the DrawCircle SUB:
SUB DrawCircle2 (Radius)
CIRCLE (320, 240), Radius, 15
END SUB
This version lets us pass in the Radius. When we do this, Radius is called a "parameter" or "argument" to our SUB. Here's how we would then pass an argument from the Main Module:
SCREEN 12
CLS
DrawCircle
DrawCircle2 20
PlayMary
We could also do something like this in our Main Module:
SCREEN 12
CLS
FOR I = 5 TO 200 STEP 5
DrawCircle2 I
NEXT I
Changing Arguments
If you need to tell the main module something, you can change one of the arguments in your SUB, and the main module will see the change.
CLS
I = 0
AddOne I
PRINT I
SUB AddOne (X)
X = X + 1
END SUB
When you run that program, it will print the value 1 on the screen. This is because the value of I is changed by the AddOne SUB. If you only need to return one value, a FUNCTION is sometimes a better choice. FUNCTIONs are described later.
Arguments That Don't Change
If you happen to put parentheses () around an argument, that argument will not be changed by the SUB. Try this change to the last program:
CLS
I = 0
AddOne I
PRINT I
' Add these two lines:
AddOne (I)
PRINT I
SUB AddOne (X)
X = X + 1
END SUB
This will print 1 and then 1 again. The parentheses around the "I" keep the SUB from making any changes to I.
Scope
What if we had variables in the Main Module and in a SUB that happen to have the same name. Would they be the same variable? Let's find out. Enter this SUB:
SUB Scope
PRINT "Scope says: "; X
X = 23
END SUB
And this Main Module:
X = 15
Scope
PRINT "Main Module says: "; X
And run it. What happened? Scope said "0" because to Scope, X was a new variable. Main Module said 15, because Scope didn't change Main Module's X, it changed it's own X. Scope's X and Main Module's X are different variables.
Variables that you create in a SUB cannot be seen by the Main Module. Variables in the Main Module cannot be seen by a SUB. If you need to share variables, you can pass them as arguments to the SUB.
[Note to self: At this point, instead of going into global data, which is evil, go into functions, and then show how to communicate between subs and functions without global data. Introduce global data and statics last instead of functions last. In fact, quarantine SHARED and STATIC into a separate chapter and move it to the end of the book. What I've done here is like introducing GOTO first. See the email I sent to a reader on 12/28/2012 for more. Also write a GOTO chapter and put it at the end of the book as well.]
Global Data
It is possible to make variables in the Main Module available to SUBs without passing them as arguments. Add a "SHARED X" to the Scope SUB like this:
SUB Scope
SHARED X
PRINT "Scope says: "; X
X = 23
END SUB
Now when you run it, you'll see that the Scope SUB can now see the Main Module's X. Scope no longer has its own X. This is called "Global Data" (since it can be seen by everyone) and should be avoided if you can. Most programmers consider this dangerous since it is hard to know which SUB might change a global variable.
You can also make a variable global to all SUBs from the Main Module by adding a "DIM SHARED" to the main module before you set X to 15:
DIM SHARED X
X = 15
Scope
PRINT "Main Module says: "; X
This makes it easier to see which variables are global since they can be found in the Main Module. The problem is that this makes a variable global to every SUB in your program. Usually, only some SUBs need to see a global variable. It is better to use SHARED within your SUB in that case.
Object Oriented Programming
When you start worrying about SUBs and the Main Module sharing variables, you are probably ready to begin learning Object Oriented programming. Object Oriented programming makes it easier to share variables between SUBs and still write code that is easy to understand. Visual BASIC, Java, and C# are programming languages that provide Object Oriented programming features, and are fairly easy to learn.
STATIC
Notice that each time you call a SUB, its variables are lost after the SUB is over. Here's an example:
SUB Counter
C = C + 1
PRINT C
END SUB
CLS
Counter
Counter
Counter
Not a very good counter, since it always prints "1". We can use STATIC to tell QBASIC that we don't want C to go away after the SUB is over. Then we will get the behavior we expect. Change the Counter SUB like this:
SUB Counter
STATIC C
C = C + 1
PRINT C
END SUB
That's much better.
Object Oriented programming languages offer many ways to avoid the use of STATIC variables. If you find yourself making lots of STATIC variables, it is probably time to learn an Object Oriented programming language.
Functions
Functions are just like SUBs, but they return a value. Here's an example:
FUNCTION Add (X, Y)
Add = X + Y
END FUNCTION
And here's a Main Module to go with it:
PRINT Add(3, 4)
Well, I DECLARE!
As you've been entering the example programs in this chapter, you may have noticed that the QBASIC editor adds "DECLARE" statements to the programs. Why does it do this? The DECLARE statement is a warning to QBASIC to let it know that there are SUBs or FUNCTIONs in this program. Without this warning, QBASIC would have no idea what we mean when we call a SUB or FUNCTION. It would think it had found a syntax error and the program would stop.
Fortunately, QBASIC handles making DECLAREs for us. Unfortunately, in larger programs, it might put the DECLAREs someplace that looks ugly. Fortunately, you can move the DECLAREs anywhere you want (as long as it is before the FUNCTION or SUB is first used) and QBASIC still takes care of the rest.
Chapter 26 - Data Structures
Built-In Types
QBASIC offers five built-in types. Each of these types can be specified by a single character after the variable name. You can also specify a type using a DIM statement. It is important to pick the right types when you are writing a program. The following descriptions of each type will help you make the right decisions.
Single-Precision
The single-precision type handles numbers with decimals. You can go up to seven digits with a single-precision variable. In a DIM statement, use "SINGLE" to create a single-precision variable. The type-character for a single-precision variable is "!". Unless you do something special, any variable without a type character is single-precision. Here are some examples of creating and using single-precision variables:
X = 1.5
DIM Y AS SINGLE
Y = 2.1
Z! = 2.5
PRINT X; Y; Z!
Notice that the DIM statement can be used to tell QBASIC the type of a variable. Then you don't need to use a type character for that variable.
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