C do-while Loop

What You’ll Learn

The do-while loop is C’s post-test loop: the body runs first, then the condition is checked. That guarantees at least one execution—perfect for menus and input validation where you must prompt before you can test whether to continue.

Definition and Usage

A do-while loop executes its body, then evaluates the condition. If the condition is true, the loop repeats; if false, control moves to the next statement. Because the test comes after the body, the body always runs at least once.

Compare with while: if the condition is false before the first test, a while loop skips the body entirely. do-while still runs the body once. That difference matters for prompts, menus, and “try at least once” logic.

📝 Syntax

General form of the C do-while loop:

do {
    // statements to repeat
} while (condition);

Parts

• do — starts the loop; body runs immediately.
• { ... } — block executed on each iteration.
• while (condition); — tested after the body; semicolon required.

Equivalent counting pattern

int i = 0;
do {
    printf("%d\n", i);
    i++;
} while (i < 5);

Compile

gcc program.c -std=c11 -o program

Examples Gallery

Compile with gcc example.c -std=c11 -o example. Notice how each pattern benefits from running the body before the first condition test.

#include <stdio.h>

int main(void) {
    int i = 0;

    do {
        printf("%d\n", i);
        i++;
    } while (i < 5);

    return 0;
}

0
1
2
3
4

#include <stdio.h>

int main(void) {
    int i = 10;

    do {
        printf("Body runs: i = %d\n", i);
        i++;
    } while (i < 5);

    printf("After loop: i = %d\n", i);
    return 0;
}

Body runs: i = 10
After loop: i = 11

#include <stdio.h>

int main(void) {
    int num;

    do {
        printf("Enter a number between 1 and 10: ");
        if (scanf("%d", &num) != 1) {
            return 1;
        }
    } while (num < 1 || num > 10);

    printf("You entered: %d\n", num);
    return 0;
}

Enter a number between 1 and 10: 0
Enter a number between 1 and 10: 15
Enter a number between 1 and 10: 7
You entered: 7

#include <stdio.h>

int main(void) {
    int choice;

    do {
        printf("\nMenu:\n");
        printf("1. Option 1\n");
        printf("2. Option 2\n");
        printf("3. Exit\n");
        printf("Enter your choice: ");

        if (scanf("%d", &choice) != 1) {
            break;
        }

        switch (choice) {
            case 1:
                printf("Option 1 selected.\n");
                break;
            case 2:
                printf("Option 2 selected.\n");
                break;
            case 3:
                printf("Exiting...\n");
                break;
            default:
                printf("Invalid choice, try again.\n");
        }
    } while (choice != 3);

    return 0;
}

Menu:
1. Option 1
2. Option 2
3. Exit
Enter your choice: 1
Option 1 selected.

Menu:
1. Option 1
2. Option 2
3. Exit
Enter your choice: 3
Exiting...

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
    char again;

    srand((unsigned)time(NULL));

    do {
        int secret = rand() % 10 + 1;
        int guess;

        printf("Guess a number 1-10: ");
        if (scanf("%d", &guess) != 1) {
            break;
        }

        if (guess == secret) {
            printf("Correct! It was %d.\n", secret);
        } else {
            printf("Wrong. The number was %d.\n", secret);
        }

        printf("Play again? (y/n): ");
        if (scanf(" %c", &again) != 1) {
            break;
        }
    } while (again == 'y' || again == 'Y');

    printf("Thanks for playing!\n");
    return 0;
}

Guess a number 1-10: 4
Wrong. The number was 7.
Play again? (y/n): n
Thanks for playing!

🚀 Common Use Cases

• Input validation — prompt, read, retry until valid.
• Menus — show options before checking exit choice.
• Play again — run activity, then ask to repeat.
• Hardware polling — attempt operation once, retry while busy.
• Password retry — at least one login attempt.
• Parser peek — read token, continue while more input exists.

📝 Notes

• Semicolon after while (condition); is mandatory.
• Body runs at least once even if the condition starts false.
• Update variables inside the body so the condition can eventually fail.
• break and continue work inside do-while like other loops.
• Prefer while when zero iterations is correct; use do-while when it is not.
• Check scanf return values in validation loops.

⚡ Optimization

do-while has the same runtime cost as an equivalent while loop for repeated iterations. Choose based on clarity, not speed. For validation, combine the prompt and read in the body once per iteration rather than duplicating code before a while loop.

💡 Best Practices