for loop - cppreference.com (original) (raw)

Executes a loop.

Used as a shorter equivalent of while loop.

[edit] Syntax

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

[edit] 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.

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.

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)

[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'); }

Possible output:

Array filled! 1 0 1 1 1 1 0 0

[edit] References

[edit] See also