QBasic/Full Book View

Text OutputEdit

Text Output

PrintEdit

The text output command is PRINT. This is the command that we will be exploring through this section. PRINT takes a series of arguments. What that means is that in QBasic, after typing PRINT (note, for a short cut, just type a question mark '?') a programmer types the things he wants displayed and PRINT will display them. The first program will display the words "Hello World" on the screen.

PRINT [Text to screen]

1HELLO.BASEdit

 PRINT "Hello World"

Understanding 1HELLO.BASEdit

First, copy everything (in this case, one line, but later programs will be much longer) into a text editor or into QBasic itself and save it as '1HELLO.BAS'. Next open it in QBasic (if you are not already there) and press F5. This will run the program. A black screen with the words "Hello World" in white on your left hand side of the screen should appear. This is your first QBasic program. Now, Press F5 again. Now there should be 2 lines saying "Hello World". This is because QBasic does not clear the screen every time a program is run. To change this we will use the command CLS, which stands for CLear Screen. We will be looking at some other new concepts in the next program as well.

2HELLO.BASEdit

 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

Commas and Semicolons in PRINT and CLSEdit

Here is what the program output should look like:

Hello World   Hello Jupiter
Good Bye                    For Now
 1             2             3              4              5

Addressing 2HELLO.BAS in line order, the first line prints "This line will be erased." to the screen, but in the second line, the CLS command clears the screen immediately after, so it will not be seen (technically it flashes at the bottom form columns). "Hello Jupiter" should line up with '2' at the bottom. More than one comma can be used in a row. 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.BASEdit

 CLS
 hello$ = "Hello World"
 number = 12
 PRINT hello$, number

VariablesEdit

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 number, letter 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.BASEdit

 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 statementEdit

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.

5FACE.BASEdit

 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"

ColorsEdit

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. The format for the COLOR statement is:
COLOR [foreground]
or
COLOR [foreground],[background]
The colors are designated by numbers which will be discussed in the next section.

Color by NumberEdit

There are 16 colors, numbered from 0 to 15.

0 Black 8 Dark Grey (Light Black)
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 colours 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).

It is possible to switch the blinking foreground text with an intense background, but this task is beyond the scope of the QBasic textbook, and may not work when Windows displays the console in windowed mode.

SummaryEdit

In this section we looked at several methods to manipulate 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. There was also a basic introduction to variables which will be expanded upon in later sections.

Basic InputEdit

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] ' Prints questionmark before prompt
or
INPUT "[text to user]", [variable] ' No additional question mark

Example:

INPUT "What is your name"; NAME$
INPUT "What is your age"; age

When an semicolon (;) is used between the text to the user and the variable, a question mark and a 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.BASEdit

CLS
INPUT "What is your name"; NAME$
PRINT "Hello, "; NAME$; "! ";
INPUT "How old are you"; age
PRINT "The user, "; NAME$; " is "; age; " years old"

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).

Basic MathEdit

There are four 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

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 SetupEdit

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.BASEdit

 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.BASEdit

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.BASEdit

 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.BASEdit

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 it's current value is common and a good way to keep the number of variables down.

9TIP.BASEdit

 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 CalculatorEdit

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.BASEdit

 '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 OperationsEdit

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 NumbersEdit

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.BASEdit

 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 RNDEdit

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 ControlEdit

Conditional executionEdit

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 FalseEdit

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.

IFEdit

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...ELSEEdit

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

13IFELSE.BASEdit

 CLS
 RANDOMIZE TIMER
 
 num = INT(RND * 20) + 1
 INPUT "Pick a number between 1 and 20: ", answer
 
 IF answer = num THEN PRINT "You Win" ELSE PRINT "You Lose"

FOR...NEXTEdit

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.BASEdit

 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...WENDEdit

 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.BASEdit

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...LOOPEdit

 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


12IF.BASEdit

 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 CASEEdit

 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 ANSWER FOR ANY OF YOUR QUESTION"
INPUT "WRITE YOUR QUESTION AND I'L GIVE YOU ANSWER ", question$
10 RANDOMIZE TIMER
PRINT
answer=INT(RND*10+1)
SELECT CASE answer
 
 PRINT "pls rephrase your question ."
  CASE 2
    PRINT "Your question is meaningless ."
  CASE 3
    PRINT "do you think i cant answer this."
  CASE 4 
    PRINT "the question looks funny."
END SELECT
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 InputEdit

INKEY$Edit

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 bufferEdit

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 where 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).

ScancodesEdit

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 FunctionsEdit

PurposeEdit

Subroutines and functions are ways to break up your code into a reusable form. They allow the programmer to do a large set of operations just by calling the appropriate procedure or function. For example, lets say you need to PRINT a lot of lines, such as instructions. One way to do this is to just enter all your PRINT commands directly into where you need them. At some point, you may want to move the PRINT commands, or you may use the exact set of PRINTs elsewhere in the program. To move or change all of the PRINT commands would be quite a hassle. A simpler way would be to create a procedure and enter all of the PRINT commands there; then, when you need to execute the commands, you may simply call your procedure and it will execute each line in its body.

Procedure vs. FunctionEdit

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 GOSUBEdit

The GOTO and GOSUB statements were the original methods at 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.

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 moves the execution point to a given label:

 

The GOSUB statement transfers control to a given label, and is paired with a corresponding RETURN statement.

 

ON ERROREdit

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 handeling 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 subroutineEdit

A superior method of declaring a subroutine is using the SUB statement block. Under the QBasic IDE, doing so moves the SUB block to it's own section in the window to prevent accidental deletion of the module, and allows the easier organization of the program code.

