for loop

Executes a loop.

Used as a shorter equivalent of while loop.

[edit] Syntax

[edit] 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.

[edit] Keywords

for

[edit] 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.)

[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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