Author: Maxim Kakitsev
maksim@freeuk.com
Created on 26.03.01
Source URL: http://www.geocities.com/maxspaceuk
Qbasic Tutorial Chapter IV
Looping with QBasic
To make interesting and efficient programs, you can make QBasic to execute a part of a program more
than once. This is called looping, when QBasic goes through a part of a program over and over again.
This can be done with the GOTO command, but in QBasic there are some good ways to loop the program.
One of them is FOR...NEXT command.
FOR...NEXT
This command allows you to execute a part of a program a certain number of times. It looks like this:
FOR i = 1 TO 4
PRINT "I am looping!"
NEXT i
This little stupid program will print on the screen:
I am looping!
I am looping!
I am looping!
I am looping!
The letter i can be anyother letter, c for example. It is actually a variable, which changes its value
each time the program loops (in this example - from 1 to 4). So, if you make a program like this:
FOR a = 1 TO 5
PRINT "This is loop number"; a
NEXT a
this will print:
This is loop number 1
This is loop number 2
This is loop number 3
This is loop number 4
This is loop number 5
With FOR...NEXT you can use the STEP command, which tells QBasic how to count from one number to another.
If you type:
FOR j = 0 TO 12 STEP 2
~ ~ ~
NEXT j
it will count by two:
0, 2, 4, 6, 8, 10, 12
FOR j = 0 TO 6 STEP 1.5
~ ~ ~
NEXT j
This will count:
0, 1.5, 3, 4.5, 6
You can also count backwards:
FOR d = 10 TO 1 STEP -1
~ ~ ~
NEXT d
When you want QBasic to count backwards, always put STEP -1 (or -whatever)!
DO...LOOP
Imagine that you have a program that works like an ordinary calculator: you enter numbers, QBasic
calculates and shows the result, and the program ends. The program may be good, but one problem
is that you have to run the program each time you want to calculate!
That’s where the handy DO...LOOP comes in. It’s a block of comands, where the program doesn’t have
to loop a certain number of times, like in FOR...NEXT. It can loop indefinitely, while the condition
is met (and when it’s not met, the loop stops), or until the condition is met (so, when it’s met, the
loop stops). Condition is basically the same as an argument, for example f < 20
Here is an example:
DO
PRINT "Enter a number."
PRINT "When you want to quit, press 0."
INPUT n
r = n / 2
PRINT n; "/2 ="; r
LOOP WHILE n > 0
END
When you run this program, you can enter numbers and get the result as many times as you like. The
program loops while numbers you enter are more than 0. Once you’ve entered 0, the program ends. The
condition WHILE n > 0 is put by the LOOP command but you can stick it to the DO command, like that:
DO WHILE n > 0
~~~
LOOP
Or you can use the word UNTIL instead, and put it either by DO or LOOP, like that:
DO UNTIL n = 0
~~~
LOOP
All these examples have the same effect: the program loops while numbers you enter are more than 0
(or, you can say - until the number you’ve entered is 0). Then QBasic stops looping and goes to execute
commands you put after the DO...LOOP block (if it’s END command, the program just ends).
