Contents
Basic Input
The INPUT command is used to gather input from the user. This section will attempt to teach you how to gather input upon request from the user. For real-time input, see QBasic/Advanced Input.
Here is the syntax of the input command:
INPUT "[text to user]"; [variable] ' Question mark added
or
INPUT "[text to user]", [variable] ' No question mark added
Example:
INPUT "What is your name"; name$
or
 INPUT "What is your age", age
When a semicolon (;) is used after the text output to the user, a question mark (?) and 'space' are added to the output. When a comma (,) is used no question mark is added.
If a string is specified (eg 'name$'), anything the user enters before pressing the 'return' key will be accepted. If a numeric variable (eg 'age') is specified, the user must enter a number (if any non-numeric key is entered, the error message "Redo from start" will be output and the INPUT command rerun)
6INPUT.BAS
1 CLS
2 INPUT "What is your name"; name$
3 PRINT "Hello, "; name$; "!"
4 INPUT "How old are you"; age
5 INPUT "what is your best computer game?", game$
6 PRINT "     name:"; name$
7 PRINT "      age:"; age; " years old"
8 PRINT "best game:"; game$
Please note: In the PRINT command, the (;) function concatenates (joins) the contents of the string variables with the text between the quotes (" "). Note the use of spaces so that the final printed text reads properly.
If a numerical variables is specified within the PRINT command, an additional space is automatically added both before and after the number.
See also: LINE INPUT command to read a line of text from a file (and place the result in a string variable) or to input a series of variables (in which case any commas found will be treated as delimiters between fields).
Text Output
Text Output
Your first QBasic program: 1HELLO.BAS
The following paragraph requires a computer with QBasic installed
To begin, copy everything from the program below ("PRINT "Hello World") into a text editor or into the QBasic IDE (Integrated Development Interface) itself and save it as "1HELLO.BAS". Next open the file in QBasic (unless you used QBasic IDE in which case it is already open) and press F5. Optionally you can use the "RUN" menu located on the menu bar at the top of the IDE window. This will execute (run) the program. The words "Hello World" should appear on the upper left hand side of the screen. You have just executed your first QBasic program. If you press F5 again, another line of text saying "Hello World" will appear on the upper left hand side of the screen pushing the first one down to the second row of the screen. You can follow the same procedure for the rest of the example programs in this wikibook.
1HELLO.BAS
 PRINT "Hello World"
PRINT
PRINT is QBasic's text output function. It is the command that we will be exploring through this section. PRINT is a QBasic function that requires arguments. The argument in the Hello World program we just ran were the words "Hello World". So, PRINT is the function and "Hello World" is the argument we pass-to the function.
PRINT [Text to screen]
*note, for a short cut, just use a question mark "?" in place of the command "PRINT". Likewise you can use a single quote " ' " in place of the key word REM to insert comments in your code
2HELLO.BAS
 PRINT "This line will be erased"
 CLS
 PRINT "Hello";
 PRINT " World",
 PRINT "Hello Jupiter"
 PRINT "Good Bye",,"For";" Now"
 PRINT 1,2,3,4,5
PRINT "Hello daddy"
PRINT, Commas, Semicolons and CLS
This is what the program output should look like:
Hello World   Hello Jupiter
Good Bye                    For Now
 1             2             3              4              5
The first line of 2HELLO.BAS outputs "This line will be erased." to the screen. However, in the second line, the CLS command clears the screen immediately after. So, it will only flash momentarily. The text "Hello Jupiter" should line up with '2' under it. More than one comma can be used consecutively. In this example, after "Good Bye" two commas are used to move "For Now" over two tab columns. "For Now" should line up with '3'.
My final statement on this topic is to play around with it. Try using commas and semicolons in a program.
3HELLO.BAS
 CLS
 hello$ = "Hello World"
 number = 12
 PRINT hello$, number
Variables
Variables are used to store information. They are like containers. You can put information in them and later change the information to something else. In this first example they may not seem very useful but in the next section (Input) they will become very useful.

In this example we use two types of variables - string variables and numeric variables. A string variable holds words, a string of characters (a character is a letter, digit or symbol). In this case the characters are letters. A string variable is denoted by ending the name of the variable with a dollar sign. The string variable in this program is hello$. What ever you set hello$ equal to will be displayed in the PRINT statement. The numeric variable is number. Numeric variables do not have a special ending like string variables.
4FACE.BAS
 CLS
 LOCATE 14, 34     'position the left eye
 PRINT "<=>"       'draw the left eye
 LOCATE 14, 43     'position the right eye
 PRINT "<=>"       'draw the right eye
 LOCATE 16, 39     'position the nose
 PRINT "o|o"       'draw the nose
 LOCATE 18, 36     'position the mouth
 PRINT "\_______/" 'draw the mouth
 LOCATE 19, 42     'the bottom
 PRINT "The Face by QBasic"
LOCATE statement
LOCATE allows you to position the cursor for the next piece of text output. Contrary to Cartesian coordinates which read (X,Y), the locate statement is LOCATE Y,X. In this case Y is the distance down from the top of the screen and X is the distance from the left side of the screen. The reason that LOCATE does not follow the standard coordinate system is that it is not necessary to include the X portion, you can use the format LOCATE Y which just specifies the line to start on.
LOCATE[row,column]
LOCATE[row]
5FACE.BAS
 CLS

 LOCATE 14, 34
 COLOR 9
 PRINT "<=>"

 LOCATE 14, 43
 PRINT "<=>"

 COLOR 11
 LOCATE 16, 39
 PRINT "o|o"

 COLOR 4
 LOCATE 18, 36
 PRINT "\_______/"

 COLOR 20
 LOCATE 19, 42
 PRINT "U"

 LOCATE 1, 1
 COLOR 16, 1
 PRINT "Hello World"
COLOR statement
The program 5FACE.BAS is broken into sections to make it easier to read. This is an example of a good programming habit. Each three line piece of code each piece of code specifies what color it's part of the face should be, where it should be and what it should look like. The order of the position and the color is unimportant. The new statement COLOR allows you to change the color of the text. Once changed, all output will be in the new color until COLOR or CLS is used.
COLOR [foreground]
COLOR [foreground],[background]
The colors are designated by numbers which will be discussed in the next section.
Color by Number
There are 16 colors (in screen mode 0), numbered from 0 to 15.
0
Black
8
Gray
1
Blue
9
Light Blue
2
Green
10
Light Green
3
Cyan
11
Light Cyan
4
Red
12
Light Red
5
Purple
13
Light Purple
6
Brown/Orange
14
Yellow (Light Orange)
7
Light Grey (White)
15
White (Light White)
If you look carefully at this chart you can see that there are 8 main colors (0 through 7) and then those colors repeat, each in a lighter shade. You may also notice that the colors act as a combination of binary values (where blue=1, green=2, red=4, etc.) This makes it much easier to memorize the color scheme. Blinking colors are also available: at 16, the colors start over again with blinking black and extend through 31 (blinking white). However, the blinking option is not available for the background, only for the text (foreground). Add 16 to the color you wish to blink. eg: 2+16=18 - Blinking Green, 4+16=20 - Blinking Red.
It is possible to switch the blinking foreground text with an intense background, but this task is beyond the scope of this QBasic textbook, and may not work when MS Windows displays the console in a windowed mode.
Summary
In this section we looked at several methods to manipulate text output. All centered around the PRINT statement. LOCATE and COLOR modified where the text was displayed and how it looked. We used CLS to clear the screen and gave a brief introduction to variables which will be expanded upon in later sections.
Basic Math
There are six numerical variables within QBasic:
Type
Minimum
Maximum
Integer
-32,768
32,767
Long Integer
-2,147,483,648
2,147,483,647
Float
-3.37x10^38
3.37x10^38
Double
-1.67x10^308
1.67x10^308
64-bit Integer
-9,223,372,036,854,775,808
9,223,372,036,854,775,807
64-bit Float
±1.18E−4932
±1.18E+4932
Please note that Integer and Float type variables for 64-bit are available only in QB64.
A lot of programming is math. Don't let this scare you: a lot of the math is simple, but it's still math. In this section, we will look at doing some basic math (the same stuff you learned in the 3rd grade) and manipulating numbers.
Equation Setup
In QBasic an equation has a basic setup a right side and a left side. For instance X=5, as you can probably figure out, this sets the variable X to 5. But we can use variables on the right side too. Y=X*10 would set Y equal to 10 times X, in this situation, 50. In this next program I will show several equations to give you a feel for math.
7MATH.BAS
 CLS

