for loop

Executes a loop.

Used as a shorter equivalent of while loop .

Syntax

Explanation

Behaves as follows:

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

• An init-clause , which is an expression, is evaluated once, before the first evaluation of cond-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 evaluating iteration-expression , control is transferred to cond-expression .

init-clause , cond-expression , and iteration-expression are all optional. If cond-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 the loop-statement .

The continue statement used anywhere within the loop-statement transfers control to iteration-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 its cond-expression , iteration-expression or loop-statement . This allows the compilers to optimize out all unobservable loops without proving that they terminate. The only exceptions are the loops where cond-expression is omitted or is a constant expression; for ( ;; ) is always an endless loop.

Keywords

for

Notes

The expression statement used as loop-statement establishes its own block scope, distinct from the scope of init-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 using goto . When entering a loop in this manner, init-clause and cond-expression are not executed. (If control then reaches the end of the loop body, repetition may occur including execution of cond-expression .)

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

References

See also