for loop
Executes a loop.
Used as a shorter equivalent of while loop.
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[edit] Syntax
for ( init_clause ; cond_expression ; iteration_expression ) loop_statement
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[edit] Explanation
Behaves as follows:
- init_clause may be an expression or a declaration
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- If it is an expression, it is evaluated once, before the first evaluation of cond_expression and its result is discarded.
- (C99) If it is a declaration, it 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
andregister
storage classes are allowed for the variables declared in this declaration.
- 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:
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.
As an extension to the as-if rule, a loop with no side effects (I/O, volatile accesses, or atomic synchronizations) in any part of its cond_expression, iteration_expression or loop_statement may be optimized out even if it is an endless loop. 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) |
[edit] Keywords
[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. In this case, init_clause and cond_expression are not executed.
[edit] Example
Possible output:
Array filled! 1 0 1 1 1 1 0 0
[edit] References
- C11 standard (ISO/IEC 9899:2011):
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- 6.8.5.3 The for statement (p: 151)
- C99 standard (ISO/IEC 9899:1999):
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- 6.8.5.3 The for statement (p: 136)
- C89/C90 standard (ISO/IEC 9899:1990):
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- 3.6.5.3 The for statement
[edit] See also
C++ documentation for for loop
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