QUESTION 15 · Loops

Why is while called an entry-controlled loop?

2 marks

Answer

Its condition is tested before the body executes on each iteration.

If the condition is initially false, the body executes zero times.
The body must make progress towards stopping when termination is required.

Explain the while loop with syntax and an example.

5 marks

Answer in brief

The while loop repeats a body while its condition is true. It is an entry-controlled loop because the test comes before the body.

Syntax

while (condition) {
    /* repeated statements */
}

Working

Set the initial values before the loop.
Test the condition. If false, leave the loop.
Execute the body when true, then return to the condition.

Example trace

The example starts with i = 1. The test i <= 3 succeeds, so 1 is printed and i becomes 2. The next iterations print 2 and 3. After the third update, i becomes 4. The condition is now false and the loop ends. The update is essential: without it, this example would continue printing 1.

Uses and important points

while is useful when repetition is controlled mainly by a condition, such as reading values until a sentinel is entered. The body can execute zero times. Use braces for multiple statements. An accidental semicolon after the condition creates an empty body. When continue is used, make sure an essential update is not skipped.

Complexity

A loop that performs constant work once for each of n values takes O(n) time. This example prints exactly three values, so the displayed run has fixed work. Generalising its limit to n gives O(n) time and O(1) auxiliary space because only a counter is needed.

Flowchart

Control flow for while with labelled true and false paths. — Explain the while loop with syntax and an example. · Download diagram

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
int main(void)
{
    int i = 1;
    while (i <= 3) {
        printf("%d\n", i);
        ++i;
    }
    return 0;
}

Sample input and output

This example uses values already written in the program.

Output

1
2
3