 'Set a-d to initial values
 a = 10
 b = 6
 c = 3.1415
 d = 3.333333

 e = a + b
 PRINT a; "+"; b; "="; e

 f = c * d
 PRINT c; "*"; d; "="; f

 g = b - c
 PRINT b; "-"; c; "="; g

 h = b / d
 PRINT b; "/"; d; "="; h

 i = INT(d)
 PRINT "Remove the decimal from "; d; "="; i
Understanding 7MATH.BAS
The most important thing you can take away from this is the setup for math equations. I think you can figure out what all the symbols are and what they do, but QBasic is picky about equations. For 'e=a+b', if you try 'a+b=e' it will not work. The final thing I would like to address in 7MATH.BAS is the INT() function. As far as vocabulary, a function is something that takes in a piece of information and gives you another piece of information back. So PRINT, was a statement, and INT() is a function. The INT() function takes a number and truncates it's decimal, it does not round. So INT(5.1) is 5 and INT(5.999) is still 5. If you want to round a number use CINT().
8MATH.BAS
 CLS
 INPUT "Enter a number: ", x
 PRINT

 x = x + 5
 PRINT "X is now: "; x

 x = x * x
 PRINT "X is now: "; x

 x = x / 5
 PRINT "X is now: "; x

 x = x - 4
 PRINT "X is now: "; x

 x = x / x
 PRINT "X should be 1: "; x
Understanding 8MATH.BAS
8MATH.BAS shows one simple concept that is very important in programming, but impossible in math. The way that the computer calculates the equation is it does all the math on the right side of the equation and then sticks it in the variable on the left side. So the equation x=x+5 makes perfect sense, unlike math where it is a contradiction. Reassigning a value to a variable based on its current value is common and a good way to keep the number of variables down.
9TIP.BAS
 CLS
 INPUT "How much is your bill: ", bill
 INPUT "What percent tip do you want to give: ", tip

 tip = tip / 100   'change percent to decimal
 tip = tip * bill  'change decimal to money

 PRINT
 PRINT "The tip is"; tip; "$."
 PRINT "Pay"; tip + bill; "$ total."
Tip Calculator
9TIP.BAS calculates your tip and total bill from the bill and percent tip you wish to give. The first three lines clear the screen and get the information from the user. The fifth line changes the tip from a percent to the correct decimal by dividing by 100 (ex. 20%=.2 because 20/100=.2) the next line takes that percent and turns it into a dollar value by multiplying the decimal value by the bill. So if your bill is $20.00 and you leave a 20% tip, it multiplies 20*.2 which is 4 or $4.00. The last three lines format the output.
This is a good example of a complete program. It collects information from the user, it processes the information and it gives the user feedback. Also, the middle section of the program is a good example of variable conservation. This is subject that will take some practice to get used to. In writing a program, if you use too many variables, it will become difficult to keep track of all of them. If you try and conserve too much, you code may become difficult to understand.
You may notice that the program may print more than two decimal places if you enter a bill that is not an exact dollar value. As an exercise, try modifying the program so that it only displays two decimal places - you can use the CINT() function or any other rounding method you intend to use.
10OROP.BAS
 'ORder of OPerations
 CLS
 a = 15
 b = 10
 c = 12.2
 d = 1.618

 PRINT a * b + c   'these two are different
 PRINT a * (b + c)

 PRINT

 PRINT b - c / d   'these two are different
 PRINT (b - c) / d

 PRINT

 PRINT a * b - c * d / a + d   'these two are the same
 PRINT (a * b) - ((c * d) / a) + d
Parentheses and Order of Operations
10OROP.BAS is an example of order of operations and how parentheses can be used to manipulate it. I do not want to go into an indepth explanation of the order of operations here. The best advice I can give is unless you are sure of the order of operations, use parentheses to make sure the equation works how you want. All you need to know about parentheses is that the deepest nested parentheses calculate first. If you wish to know more, there are plenty of algebra resources available. On that note, you may wish to brush up on algebra. While it is not necessary for programming, it can help make programming easier and it can allow you to create more advanced programs.
Random Numbers
Though we will not go into their use until the next section, I would like to discuss the generation of random numbers. QBasic has a random number statement, RND, that generates a random decimal between 0 and 1. You can think of it as a random percent. At first, this may seem like an odd way to generate random numbers. However, with a little math it is very easy to manipulate this to provide numbers in whatever range you want.
The first step is to multiply RND by a number (the range you want). For instance 'RND*10'. This will return random numbers (decimal numbers) between 0 and 10(both included). So, to pick a random number between zero and ten we would say '(RND*10)'
11RND.BAS
 CLS
 RANDOMIZE TIMER

