Executes a loop.

Used as a shorter equivalent ofwhile loop.

[edit]Syntax

[edit]Explanation

Behaves as follows:

• init-clause may be an expressionor a declaration(since C99).

• Aninit-clause, which is an expression, is evaluated once, before the first evaluation ofcond-expression and its result is discarded.

• cond-expression is evaluated before the loop body. If the result of the expression is zero, the loop statement is exited immediately.
• iteration-expression is evaluated after the loop body and its result is discarded. After evaluatingiteration-expression, control is transferred tocond-expression.

init-clause,cond-expression, anditeration-expression are all optional. Ifcond-expression is omitted, it is replaced with a non-zero integer constant, which makes the loop endless:

for(;;){printf("endless loop!");}

loop-statement is not optional, but it may be a null statement:

for(int n=0; n<10;++n,printf("%d\n", n));// null statement

If the execution of the loop needs to be terminated at some point, a break statement can be used anywhere within theloop-statement.

The continue statement used anywhere within theloop-statement transfers control toiteration-expression.

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 itscond-expression,iteration-expression orloop-statement. This allows the compilers to optimize out all unobservable loops without proving that they terminate. The only exceptions are the loops wherecond-expression is omitted or is a constant expression;for(;;) is always an endless loop.

[edit]Keywords

for

[edit]Notes

The expression statement used asloop-statement establishes its own block scope, distinct from the scope ofinit-clause, unlike in C++:

for(int i=0;;){long i=1;// valid C, invalid C++// ...}

It is possible to enter the body of a loop usinggoto. When entering a loop in this manner,init-clause andcond-expression are not executed. (If control then reaches the end of the loop body, repetition may occur including execution ofcond-expression.)

[edit]Example

#include <stdio.h>#include <stdlib.h>enum{ SIZE=8};int main(void){int array[SIZE];for(size_t i=0; i< SIZE;++i)        array[i]=rand()%2;printf("Array filled!\n");for(size_t i=0; i< SIZE;++i)printf("%d ", array[i]);putchar('\n');}

Array filled!1 0 1 1 1 1 0 0

[edit]References

[edit]See also

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