QUESTION 19 · Loops

Why does a do-while loop execute its body at least once?

2 marks

Answer

It executes the body before testing the condition.

The condition is checked at the end of each iteration.
A semicolon is required after while (condition).

Explain the do-while loop and show why it executes at least once.

5 marks

Answer in brief

The do-while loop executes its body first and tests its condition afterwards. It is an exit-controlled loop.

Syntax

do {
    /* loop body */
} while (condition);

Working

Enter the loop and execute the body.
Test the condition after the body.
Repeat the body when the condition is non-zero; otherwise continue after the loop.

Example trace

The example starts with i = 5. The body prints 5 and increments i to 6. Only then is i <= 3 tested. It is false, so the loop stops. This produces one output line even though the starting value would fail an equivalent while loop test.

Uses

do-while is useful when an action must happen before a repeat decision, such as displaying a menu and then checking whether the user requested another operation. It does not automatically validate input: the body must still check whether reading a value succeeded and whether the value is allowed.

Important points and complexity

The final semicolon is part of the syntax. A continue goes to the condition test, not directly to the beginning of the body. A break leaves the loop immediately. If the body performs constant work for n iterations, time is O(n), with at least one execution. A fixed number of variables gives O(1) auxiliary space. The sample deliberately demonstrates a single iteration.

Flowchart

Control flow for do-while with labelled true and false paths. — Explain the do-while loop and show why it executes at least once. · Download diagram

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
int main(void)
{
    int i = 5;
    do {
        printf("%d\n", i);
        ++i;
    } while (i <= 3);
    return 0;
}

Sample input and output

This example uses values already written in the program.