 PRINT "Random number from 0-9:"; RND * 10
 PRINT

 PRINT "Random number from 1-10:"; (RND * 10) + 1
 PRINT

 PRINT "Random integer from 1-10:"; INT(RND * 10) + 1
 PRINT

 PRINT "Random even integer from 50-100:"; INT(RND * 25) * 2 + 50
More on RND
A few notes on 11RND.BAS, the second line, RANDOMIZE TIMER, sets it so that the computer uses the current time to pick random number. If you don't do this, it picks the same random number every time (try it, write a one line program, PRINT RND, and run it over and over, your screen will fill up with the same number) this can prove useful for some applications, but not most. Stick RANDOMIZE TIMER in at the top of all your programs that use the RND statement and they will be far less predictable. This program just show some ways to choose what you want from your random number generator. The last line shows that you can be very specific in what you get. Make sure to run this program several times to see the different results.
Flow Control
Conditional execution
To choose between two or more sections of the program to execute, the IF statement can be used. It is also possible to use the WHILE, DO UNTIL and CASE statements. All of these control conditional execution by using a Boolean logic 'test', the result of which is either TRUE or FALSE. To repeat a section of code for a set number of times, the FOR statement is used.
The IF test can be executed in a single line, however it can also be used like the others to control a block of code.
True or False
Boolean logic is a test that yields one of only two possible results, true or false. The tests are always mathematical in nature .. when two characters (or strings) are 'compared' it is their ASCII codes that are used (thus a < b and b < A).
The comparison operators used in qbasic are: = true if two variables are equal< true if the first is less than the second =< true if the first is less than or equal to the second> true if the first is greater than the second> = true if the first is greater than or equal to the second<> true if the two are unequal
Multiple tests can be linked together in the comparison, using the 'AND', 'OR' and 'NOT' operators. We will cover exactly what these mean later on, but you probably understand the first two already.
IF
One of the most useful statements in QBasic is the IF statement. It allows you to choose what your program will do depending on the conditions you give it. The next few programs will be taking a look at ways to use the IF statement.
IF [conditional] THEN [do this]
The single line IF is the simplest example. To execute a block of code, the END IF is used
 IF [conditional] THEN
   [do this]
   [and do this]
    ...
   [and also do this]
 END IF
IF...THEN...ELSE
IF [conditional] THEN [do this] ELSE [do that]
To choose between two different code blocks, the ELSE statement is used.
IF [conditional] THEN
  [do this]
  ..
  [and do this]
ELSE
  [do that]
  ..
  [and also that]
END IF
13 ELSEIF
As an alternative to starting an entirely new IF THEN ELSE statement sequence. You can follow the THEN statement(s) with ELSEIF [conditional] THEN. This does not create a new level of nesting.
IF [conditional] THEN
 [do this]
 ..
 [and do this]
ELSEIF [conditional] THEN
 [do that]
 ..
 [and also that]
ELSEIF [conditional] THEN
 [do the other]
 ..
 [and also ...]
ELSE
 [do this final thing]
END IF
FOR...NEXT
FOR <variable name> = <start value> TO <end value> [STEP <increment>]
 [do this]
 ...
 [and do this]
NEXT
<increment> may be + or - and is optional. If omitted the default is +1. The code contained within the FOR loop will always be executed at least once because it is only at the 'NEXT' statement that the value of the variable is checked against the end value.
When the NEXT statement executes, the variable is modified by STEP value and compared against the end value. If the variable has not yet exceeded the end value, control is returned to the line following the FOR.
You can exit a FOR loop early with the EXIT FOR command.
14FOR.BAS
 CLS
 RANDOMIZE TIMER

 num = INT(RND * 20) + 1

 FOR count = 1 TO 5
  INPUT "Pick a number between 1 and 20: ", answer
  IF answer = num THEN PRINT "You win after";count;"guesses!": END
 NEXT
 PRINT "You lose"
WHILE...WEND
 WHILE <condition is true>
   [do this]
   ..
   [and this]
 WEND
If the condition is true, the code following the WHILE is executed. When the WEND command is executed, it returns control to the WHILE statement (where the condition is tested again). When the condition evaluates to FALSE, control is passed to the statement following the WEND.
15WHILE.BAS
PRINT "Press any key to continue"
WHILE INKEY$=""
WEND
In the example above, you see a press any key prompt that waits until the user presses a key. (The INKEY$ feature will be described under Advanced Input.)

DO...LOOP
 DO
   [this]
   ..
   [and this]
 LOOP WHILE <condition is true> / LOOP UNTIL <condition is true>
The DO...LOOP construct is a more advanced of the WHILE loop - as with other flow control blocks, it is marked by DO and LOOP to denote the boundaries.
It relies on a conditional statement placed after either DO or LOOP:
DO
  a$ = inkey$
LOOP WHILE a$=""
As an alternative, you can instead replace WHILE with UNTIL have the loop continue until a specific condition is met:
DO
  x=x+1
LOOP UNTIL x >= 10
In some versions of BASIC the UNTIL or WHILE condition can follow the DO statement rather than the LOOP statement (pre-test) as apposed to the above shown (post-test).
12IF.BAS
 CLS
 RANDOMIZE TIMER

 num = INT(RND * 100) + 1
 DO
      INPUT "Pick a number between 1 and 100: ", answer

