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for loop

Executes a loop.

-

Used as a shorter equivalent of while loop.

-

Syntax

- -
attr-spec-seq(since C23)(optional) for ( init-clause ; cond-expression ; iteration-expression ) loop-statement

Explanation

Behaves as follows:

- - -
  • An init-clause, which is a declaration, is in scope in the entire loop body, including the remainder of init-clause, the entire cond-expression, the entire iteration-expression and the entire loop-statement. Only auto and register storage class specifiers are allowed for the variables declared in this declaration.
(since C99)

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.

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The continue statement used anywhere within the loop-statement transfers control to iteration-expression.

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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.

- - -

As with all other selection and iteration statements, the for statement establishes block scope: any identifier introduced in the init-clause, cond-expression, or iteration-expression goes out of scope after the loop-statement.

-		 (since C99)
- -

attr-spec-seq is an optional list of attributes, applied to the for statement.

-		 (since C23)

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');
-}

Possible output:

-
Array filled!
-1 0 1 1 1 1 0 0

References

See also

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C++ documentation for for loop
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/language/for -

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