Oppiskele The Do-While Loop in C

Definition

A post-test loop, like the do-while loop, checks its condition after executing the loop body. This is different from a pre-test loop, such as the while loop, which checks the condition before running the body.

The do-while loop in C allows you to run a block of code at least once before checking the condition. Unlike the while loop, which checks the condition before entering the loop body, the do-while loop executes its block first, then checks if it should repeat.

This makes the do-while loop useful when you want to ensure a section of code runs at least once — for example, when performing an action that must happen before a condition can be verified.

main.c


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#include <stdio.h>

int main() {
    // Try to change `number` to any positive value
    int number = -3;  // Starts with a negative `number`

    do {
        printf("Current number: %d\n", number);
        number++;
    } while (number <= 0);

    printf("Loop finished, final number: %d\n", number);
    return 0;
}

The do-while loop is ideal when you need to guarantee execution before validation. In this example, the loop prints and increments the variable at least once, even though the condition (number <= 0) is checked only after the body runs.

It's perfect for tasks that require one guaranteed action before rechecking, such as repeating an operation until a valid state or condition is reached.

Note

Post-test loops always run the body at least once, while pre-test loops might not run at all if the condition is false from the start.

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