      IF num = answer THEN PRINT "You Got It!"
      IF num > answer THEN PRINT "Too Small"
      IF num < answer THEN PRINT "Too big"
LOOP UNTIL num = answer
PRINT "Game Over."
SELECT CASE
 SELECT CASE <variable expression>
   CASE <value>
     [do this]
   CASE <value 2>
     [do instead]
   ...
   CASE ELSE
   ...
 END SELECT
The select statement is a substitute for repeated use of IF statements. The <variable expression> is evaluated and compared against each CASE <value> in turn. When a CASE <value> is found to match, the [do this] code following is executed. If an EXIT CASE is executed, control passes to the line following the END SELECT, otherwise the next CASE <value> is checked. If no matches are found, the CASE ELSE is executed. Note that <value> may be a number, character or string or logical expression (eg '>0', '<>1'). Note also that multiple CASE matches may be found and executed (so, for example, if two CASE <values> are 'CASE >1' and 'CASE >10', a <variable expression> that evaluates to 11 (or more) will result in both CASE >1 and CASE >10 being executed)
CLS
PRINT "WELCOME"
PRINT "I HAVE AN ANSWER FOR ANY OF YOUR QUESTIONS"
10 INPUT "WRITE YOUR QUESTION AND I'LL GIVE YOU AN ANSWER ", question$
RANDOMIZE TIMER
PRINT
answer = INT(RND * 4 + 1)
SELECT CASE answer
    CASE 1
        PRINT "PLEASE REPHRASE YOUR QUESTION."
    CASE 2
        PRINT "YOUR QUESTION IS MEANINGLESS."
    CASE 3
        PRINT "DO YOU THINK I CAN ANSWER THIS?"
    CASE 4
        PRINT "THIS QUESTION LOOKS FUNNY."
END SELECT
PRINT
PRINT "ENTER ANOTHER QUESTION", K$
GOTO 10
If a parameter would be covered by more than one case statement, the first option will take priority.
Advanced Input
INKEY$
Getting real time information from the user is a little more difficult. To do so, we will use the INKEY$ command, which checks to see if a user typed a key and provides the keypress to the program.
Look at this code and then we will look at it in depth:
 DO
   LET k$ = INKEY$
 LOOP UNTIL k$ <> ""
 SELECT CASE k$
   CASE "q"
     QuitProgram
   CASE "c"
     MakeCircle
   CASE "s"
     MakeSquare
 END SELECT
The first part is the DO-LOOP which constantly polls INKEY$ for a return value. In the basic use, INKEY$ returns an empty string if no keys are being pressed and continues with the program. Once a key is pressed, INKEY$ will return that key immediately.
The keyboard buffer
What is INKEY$ doing and how does it work?
While the INKEY$ command looks like it returns the key currently being pressed, this is not the case. It is used by the program to answer the question, "What is IN the KEYboard buffer?" To understand this you will need to understand what a basic buffer is and why it is used.
In older systems (not necessarily the IBM PC) a single chip processed keyboard input, and controlled the LED lights for caps lock and number lock. Because a computer does many things at once (e.g., take input from the mouse, crunch numbers, call subroutines, display new information on the screen), it needs to be able to remember what was pressed on the keyboard while it is busy. This chip contained some memory (called a buffer) that allow keeping track of a limited number of keypresses.
Within the Dos platform under IBM PCs, the hardware has changed slightly. Instead of a hardware buffer, pressing or releasing a key will interrupt the running program to add a keystroke to a software buffer located in the BIOS. This procedure is usually unnoticed by the user and has minimal impact on system performance. However, this buffer allows for 15 characters - attempting to overflow it when the computer is busy will cause a short beep and drop any further characters.
The INKEY$ command uses this buffer as a FIFO (First In First Out) buffer. As an example let's say you have a game that has a bouncing ball on the screen and a paddle at the bottom. The computer program constantly has to update the screen to show the movement of the ball. While it does this the program passes by an INKEY$ command to see what value is returned. If the user has pressed a key since the last time the command was invoked it will return that key. Let's say the ball is moving over to the right and the user needs to press the "R" key to tell the program to move the paddle right. Since the program is busy moving the ball and updating the screen, it does not instantaneously notice that the user has pressed the key. Instead, the key press is stored in the keyboard buffer, and retrieved a few milliseconds (or microseconds) later when the INKEY$ command is used.
In many programs (as above), INKEY$ will appear nested in a loop. It is requested over and over again. This allows the program to get user input one character at a time. Using our example above, the user may need to press R over and over again until the paddle is under the ball. On the other hand, the user may press R too many times and need to press L to move it left. Because the INKEY$ command is using a FIFO buffer it will always retrieve the keys pressed in the same order as they were typed.
In summary, the INKEY$ command will always return and remove the first character in the keyboard buffer. Generally speaking, it is used over and over to retrieve every key that has been pressed, and to allow a user to interact with a program in a close approximation to "real time." If there is no key in the keyboard buffer, INKEY$ it will return an empty string (no character).
Scancodes
Some keypresses are not associated with an ASCII character. When one of these keys is pressed, INKEY$ returns a string with two characters = the first character is a null (ASCII code 0), and the second is the raw scan code for the keyboard. A full listing of the scancodes can be found within the QBASIC help file - you can also determine the scan codes by examining the results of INKEY$ as you press those keys in question.
Note:
Some keys cannot be directly detected by INKEY$.
Subroutines and Functions
Purpose
Subroutines and functions are ways to break up your code into reusable 'lumps'. They allow the programmer reuse a large set of common instructions just by calling the appropriate procedure or function.
For example, lets say you need to PRINT multiple Tables of values. One way to do this is to just enter all the Table PRINT commands directly into where you need them. However this not only makes the program very large but also makes it harder to debug or change the 'style' of the table. A simpler way is to create a single 'Print Table' procedure and enter all of the PRINT commands there. Then, each time need to print a Table, you would simply 'call' the 'Print Table' procedure with a list of the values to be printed.
Procedure vs. Function
A procedure does something and does not return anything for the programmer. For example, a procedure might be used to set the screen mode and palette.
A function does something and RETURNS a value. For example, if you need to find the average of two values, you might write a function that takes in two numbers and returns the average.
GOTO and GOSUB
The GOTO and GOSUB statements were the original methods by which functions were created. They were the most common on older basic implementations and are kept around for compatibility reasons; however, their use is not recommended in other programming languages or in large scale projects, both because GOTO's make it harder to 'follow' the program flow and because GOSUB's do not 'isolate' the changes made to any variables.
These two commands depend on Labels, which come in one of two forms. The first and older form involves writing line numbers at the beginning of each line (usually in increments of 10). The newer method looks similar to other programming languages, which is a single word followed by a colon.
The GOTO statement is simple; it just moves the execution point to a given Label:
The GOSUB statement transfers control to a given Label, however whan a RETURN statement is encountered, execution returns to the line following the GOSUB statement. Any changes made within the GOSUB will be to actual variables used within the 'main' code.
ON ERROR
The ON ERROR allows you to define an error handler for your program; when an error occurs, it immediately jumps to the given label. The control returns once the program reaches a RESUME statement, which can either return control to the same point, the next statement, or any other desired label.
Within Qbasic, the error handler cannot be located within any subroutines. As such, any error checking or flags will have to be handled through the use of variables that are shared with the main module.
Note:
While the QBasic documentation states ON ERROR RESUME NEXT is a valid statement, this is incorrect.
NOTE: If your error handling routine does not have a "resume" statement in it (IOW you try to do it all with gotos) error handling will only work once - the next "on error" will be ignored and the program ends as if you had no "on error" statement at all. This problem does not seem to be mentioned in any of the documentation. It took me three hours to figure out why two nearly identical program portions acted so differently.
Declaring a subroutine
A superior method of declaring a subroutine is using the SUB statement block, because (by default) any new variables used within the subroutine are discarded on exit.
Under the QBasic IDE, doing so moves the SUB block to its own section in the window to prevent accidental deletion of the module, and allows the easier organization of the program code.
Calling a subroutine is as simple as writing the name of the subroutine (passing any required parameters). If you want, you can use the CALL statement to indicate to other programmers that it is a subroutine.
SUB name (params)
{SHARED variables 'if any}
'{code to execute}
'  ...
'  ...
{STATIC variables 'if any, to be saved for use next time}
END SUB
Whilst the Parameters passed into subroutines are passed by 'reference' (i.e. they take on a new name within the SUB), any changes that are made to the values are 'reflected back' into the originals. By default, all other variables used within the SUB are discarded when the END SUB is reached (or an EXIT SUB is executed), except as below :-
To 'preserve' the values of variables used within the SUB for re-use on the next CALL, use the STATIC keyword at the end.
If you need access to a variable (that has not been passed as a parameter), use the SHARED keyword to define each at the start of the subroutine (a SHARED variable retains its name).
Declaring a function
A function is a form of subroutine that returns a value. Everything that applies in defining a subroutine also applies to a function. Within the function, the return value is created by using the function name as a variable - the return value is then passed to the calling expression.
FUNCTION name (params)
  ' Shared variable declarations
  name = result
  ' ...
END FUNCTION
