do-while loop

 

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Atomic operations (C11)

Thread support (C11)

Technical Specifications

 

C language

 

Statements

Labels

label : statement

Expression statements

expression ;

Compound statements

{ statement... }

Selection statements

if

switch

Iteration statements

while

do-while

for

Jump statements

break

continue

return

goto

 

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

Syntax

do statement while ( expression ) ;

expression	-	any expression of scalar type. This expression is evaluated after each iteration, and if it compares equal to zero, the loop is exited.
statement	-	any statement, typically a compound statement, which is the body of the loop

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.

As with all other selection and iteration statements, the do-while statement establishes block scope: any identifier introduced in the expression goes out of scope after the statement.

(since C99)

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

Run this code

#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);
}

Possible output:

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

References

• C11 standard (ISO/IEC 9899:2011):

• 6.8.5.2 The do statement (p: 151)

• C99 standard (ISO/IEC 9899:1999):

• 6.8.5.2 The do statement (p: 136)

• C89/C90 standard (ISO/IEC 9899:1990):

• 3.6.5.2 The do statement

See also