for loop

 

C++

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label : statement

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expression ;

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{ statement... }

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range for(C++11)

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try compound-statement handler-sequence

 

Executes init-statement once, then executes statement and iteration_expression repeatedly, until the value of condition becomes false. The test takes place before each iteration.

Syntax

attr(optional) for ( init-statement condition(optional) ; iteration_expression(optional) ) statement

attr(C++11)	-	any number of attributes
init-statement	-	either an expression statement or a declaration statement, typically a declaration of a loop counter variable with initializer. Note that an init-statement must end with a semicolon.
condition	-	any expression which is contextually convertible to bool or a declaration of a single variable with a brace-or-equals initializer. This expression is evaluated before each iteration, and if it yields false, the loop is exited. If this is a declaration, the initializer is evaluated before each iteration, and if the value of the declared variable converts to false, the loop is exited.
iteration_expression	-	any expression, which is executed after every iteration of the loop and before re-evaluating condition. Typically, this is the expression that increments the loop counter
statement	-	any statement, typically a compound statement, which is the body of the loop

Explanation

The above syntax produces code equivalent to:

{ init_statement while ( condition ) { statement iteration_expression ; } }

Except that

1) Names declared by the init-statement (if init-statement is a declaration) and names declared by condition (if condition is a declaration) are in the same scope.

2) continue in the statement will execute iteration_expression

3) Empty condition is equivalent to while(true)

If the execution of the loop needs to be terminated at some point, break statement can be used as terminating statement.

If the execution of the loop needs to be continued at the end of the loop body, continue statement can be used as shortcut.

As is the case with while loop, if statement is a single statement (not a compound statement), the scope of variables declared in it is limited to the loop body as if it was a compound statement.

for(;;)
    int n;
// n goes out of scope

Keywords

for

Example

Run this code

#include <iostream>
#include <vector>
 
int main()
{
    // typical loop with a single statement as the body
    for (int i = 0; i < 10; ++i)
        std::cout << i << ' ';
    std::cout << '\n';
 
    // init-statement can declare multiple names, as long as they
    // can use the same decl-specifier-seq
    for (int i=0, *p = &i; i < 9; i+=2) {
        std::cout << i << ':' << *p << ' ';
    }
    std::cout << '\n';
 
    // condition may be a declaration
    char cstr[] = "Hello";
    for(int n = 0; char c = cstr[n]; ++n)
        std::cout << c;
    std::cout << '\n';
 
    // init-statement can use the auto type specifier
    std::vector<int> v = {3, 1, 4, 1, 5, 9};
    for (auto iter = v.begin(); iter != v.end(); ++iter) {
        std::cout << *iter << ' ';
    }
    std::cout << '\n';
 
   // init-statement can be an expression
    int n = 0;
    for (std::cout << "Loop start\n";
         std::cout << "Loop test\n";
         std::cout << "Iteration " << ++n << '\n')
        if(n > 1)
            break;
    std::cout << '\n';
}

Output:

0 1 2 3 4 5 6 7 8 9 
0:0 2:2 4:4 6:6 8:8 
Hello
3 1 4 1 5 9 
Loop start
Loop test
Iteration 1
Loop test
Iteration 2
Loop test

See also

range-for loop

executes loop over range (since C++11)