Functions are declared in the same way as variables - it returns the variable type it's defined to return, in the same way variables are defined to contain their specified type. By default, it is a number, but appending a dollar sign indicates that it is returning a string.
Functions can only be called within an expression; unlike subroutines, they are not a standalone statement.
Arrays and Types
Q basic is an IDE(integrated development environment) developed by Microsoft to create, edit, debug and execute basic program.
Built-in Types
QBasic has five built-in types: INTEGER (%), LONG(&) integer, SINGLE(!) float, DOUBLE(#) float and STRING($). QB64 has two more built-in types: _INTEGER64 (&&) and _FLOAT (##)
Implicit declaration is by adding the type character to the end of the variable name (%, &, !, #, $, &&, ##). See QBasic/Basic math for more.
Explict declaration is by using the DIM statement before first use:
DIM a AS STRING
DIM b AS INTEGER
DIM c AS LONG
DIM d AS SINGLE
DIM e AS DOUBLE
DIM f AS _INTEGER64 'QB64 only
DIM g AS _FLOAT 'QB64 only
If you do not use either implicit or explicit declaration, QBASIC interpreter assumes SINGLE type.
User-defined type
To do:
Make this example more suitable for textbook usage.
A user defined type allows you to create your own data structures. Please note that custom types are similar to arrays.
 TYPE playertype
  name AS STRING
  score AS INTEGER
 END TYPE
You can then declare variables under this type, and access them:
DIM playername AS playertype
playername.name = "Bob"
playername.score = 92
This above example shows how a custom type can be used for maintaining data, say on a player who plays a game.
Array
An array is a collection of values stored in a single variable. A STRING is an array of characters (so, for example, char$(1) means 1st character in string char$). Arrays of numbers should be defined using the DIM instruction (unless you DIM them, they are limited to 10 elements on each dimension).
By default, arrays in QBasic are static in size and cannot be changed later in the program. Code that will set up this type of array is as follows:
DIM myArray(10) as TYPE 'this is explaining the datatype to be used during program execution in array'
TYPE can be any of the built in QBasic (INTEGER, LONG, SINGLE, DOUBLE, STRING) or user-defined type. If this is not specified, the array takes the Type defined by the variable name suffix - INTEGER (%), LONG(&) integer, SINGLE(!) float, DOUBLE(#), STRING($) - or INTEGER if none.
WARNING: If your data Type is string, DIM string(10) defines a SINGLE string of 10 characters, NOT 10 strings of arbitary length ! (10 strings of up to 128 chars each would be defined as DIM string(10,128)
By issuing the Meta Command '$DYNAMIC at the beginning of your program you can cause your arrays to be dynamic:
 ' $DYNAMIC
 DIM myDynamicArray(5) as INTEGER
 REDIM myDynamicArray(10) as INTEGER
This is now perfectly legal code.
To free up space occupied by an array, use the ERASE statement.
Multidimensional array
An array isn't restricted to one dimension - it's possible to declare an array to accept two parameters in order to represent a grid of values.
 DIM housenames(25,25) as STRING
You cannot use the REDIM statement to change the number of dimensions on the array, even with dynamic allocation.
Non-zero base
In most languages, arrays start at the value 0, and count up. In basic, it's possible to index arrays so that they start at any value, and finish at any other value.
 DIM deltas(-5 TO 5)
You can change the default lower bound with the OPTION BASE statement.
Appendix
Q Basic uses REM to insert descriptions and comments in source program
Commands
ABS()
N = ABS(expression returning a numerical value)
Returns the 'absolute' value of the expression, turning a negative to a positive (e.g. -4 to 4)
PRINT ABS(54.345) 'This will print the value ABS now as it is (54.345)
PRINT ABS(-43)    'This will print the value as (43)
.
ACCESS
OPEN "file.txt" FOR APPEND ACCESS WRITE
This sets the access of a file that has been declared into the program. There are three settings that the programmer can set. These are:
READ - Sets up the file to be read only, no writing.
WRITE - Writes only to the file. Cannot be read.
READ WRITE - Sets the file to both of the settings above.
APPEND means 'add' to the 'end of' the existing "file.txt". If omitted, any existing "file.txt" is over-written without warning
ASC()
PRINT ASC("t")  'Will print 116
Prints the ASCII code number of the character found within the brackets. If the programmer put in a string into brackets, only the first character of the string will be shown.
ATN()
ATN(expression)
Part of the inbuilt trigonometry functions. An expression that evaluates to a numeric vale is converted to it's Arc-Tangent.
angle = ATN( B )
angle2 = ATN( 23 / 34 )
BEEP
BEEP
The BIOS on the motherboard is instructed to emit a "Beep" sound from the PC 'speaker'. See also SOUND and PLAY.
CASE
SELECT CASE [variable]
CASE [value]: [command]
CASE ELSE: [command]
END SELECT
Use this when using multiple values in your program and assigning them separate paths. This is an example of a program with no CASE commands that assigns different paths to values:
10 PRINT "1. Print 'path'"
PRINT "2. Print 'hello"
PRINT "3. Quit"
INPUT "Enter a choice: "; a$
IF a$ = "1" THEN
PRINT "path"
GOTO 10
IF a$ = "2" THEN
PRINT "hello"
GOTO 10
IF a$ = "3" THEN END
PRINT "That is not a valid choice"
GOTO 10
This is what a program looks like with the CASE command:
PRINT "1. Print 'path'"
PRINT "2. Print 'Hello'"
PRINT "3. Quit"
INPUT "Enter a choice: "; a$
SELECT CASE a$
CASE "1":
PRINT "path"
CASE "2":
PRINT "Hello"
CASE "3":
END
CASE ELSE:
PRINT "That is not a valid choice"
END SELECT
CHAIN
CHAIN {filename.format (parameters list)}
This 'calls' (transfers execution) to another program (typically another .bas file, a command (.bat or .cmd) script or an executable (.com, .exe) ).
Values may be passed directly in the invoking command (to files accepting command line parameters). Values may be passed to another basic script by using the 'COMMON' statement before the CHAIN command.
Control returns to the basic interpreter when the 'chained' program terminates.
CHDIR
CHDIR [directory name]
This is used for changing a directory. Used in conjunction with MKDIR and RMDIR. The directory name is declared exactly like in DOS PROMPT. For example:
CHDIR "c:/Program Files/QBasic_FIles/Testing_Files"
To use this command, the programmer will have to know how MS DOS and COMMAND PROMPT work.
CHR$()
This returns the string character symbol of an ASCII code value.
name$ = CHR$([ascii character code])
Often used to 'load' characters into string variables when that character cannot be typed (e.g. the Esc key or the F{n} (Function Keys) or characters that would be 'recognised' and acted upon by the QBASIC interpreter (e.g. ", the double quote). Here is a list of some character codes :-
07 Beep (same as BEEP)
08 Backspace
09 Tab
27 Esc
34 Double quote
72 Up Arrow
75 Left Arrow
77 Right Arrow
80 Down Arrow
CINT()
This rounds the contents of the brackets to the nearest integer.
PRINT CINT(4573.73994596)
4574
CIRCLE
CIRCLE ([X Coordinate], [Y Coordinate]), [Radius], [Colour],[Start],[End],[Aspect]
Lets the programmer display a circle. Like all graphics commands, it must be used with the SCREEN command.
CLEAR
CLEAR
Resets all variables, strings, arrays and closes all files. The reset command on QBasic.
CLOSE
CLOSE
Closes all open files
CLOSE #2
Closes only the file opened as data stream 2. Other files remain open
CLS
CLS
Clears the active screen. Erases all text, graphics, resets the cursor to the upper left (1,1), and also applies the current background color (this has to be set using the COLOR command) to the whole screen.
COLOR
COLOR [Text Colour], [Background Colour]
This lets you change the colour of the text and background used when next 'printing' to the current output window. It can be done like this:
COLOR 14, 01
PRINT "Yellow on Blue"
You have a choice of sixteen colours:
00: Black            08: Dark Grey
01: Dark Blue        09: Light Blue
02: Dark Green       10: Light Green
03: Dark Cyan        11: Light Cyan
04: Dark Red         12: Light Red
05: Dark Purple      13: Magenta
06: Orange Brown     14: Yellow
07: Grey             15: White
These values are the numbers that you put in the COLOR command.
Note Only screen modes 0, 7, 8, 9, 10 support a background color. To 're-paint' the whole screen in a background colour, use the CLS command.
COMMON
Declares a variable as 'global', which allows its value to be accessed across multiple QBasic programs / scripts (see also the CHAIN command)
COMMON SHARED [variablename]
Each program that declares 'variablename' as COMMON will share the same value.
NOTE. All COMMON statements must appear at the start of the program (i.e. before any executable statements).
CONST
Fixes a variable so it can not be changed within the program.
CONST (name) {AS (type = INTEGER / STRING)} = (expression or value)
For example :-
CONST PI = 3.14159265
Assigns the value 3.14159265 to PI.
CONST PI2 = 2 * PI
PI must be assigned a value before it is used to calculate PI2. Typically all CONST are declared at the beginning of a program.
DATA
DATA [constant]
Use in conjunction with the READ and RESTORE command. Mostly used in programs dealing with graphics, this command lets QBasic read a large number of constants. The READ command accesses the data while the RESTORE command "refreshes" the data, allowing it to be used again.
DATE$
A system variable that always contains the current date as a string in mm-dd-yyyy format. Use it like this:
a$ = DATE$
====DIM====kko This is used to declare an array (early versions of QBasic required all variables to be defined, not just arrays greater than 10)
DIM [Array Name] ([count],[count], ..)
The Array name can be of any type (Integer, Double, String etc). If not declared, single precision floating point is assumed. Strings can be 'declared' using $ sign (Integers with the '%' sign). The QBASIC interpreter tolerates numeric arrays of up to 10 count without these needing to be declared.
NOTE Early versions of QBasic did not explicitly set the contents of an array to zero (see CLEAR command)
DIM table%(100,2)
Create an integer array called table% containing 100x2 = 200 entries
DIM form$(5)
Create a string array called form$ containing 5 strings
DIM quotient(20) AS DOUBLE
Create an array called quotient that contains 20 double precision numbers
DO .. LOOP
DO
[program]
LOOP UNTIL [test condition becomes TRUE]
Used to create a loop in the program. The [condition] is tested only after the [program] code is executed for the first time (see also WHILE). For example:
num$ = 1
sum$ = 0
DO
sum$ = 2 * num$
PRINT sum$
num$ = num$ + 1
LOOP UNTIL num$ = 13
This does not work But the following does
num = 1
sum = 0
DO
sum = 2 * num
PRINT sum
num = num + 1
LOOP UNTIL num = 13
DRAW
DRAW "[string expression]"
Used to draw a straight line from the current 'cursor' position in the current colour. DRAW defines the direction (up, down etc.) and the length of the line (in pixels). For example:-
SCREEN 7
PSET (50, 50), 4
DRAW "u50 r50 d50 l50"

