The process of repeating or doing same task many times until the given condition is true is called looping or iteration. There are different looping statements are used in QBASIC such as FOR … NEXT, WHILE …. WEND, DO … LOOP, etc. It allows a specified group of statements to be executed a certain number of times while certain condition is true. Among these looping statements FOR … NEXT is the most common and popular looping statement.

FOR … NEXT Loop

The FOR … Next is a most popular and mostly used looping statement which is used to execute the set of statements repeatedly for a given number of times.

Syntax:
FOR = TO STEP n
[statements]
NEXT

Example 1
WAP to print the natural numbers from 1 to 1. 

CLS
FOR x = 1 TO 10
    PRINT x;
NEXT x
END

Example 2
WAP to print even numbers from 1 to 20 

CLS
FOR i = 2 TO 20 STEP 2
    PRINT i;
NEXT i
END

Example 3
WAP to print the numbers from 20 to 1 in descending order. 

CLS
FOR a = 20 TO 1 STEP -1
    PRINT a;
NEXT a
END

Example 4
WAP to print your name 10 times 

CLS
FOR i = 1 TO 10
    PRINT "Your Name"
NEXT i
END

Example 5
WAP to print the multiplication table of given number. 

CLS
INPUT "Enter a number "; n
FOR c = 1 TO 10
    PRINT n; "x"; c; "="; n * c
NEXT c
END

WHILE …. WEND

In a WHILE … WEND loop, if the condition is True, all statements are executed until WEND keyword is encountered. If the condition is false, the loop is exited and the control jumps to very next statement after WEND keyword.

Syntax:
WHILE
[statements]
WEND

Example 1
WAP to print the natural numbers from 1 to 10 

CLS
n = 1
WHILE n <= 10
    PRINT n;
    n = n + 1
WEND
END

Example 2
WAP to print even numbers from 1 to 20 

CLS
n = 2
WHILE n <= 20
    PRINT n;
    n = n + 2
WEND
END

Example 3
WAP to print the numbers from 20 to 1 in descending order. 

CLS
n = 20
WHILE n >= 1
    PRINT n;
    n = n - 1
WEND
END

Example 4
WAP to print your name 10 times 

CLS
n = 1
WHILE n <= 10
    PRINT "Your Name"
    n = n + 1
WEND
END

Example 5
WAP to print the multiplication table of given number. 

CLS
i = 1
INPUT "Enter any number "; n
WHILE i <= 10
    PRINT n; "x"; i; "="; i * n
    i = i + 1
WEND
END

DO … LOOP

It is another type of looping statement in QBASIC. Sometime it is also called DO WHILE LOOP. In this loop, the instructions within the loop are performed if the comparison is true. We can use this loop by 4 different methods. Syntax are given below.

Syntax 1 Syntax 2
DO … WHILE
[statements]
LOOP		 DO
[statements]
LOOP WHILE
Syntax 3 Syntax 4
DO … UNTIL
[statement]
LOOP		 DO
[statement]
LOOP UNTIL

While writing a program we can fallow any one syntax among them.

Example 1
WAP to print the natural numbers from 1 to 10 

CLS
CLS
n = 1
DO
    PRINT n;
    n = n + 1
LOOP WHILE n <= 10
END

Example 2
WAP to print even numbers from 1 to 20 

CLS
n = 2
DO
    PRINT n;
    n = n + 2
LOOP WHILE n <= 20
END

Example 3
WAP to print the numbers from 20 to 1 in descending order. 

CLS
n = 20
DO
    PRINT n;
    n = n - 1
LOOP UNTIL n < 1
END

Example 4
WAP to print your name 10 times 

CLS
n = 1
DO WHILE n <= 10
    PRINT "Your Name"
    n = n + 1
LOOP
END

Example 5
WAP to print the multiplication table of given number. 

CLS
i = 1
INPUT "Enter any number "; n
DO WHILE i <= 10
    PRINT n; "x"; i; "="; i * n
    i = i + 1
LOOP
END

NESTED LOOP

A nested loop is a loop within a loop, an inner loop within the body of an outer one. This repeats until the outer loop finishes. QBASIC allows to use one loop inside another loop.  To understand nested loop following section shows a few examples.

Example 1 

CLS
FOR i = 1 TO 4
    PRINT "This is an outer loop "; i
    FOR j = 1 TO 3
        PRINT "Inner Loop "; j
    NEXT j
    PRINT
NEXT i
END

Example 2 

CLS
FOR i = 1 TO 5
    FOR j = 1 TO i
        PRINT j;
    NEXT j
    PRINT
NEXT i
END

Example 3 

CLS
FOR i = 1 TO 5
    FOR j = 1 TO i
        PRINT i;
    NEXT j
    PRINT
NEXT i
END

Example 4 

CLS
FOR i = 5 TO 1 STEP -1
    FOR j = 1 TO i
        PRINT j;
    NEXT j
    PRINT
NEXT i
END

Example 5 

CLS
FOR i = 5 TO 1 STEP -1
    FOR j = 1 TO i
        PRINT i;
    NEXT j
    PRINT
NEXT i
END

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