while loop

 

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Strings library

Algorithms

Numerics

Date and time utilities

Input/output support

Localization support

Concurrency support (C11)

Technical Specifications

Symbol index

 

C language

Basic concepts

Keywords

Preprocessor

Statements

Expressions

Initialization

Declarations

Functions

Miscellaneous

History of C

Technical Specifications

 

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 expression becomes equal to zero. The test takes place before each iteration.

Syntax

attr-spec-seq(optional) while ( expression ) statement

expression	-	any expression of scalar type. This expression is evaluated before each iteration, and if it compares equal to zero, the loop is exited.
statement	-	any statement, typically a compound statement, which serves as the body of the loop
attr-spec-seq	-	(C23)optional list of attributes, applied to the loop statement

Explanation

A while statement causes the statement (also called the loop body) to be executed repeatedly until the expression (also called controlling expression) compares equal to zero. 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 before each execution of statement (unless entered by a goto). If the controlling expression needs to be evaluated after the loop body, the do-while loop may be used.

If the execution of the loop needs to be terminated at some point, break statement can be used as a 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; while(true) is always an endless loop.

As with all other selection and iteration statements, the 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

while

Example

Run this code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
enum { SIZE = 8 };
int main(void)
{
    // trivial example
    int array[SIZE], n = 0;
    while(n < SIZE) array[n++] = rand() % 2;
    puts("Array filled!");
    n = 0;
    while(n < SIZE) printf("%d ", array[n++]);
    printf("\n");
 
    // classic strcpy() implementation
    // (copies a null-terminated string from src to dst)
    char src[]="Hello, world", dst[sizeof src], *p=dst, *q=src;
    while(*p++ = *q++)
        ; // null statement
    puts(dst);
}

Output:

Array filled!
1 0 1 1 1 1 0 0 
Hello, world

References

• C17 standard (ISO/IEC 9899:2018):

• 6.8.5.1 The while statement (p: 109)

• C11 standard (ISO/IEC 9899:2011):

• 6.8.5.1 The while statement (p: 151)

• C99 standard (ISO/IEC 9899:1999):

• 6.8.5.1 The while statement (p: 136)

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

• 3.6.5.1 The while statement