The letter in front of each number is the direction:
U = Up    E = Upper-right
D = Down  F = Lower-right
L = Left  G = Lower-left
R = Right H = Upper-left
The drawing 'cursor' is left at the position where the line ends. u50 draws from 50,50 upwards ending at 50,0 r50 draws from 50,0 to the right, ending at 100,0 d50 draws from 100,0 downwards, ending at 100,50 l50 draws from 100,50 to the left, ending at 50,50
The example shown will thus draw a red 'wire frame' square.
See also LINE and CIRCLE commands.
Note: Pixels are square. The diagonal from 0,0 to 100,100 will be 100 * root(2) pixels long (i.e. 141)
END
END
Signifies the end of the program. When QBasic sees this command it usually comes up with a statement saying: "Press Any Key to Continue".
END IF
END IF
Ends the program if a condition is reached.
ENVIRON
ENVIRON [string expression]
NOTE: If you are running QBasic on a Windows system, you will not be able to use this command.
This command helps you set an environment variable for the duration of the session. On exit from the QBasic.exe interpreter, the variables revert to their original values.
EOF()
This checks if there are still more data values to be read from the file specified in (). EOF() returns a boolean / binary value, a one or zero. 0 if the end of file has not been reached, 1 if the last value in the file has been read (see also LINE INPUT)
OPEN File.txt FOR INPUT AS #1
DO
INPUT #1, text$
PRINT text$
LOOP UNTIL EOF(1)
END
Note that, since the INPUT is executed before UNTIL is reached, File.txt must contain at least one line of text - if the file is empty, you will receive an 'ERROR (62) Input past end of file'.
ERASE
ERASE [arrayname] [,]
Used to erase all dimensioned arrays.
ERROR
System variable holding a numeric value relating to the processing of the previous line of code. If the line completed without error, ERROR is set to 0. If the line failed, ERROR is set to one of the values shown below. Most commonly used to redirect program flow to error handling code as in :-
ON ERROR GOTO [line number / label]
If ERROR is non=zero, program flow jumps to the line number or label specified. If ERROR is zero, program flow continues with the next line below.
To manually test your program and check to see if the error handling routine runs OK, ERROR can be set manually :-
ERROR [number]
Set ERROR = number
The error numbers are as follows:
1 NEXT without FOR                 39 CASE ELSE expected
2 Syntax Error                     40 Variable required
3 RETURN without GOSUB             50 FIELD overflow
4 Out of DATA                      51 Internal error
5 Illegal function call            52 Bad file name or number
6 Overflow                         53 File not found
7 Out of memory                    54 Bad file mode
8 Label not defined                55 File already open
9 Subscript out of range           56 FIELD statement active
10 Duplicate definition            57 Device I/O error
11 Division by zero                58 File already exists
12 Illegal in direct mode          59 Bad record length
13 Type mismatch                   61 Disk full
14 Out of string space             62 Input past end of file
16 String formula too complex      63 Bad record number
17 Cannot continue                 64 Bad file name
18 Function not defined            67 Too many files
19 Yes RESUME                       68 Device unavailable
20 RESUME without error            69 Communication-buffer overflow
24 Device timeout                  70 Permission denied
25 Device Fault                    71 Disk not ready
26 FOR without NEXT                72 Disk-media error
27 Out of paper                    73 Advanced feature unavailable
29 WHILE without WEND              74 Rename across disks
30 WEND without WHILE              75 Path/File access error
33 Duplicate label                 76 Path not found
35 Subprogram not defined
37 Argument-count mismatch
38 Array not defined
Note that ERROR is set when execution fails, not when the code is 'read' - so, for example, a 'divide by 0' will be found before the result is assigned to a non-existent array variable or written to a non-existent file.
EXIT
Allows the immediate exit from a subroutine or a loop, without processing the rest of that subroutine or loop code
EXIT DEF
Exits from a DEF FN function.
EXIT DO
Exits from a DO loop, execution continues with the command directly after the LOOP command
EXIT FOR
Exits from a FOR loop, execution continues with the command directly after the NEXT command
EXIT FUNCTION
Exits a FUNCTION procedure, execution continues with the command directly after the function call
EXIT SUB
Exits a SUB procedure.
FOR .. NEXT
FOR [variable name] = [start value] TO [end value] {STEP n}
[program  code]
NEXT [variable name]
The variable is set to the [start value], then program code is executed and at the Next statement the variable is incremented by 1 (or by the STEP value, if any is specified). The resulting value is compared to the [end value] and if not equal program flow returns to the line following the FOR statement.
For example:
FOR a = 200 TO 197 STEP-1
PRINT a
NEXT a
200 199 198
Care must be taken when using STEP, since it is quite possible to step past the (end value) with the result that the FOR loop will run 'for ever' (i.e. until the user aborts the interpreter or an error occurs), for example :-
FOR a = 200 TO 197 STEP-2
PRINT a
NEXT a
200 198 196 194 192 ... 0 -2 -4 ... -32768 ERROR overflow
GOSUB
GOSUB [subroutine line number / label]
Command processing jumps to the subroutine specified. When the RETURN command is encountered, processing returns to this point and continues with the line below the GOSUB.
IF
IF [variable or string] [operator] [variable or string] THEN [command] {ELSE [command]}
Compares variables or strings. For example, if you wanted to examine whether or not a user-entered password was the correct password, you might enter:
IF a$ = "password" THEN PRINT "Password Correct"
Where a$ is the user entered password. Some operators include:
"="- equal to
"<"- less than (only used when variable or string is a number value)
">"- greater than (only used when variable or string is a number value)
"<>"- does not equal
"<="- less than or equal to (only used when variable or string is a number value)
">="- greater than or equal to (only used when variable or string is a number value)
One can also preform actions to number values then compare them to other strings or variables using the if command, such as in the below examples:
IF a+5 = 15 THEN PRINT "Correct"
IF a*6 = b*8 THEN PRINT "Correct"
INCLUDE (QUICKbasic Only)
QUICKBasic supports the use of include files via the $INCLUDE directive:
(Note that the Qbasic interpreter does NOT support this command.)
'$INCLUDE: 'foobar.bi'
Note that the include directive is prefixed with an apostrophe, dollar, and that the name of the file for inclusion is enclosed in single quotation mark symbols.
INKEY$
[variable] = INKEY$
This is used when you want a program to function with key input from the keyboard. Look at this example on how this works:
a$ = INKEY$
PRINT "Press Esc to Exit"
END IF a$ = CHR$(27)
You can use this in conjunction with the CHR$ command or type the letter (e.g. A).
INPUT
INPUT [String Literal] [,or;] [Variable]
Displays the String Literal, if a semi colon follows the string literal, a question mark is displayed, and the users input until they hit return is entered into the variable. The variable can be a string or numeric. If a user attempts to enter a string for a numeric variable, the program will ask for the input again. The String Literal is option. If the string literal is used, a comma (,) or semicolon (;) is necessary.
INPUT #
INPUT #n [String Literal] [,or;] [Variable]
Reads a string / value from the specified file stream (see also LINE INPUT #)
INPUT #1, a$, b$, n, m
Reads 4 values from the file that is OPEN as #1. a$ is assigned all text until a ',' (comma) or end of line is reached, b$ the next segment of text, then two numeric values are interpreted and assigned to n and m.
Note that, within the file, numbers can be separated by 'anything' - so, if a number is not found (for 'n' or 'm') on the current 'line' of the file, the rest of the file will be searched until a number is found. Input is then left 'pointing' at the position in the file after the last number required to satisfy the input statement (see also 'seek #' command)
INSTR
INSTR (start%, Search$, Find$)
Returns the character position of the start of the first occurrance of Find$ within Search$, starting at character position 'start%' in Search$. If Find$ is not found, 0 is returned. start% is optional (default = 1, the first character of Search$)
Pos = INSTR ("abcdefghi", "de")
returns 4
LEFT$()
A$ = LEFT$(B$,N)
A$ is set to the N left most characters of B$.
A$ =  LEFT$("Get the start only",6)
returns "Get th"
See also RIGHT$(), MID$().
LET
LET [variable] = [value]
Early versions of the QBasic.exe command interpreter required use of the 'LET' command to assign values to variables. Later versions did not.
LET N = 227 / 99
LET A$="a line of simple text"
is equliavent to :-
N = 227 / 99
A$="a line of simple text"
LINE
LINE ([X], [Y]) - ([X], [Y]), [Colour Number]
Used for drawing lines in QBasic. The first X and Y are used as coordinates for the beginning of the line and the second set are used for coordinating were the end of the line is. You must put a SCREEN command at the beginning of the program to make it work.