Calling a function is as simple as writing the name of the function (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

Parameters passed into subroutines are passed by 'reference' i.e. whilst they may take on a new name within the SUB any changes that are made to the values are 'reflected back' into the originals. 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 it's name).

Declaring a functionEdit

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 TypesEdit

Built-in TypesEdit

QBasic has five built-in types: INTEGER (%), LONG(&) integer, SINGLE(!) float, DOUBLE(#) float and STRING($).

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 NAME AS STRING

If you do not use either implicit or explicit declaration, QBASIC interpreter assumes INTEGER type.

User-defined typeEdit

Clipboard

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 similiar 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.

ArrayEdit

An array is a collection of values stored in a single variable. 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

TYPE can be any of the built in QBasic or user-defined type. If this is not specified, the array takes the default type for the variable.

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 arrayEdit

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 baseEdit

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.

AppendixEdit

CommandsEdit

ABS()Edit

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)

ACCESSEdit

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()Edit

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()Edit

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 )

BEEPEdit

BEEP

The BIOS on the motherboard is instructed to emit a "Beep" sound from the PC 'speaker'. See also SOUND and PLAY.

CASEEdit

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

CHAINEdit

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.

CHDIREdit

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$()Edit

This returns the string character symbol of an ASCII code value.

name$ = CHR$([acsii 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 QBASIC interpreter (eg ", 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()Edit

This rounds the contents of the brackets to the nearest integer.

PRINT CINT(4573.73994596)

4574

CIRCLEEdit

CIRCLE ([X Coordinate], [Y Coordinate]), [Radius], [Colour]

Lets the programmer display a circle. Like all graphics commands, it must be used with the SCREEN command.

CLEAREdit

CLEAR

Resets all variables, strings, arrays and closes all files. The reset command on QBasic.

CLOSEEdit

CLOSE

Closes all open files

CLOSE #2

Closes only the file opened as data stream 2. Other files remain open

CLSEdit

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.

COLOREdit

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.

COMMONEdit

Declares a variable as 'global' which allows it's 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).

CONSTEdit

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.

DATAEdit

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$Edit

A system variable that always contains the current date as a string in mm-dd-yyyy format. Use it like this:

a$ = DATE$

DIMEdit

This this is used to declare an array (early versions of Basic 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 .. LOOPEdit

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 program outputs the Two Times Tables up to 12. The Reason why we put 13 is that the program stops at that point and does not continue the loop. If we put LOOP UNTIL num$ = 12 the last output on the screen would be "22".

See also EXIT DO

DRAWEdit

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)

ENDEdit

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 IFEdit

END IF

Ends the program if a condition is reached.

ENVIRONEdit

ENVIRON [string expression]

NOTE: If you are running QBasic on a Windows system, you wont be able to use this command.

This command lets 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()Edit

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 UNITIL 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'.

ERASEEdit

ERASE [arrayname] [,]

Used to erase all dimensioned arrays.

ERROREdit

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 No 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.

EXITEdit

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 .. NEXTEdit

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

GOSUBEdit

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.

IFEdit

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"

INCLUDEEdit

QBasic supports the use of include files via the $INCLUDE directive:

'$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$Edit

[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).

INPUTEdit

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 #Edit

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)

INSTREdit

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 (defaul = 1, the first character of Seach$)

Pos = INSTR ("abcdefghi", "de")

returns 4

LEFT$()Edit

A$ = LEFT$(B$,N)

A$ is set to the left most characters of B$ ending at the Nth character.

A$ =  LEFT$("Get the start only",4)

returns "Get "

See also RIGHT$(), MID$().

LETEdit

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"

LINEEdit

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 #Edit

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 atempting to process the file using QBasic.

LOOPEdit

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.

LPRINTEdit

LPRINT [statement or variable]

Prints out text to a printer.


MID$Edit

 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 chracters to the right)

See also LEFT$ RIGHT$ LEN

OPENEdit

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.

PALETTEEdit

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.

READEdit

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 'Edit

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

RETURNEdit

RETURN

Signifies that it is the end of a subroutines

PLAYEdit

PLAY "[string expression]"

Used to play notes and a score in QBasic on a computer equipped with a MIDI sound-card (all modern computer motherboards with built-in sound support MIDI). 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.

PRINTEdit

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]'

PSETEdit

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.

SCREENEdit

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.

SEEKEdit

SEEK #[file number], 1

Repositions the 'input #' pointer to the beginning of the file.

SGNEdit

SGN(expression yielding a numeric value)

Yields the 'sign' of a value, -1 if < 0, 0 if 0, 1 if > 0


SLEEPEdit

SLEEP [n]

Execution is suspended for n seconds

SOUNDEdit

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$Edit

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$. Used to convert numbers into a text string.

WARNING. 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.

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.

SYSTEMEdit

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.).

THENEdit

[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.

TOEdit

[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.

USINGEdit

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 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 line.

VAL()Edit

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 + 3"
B = VAL(A$)
PRINT A$ is B

2 + 3 is 5

WHILE ... WENDEdit

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

Source CodeEdit

WAP TO PRINT ODD NUMBERS AUTOMATICALLY FROM 1 WITHIN AN ARRAY OF SIZE 10

AuthorsEdit

< Programming:QBasic

Faraaz Damji (Frazzydee)
Adam Colton
Gareth Richardson (Grich)

Last modified on 30 January 2011, at 07:39