Control Structures: do-while

Notes

do-while Loop in C

Definition: The do-while loop is an exit-controlled loop in which the loop body executes first and the condition is checked afterward.

Syntax

do
{
    statements;
}
while (condition);

How do-while Works

Example

The following program prints the value of i and increases it until i becomes greater than 5.

#include <stdio.h>

int main()
{
    int i = 1;

    do
    {
        printf("%d ", i);
        i++;                // move to the next number
    }
    while (i <= 5);

    return 0;
}

Output

1 2 3 4 5

Why is do-while Called an Exit-Controlled Loop?

The condition of a do-while loop is checked after the loop body executes. Therefore, the body always executes at least once.

Real-World Example

A menu-driven program often needs to display the menu at least once before asking whether the user wants to continue.

Practical Example — Input Until Zero

Problem Statement

Write a C program that repeatedly accepts numbers from the user and displays the entered number until the user enters 0.

Learning Outcomes

• Understand the working of the do-while loop.
• Use a sentinel value to terminate repeated input.
• Understand the difference between entry-controlled and exit-controlled loops.

Hint

Read a number inside the loop, display it when it is not zero, and continue while the number is not 0.

Theory

A do-while loop executes its body first and checks the condition afterward. Therefore, the loop body is guaranteed to execute at least once.

Program

#include <stdio.h>

int main()
{
    int num;

    do
    {
        printf("Enter a number (0 to stop): ");
        scanf("%d", &num);

        if (num != 0)              // 0 is the sentinel, so skip printing it
            printf("You entered: %d
", num);

    } while (num != 0);             // condition is checked after the body

    printf("Loop terminated.
");

    return 0;
}

Expected Output

Enter a number (0 to stop): 5
You entered: 5
Enter a number (0 to stop): 8
You entered: 8
Enter a number (0 to stop): 0
Loop terminated.

Note

Unlike a while loop, a do-while loop always executes its body at least once because the condition is checked after the body.

while vs do-while

Important Points for Exam

• do-while is an exit-controlled loop.
• The loop body executes before the condition is checked.
• The body executes at least once.
• A semicolon is required after the while(condition).
• It is useful for menus and input-driven programs.

Quick Revision

Important Exam Questions

Short Answer Questions

1. What is a do-while loop in C?
2. Why is do-while called an exit-controlled loop?
3. Write the syntax of a do-while loop.
4. How is do-while different from while?
5. Why does a do-while loop execute at least once?
6. What is a sentinel value?

Long Answer Questions

1. Explain the do-while loop with syntax and suitable example.
2. Write a C program using do-while to accept numbers until the user enters 0.
3. Differentiate between while and do-while loops.