Note. When in SCREEN 13, the Colour Number == the Palette number
LINE INPUT #
LINE INPUT #1, a$
Reads a complete line as text characters from the file OPEN as stream #1 and places it in a$.
To find the 'end of line', the QBasic interpreter seaches for the 'Carriage Return' + 'Line Feed' (0x0D, 0x0A) characters. When reading text files created on UNIX/LINUX systems (where the 'Line feed' 0x0A is used on it's own to signify 'end of line'), LINE INPUT will not recognise the 'end of line' and will continue to input until the end of file is reached. For files exceeding 2k characters, the result is an "Out of String Space" Error as a$ 'overflows'. One solution is to use a text editor able to handle UNIX files to open and 'save as' before attempting to process the file using QBasic.
LOADIMAGE (QB64 Only)
(NOTE! The commands in this section refer to a third-party program called "QB64". Neither QUICKbasic nor Qbasic support _LOADIMAGE, _NEWIMAGE, OR _PUTIMAGE commands. Both QUICKbasic and Qbasic have a "SCREEN" command, but it works diffently in those languages than in QB64.)
_LOADIMAGE("image.jpg")
Shows an image. Must be used with the commands SCREEN, _NEWIMAGE and _PUTIMAGE.
Example:
DIM rabbit AS LONG SCREEN _NEWIMAGE(800, 600, 32) rabbit = _LOADIMAGE("rabbit.jpg") _PUTIMAGE (100,100), rabbit
LOOP
DO
[Program]
LOOP UNTIL [condition]
Used to create a loop in the program. This command checks the condition after the loop has started. This is used in conjunction with the DO command.
LPRINT
LPRINT [statement or variable]
Prints out text to a printer. The LPRINT command expects a printer to be connected to the LPT1(PRN) port. If a printer is not connected to LPT1, QBasic displays a "Device fault" error message.
If your printer is connected to a COM port instead, use the MS-DOS MODE command to redirect printing from LPT1 to COMx (for example, to redirect to COM1, use the following command:
   MODE LPT1=COM1
If you need to cancel the redirection when finished, use the following command:
   MODE LPT1
MID$
 a$=MID$(string$,start%[,length%])
 MID$(string$,start%[,length%])=b$
In the first use, a$ is set to the substring taken from string$ strating with character start% taking Length% characters. If length% is omitted, the rest of the line (i.e. start% and all the characters to the right) are taken.
In the second use, length% characters of string$ are replaced by b$ starting at start%. If length% is omitted, the rest of the line is replaced (i.e. start% and all the characters to the right)
See also LEFT$ RIGHT$ LEN
MOD
 a MOD b
Returns the remainder of an integer divide of a by b
For example, 10 MOD 3 returns 1
OPEN
OPEN "[(path)\8.3 file name.ext]" (FOR {INPUT/OUTPUT} AS #{n})
This opens a file. You have to give the DOS file name, for example:
OPEN "data.txt" FOR INPUT AS #1
Opens the existing file data.txt for reading as data stream #1. Since no path is specified, the file must be in the same folder as the QBasic.exe - if not, processing halts with a 'file not found' error
OPEN "C:\TEMP\RUN.LOG" FOR OUTPUT AS #2
Opens an empty file named RUN.LOG in the C:\TEMP folder for writing data stream #2. Any existing file of the same name is replaced.
PALETTE
PALETTE[palette number, required colour]
For VGA (SCREEN mode 13) only, sets the Palette entry to a new RGB color. The palette number must be in the range 1-256. The required colour is a LONG integer created from the sum of (required Blue * 65536) + (required Green * 256) + required Red.
RANDOMIZE
RANDOMIZE TIMER
A = INT((RND * 100)) + 1
RANDOMIZE will set the seed for QBasic's random number generator. With QBasic, it's standard to simply use RANDOMIZE TIMER to ensure that the sequence remains the same for each run.
The example is a mathematical operation to get a random number from 1 to 100.
INT stands for Integer, RND for Random and "*" stands for the limit upto which the random number is to be chosen. The "+ 1" is just there to ensure that the number chose is from 1 to 100 and not 0 to 99.
Note: Subsequent calls of this function do not guarantee the same sequence of random numbers.
READ
READ [Variable]
Used in conjunction with the DATA command, this command lets QBasic read data. This is mostly used when dealing with large quantities of data like bitmaps.
REM or '
REM {comments}
' {comments}
When the interpreter encounters REM or " ' " (a single quote) at the start of a line, the rest of the line is ignored
RETURN
RETURN
Signifies that it is the end of a subroutines
RND
RANDOMIZE TIMER
A = INT((RND * 100)) + 1
RND will provide a random number between 0 and 1.
The example is a mathematical operation to get a random number from 1 to 100. RANDOMIZE TIMER will set the initial seed to a unique sequence. INT stands for Integer, RND for Random and "*" stands for the limit upto which the random number is to be chosen. The "+ 1" is just there to ensure that the number chose is from 1 to 100 and not 0 to 99.
Internally, the seed a 24-bit number, iterated in the following method: rnd_seed = (rnd_seed*16598013+12820163) MOD 2^24
PLAY
PLAY "[string expression]"
Used to play notes and a score in QBasic on the PC speaker. The tones are indicated by letters A through G. Accidentals are indicated with a "+" or "#" (for sharp) or "-" (for flat) immediately after the note letter. See this example:
PLAY "C C# C C#"
Whitespaces are ignored inside the string expression. There are also codes that set the duration, octave and tempo. They are all case-insensitive. PLAY executes the commands or notes the order in which they appear in the string. Any indicators that change the properties are effective for the notes following that indicator.
Ln     Sets the duration (length) of the notes. The variable n does not indicate an actual duration
       amount but rather a note type; L1 - whole note, L2 - half note, L4 - quarter note, etc.
       (L8, L16, L32, L64, ...). By default, n = 4.
       For triplets and quintets, use L3, L6, L12, ... and L5, L10, L20, ... series respectively.
       The shorthand notation of length is also provided for a note. For example, "L4 CDE L8 FG L4 AB"
       can be shortened to "L4 CDE F8G8 AB". F and G play as eighth notes while others play as quarter notes.
On     Sets the current octave. Valid values for n are 0 through 6. An octave begins with C and ends with B.
       Remember that C- is equivalent to B.
< >    Changes the current octave respectively down or up one level.
Nn     Plays a specified note in the seven-octave range. Valid values are from 0 to 84. (0 is a pause.)
       Cannot use with sharp and flat. Cannot use with the shorthand notation neither.
MN     Stand for Music Normal. Note duration is 7/8ths of the length indicated by Ln. It is the default mode.
ML     Stand for Music Legato. Note duration is full length of that indicated by Ln.
MS     Stand for Music Staccato. Note duration is 3/4ths of the length indicated by Ln.
Pn     Causes a silence (pause) for the length of note indicated (same as Ln).
Tn     Sets the number of "L4"s per minute (tempo). Valid values are from 32 to 255. The default value is T120.
.      When placed after a note, it causes the duration of the note to be 3/2 of the set duration.
       This is how to get "dotted" notes. "L4 C#." would play C sharp as a dotted quarter note.
       It can be used for a pause as well.
MB MF  Stand for Music Background and Music Foreground. MB places a maximum of 32 notes in the music buffer
       and plays them while executing other statements. Works very well for games.
       MF switches the PLAY mode back to normal. Default is MF.
PRINT
PRINT [Argument] [,or;] [Argument]...
Displays text to the screen. The Argument can be a string literal, a string variable, a numeric literal or a numeric variable. All arguments are optional.
PRINT #[n] [,or;] [Argument]...
Saves data to the file that is 'OPEN FOR OUTPUT AS #[n]' or we can use ? symbol for print command
PSET
PSET ([X coordinate],[Y coordinate]), [Pixel Colour]
This command displays pixels, either one at a time or a group of them at once. For the command to work, the program must have a SCREEN command in it.
SCREEN
SCREEN [Screen Mode Number]
This command is used for displaying graphics on the screen. There are ten main types of screen modes that can be used in QBasic depending on the resolution that you want. Here is a list of what screen modes you can choose from:
SCREEN 0: Textmode, cannot be used for graphics. This the screen mode that text based programs run on.
SCREEN 1: 320 x 200 Resolution. Four Colours
SCREEN 2: 640 x 200 Resolution. Two Colours (Black and White)
SCREEN 7: 320 x 200 Resolution. Sixteen Colours
SCREEN 8: 640 x 200 Resolution. Sixteen Colours
SCREEN 9: 640 x 350 Resolution. Sixteen Colours
SCREEN 10: 640 x 350 Resolution. Two Colours (Black and White)
SCREEN 11: 640 x 480 Resolution. Two Colours
SCREEN 12: 640 x 480 Resolution. Sixteen Colours
SCREEN 13: 320 x 200 Resolution. 256 Colours. (Recommended)
Note. In SCREEN 13 you have a colour Palette of 256 colours. The PALETTE is pre-set by Windows however you can change the RGB values using the PALETTE command.
SEEK
SEEK #[file number], 1
Repositions the 'input #' pointer to the beginning of the file.
SGN
SGN(expression yielding a numeric value)
Yields the 'sign' of a value, -1 if < 0, 0 if 0, 1 if > 0

