do-while loop

Executes a statement repeatedly until the value of condition becomes false. The test takes place after each iteration.

Syntax

Explanation

A do-while statement causes the statement (also called the loop body) to be executed repeatedly until the expression (also called controlling expression) compares equal to 0. The repetition occurs regardless of whether the loop body is entered normally or by a goto into the middle of statement.

The evaluation of expression takes place after each execution of statement (whether entered normally or by a goto). If the controlling expression needs to be evaluated before the loop body, the while loop or the for loop may be used.

If the execution of the loop needs to be terminated at some point, break statement can be used as terminating statement.

If the execution of the loop needs to be continued at the end of the loop body, continue statement can be used as a shortcut.

A program with an endless loop has undefined behavior if the loop has no observable behavior (I/O, volatile accesses, atomic or synchronization operation) in any part of its statement or expression. This allows the compilers to optimize out all unobservable loops without proving that they terminate. The only exceptions are the loops where expression is a constant expression; do {...} while(true); is always an endless loop.

Notes

Boolean and pointer expressions are often used as loop controlling expressions. The boolean value false and the null pointer value of any pointer type compare equal to zero.

Keywords

do, while

Example

#include <stdio.h>
#include <stdlib.h>
enum { SIZE = 8 };
int main(void)
{
    // trivial example
    int array[SIZE], n = 0;
    do array[n++] = rand() % 2; // the loop body is a single expression statement
    while(n < SIZE);
    puts("Array filled!");
    n = 0;
    do { // the loop body is a compound statement
        printf("%d ", array[n]);
        ++n;
    } while (n < SIZE);
    printf("\n");
 
    // the loop from K&R itoa(). The do-while loop is used
    // because there is always at least one digit to generate
    int num = 1234, i=0;
    char s[10];
    do s[i++] = num % 10 + '0'; // get next digit in reverse order
    while ((num /= 10) > 0);
    s[i] = '\0';
    puts(s);
}

Array filled!
1 0 1 1 1 1 0 0
4321

References