SLEEP
SLEEP [n]
Execution is suspended for n seconds
SOUND
SOUND [frequency], [duration]
Unlike the BEEP command, this produces a sound from the PC speakers that is of a variable frequency and duration. The frequency is measured in Hertz and has a range from 37 to 32767. Put in one of these numbers in the frequency section. The duration is clock ticks that is defaulted at 18.2 ticks per second.
STR$
Converts a numeric value into a text (string) character
A$ = STR$(expression yielding a numeric value)
The numeric value is converted into text characters and placed into A$. Use to convert numbers into a text string.
WARNINGS.
1) If the result is positive, a leading 'space' is added (STR$(123) = " 123" and not "123" as might be expected). If the result is negative, instead of a space you get a '-' (minus sign), i.e. STR$(-123) = "-123" and not " -123" as might be expected from the positive behaviour.
2) When converting a float (mumb!, numb#) less than 0.1, the string value may be rendered in 'scientific notation', with 'D' used rather than '*10^' (for example "5.nnnnnnD-02" rather than " .05nnnnnn" or "5.nnnnnn*10^-02"). This only occurs when the number of significant digits needs to be preserved (so .03000000 is rendered as " .03", whilst .030000001 becomes " 3.0000001D-02"), again perhaps not what you might expect.
See also CHR$ for converting an ascii value into a string character.
See also LEFT$, MID$, RIGHT$ for extracting sub-strings from a line of text.
SYSTEM
SYSTEM
The .bas exits, the QBasic.exe interpreter is closed and 'control' passes to the Command Window c:\ prompt (or next line of a calling .cmd script etc.)
NOTE!: This only works when you start your program at the command prompt using the "/run" parameter! (EX: "Qbasic /run MyProg.bas") Otherwise, Qbasic assumes you opened your program to make changes, and thus "SYSTEM" drops you back at the editor screen.
THEN
[Command] [variable] = [value] THEN GOTO [line command value]
Used in conjunction with the GOTO or IF condition commands. It tells the computer what to do if a certain condition has been met.
TO
[Command] [Variable] = [Value] TO [Value]
Usually used to input a number of variables.
FOR a = 400 TO 500
PRINT a
NEXT a
This example will print all numbers from 400 to 500. Instead of declaring all values separately, we can get them all declared in one go.
USING
USING "format";
Used to format the output of data from PRINT commands. Normally, the QBasic interpreter will print a number as 8 characters with as many leading spaces as necessary. To change this behavour, the USING command can be used to format the output. For example ..
IF n > 99 THEN PRINT #1, USING "###"; n; ELSE IF n > 9 AND n<=99
THEN PRINT #1, USING "0##"; n; ELSE PRINT #1, USING "00#"; n;
.. will output n from 0 to 999 with leading zeros. Note the ';' after the n. This means 'don't start a new line' and results in the next PRINT #1 adding data directly after the comma (',') Qbasic automatically inserts instead of a line.
VAL()
name=VAL([variable$])
Converts the [variable string] contents into a numeric value so it can be used in calculations. If (name) is an INTEGER type, the VAL is rounded down. See also STR$.
A$ = "2"
B$ = "3"
X = VAL(A$) + VAL(B$)
PRINT A$; " + "; B$; " ="; X
WHILE ... WEND
WHILE {NOT} [test condition is true]
[program code to execute]
WEND
The condition is tested and if true (or NOT true) the [program] code is executed until WEND is reached, at which point control passes back to the WHILE line.
WHILE NOT (EOF(1))
LINE INPUT #1, A$
PRINT #2, A$
WEND
While the end of file #1 has not been reached, read each complete line and write it to file #2.
Unlike FOR and DO, it is not possible to EXIT from a WHILE loop