View source for intro/control/while

{{title|The {{tt|while}} loop}}

The {{tt|while}} loop is the simplest possible loop in C++. It repeatedly executes a specific set of statements block as long as the given condition is {{c|true}}.

The syntax of a {{tt|while}} statement is as follows:

{{source|1=
while(/*condition*/) {
    /*body*/
}
}}

The code above works as follows:

@1@ {{c|/*condition*/}} is checked.
:@a@ If it returns {{c|true}}, the {{c|/*body*/}} block runs. Return to (1).
:@b@ Otherwise, execution continues after the closing bracket of the {{c|/*body*/}} block.

This includes two special cases:
@1@ If the condition is already {{c|false}} before the loop begins, it never executes.
@2@ If the condition is {{c|true}} and this value never changes, the loop executes forever (endless loop). The programmer must ensure this never happens.

{{example| The example below prints the sequence of numbers from 0 to 10.
|code=
#include <iostream>

int main()
{
    int i = 0;
    while (i < 10) {
        std::cout << i << ' ';
        i = i + 1; // or ++i, which is equivalent
    }
    std::cout << '\n';
}
|output=
0 1 2 3 4 5 6 7 8 9
}}

===More on the syntax===

{{c|/*condition*/}} must be an expression that evaluates to {{c|true}} or {{c|false}}. Often, comparison operators {{c|1===}}, {{c|1=!=}}, {{c|<}}, {{c|<{{=}}}}, {{c|>}}, {{c|>{{=}}}} are used, e.g. like {{c|1=i != j}}. Also, the condition may be just a number: non-zero value is interpreted as {{c|true}}, wheres {{c|0}} will be interpreted as {{c|false}}.

{{c|/*body*/}} must be one or more statements. If {{c|/*body*/}} contains a single statement, the curly braces may be omitted, but this is not recommended practice as it may lead to bugs in the code.

{{example| The example below shows some more complex ways to use a while loop.
|code=
#include <iostream>

int main()
{
    int i = 10;

    // print a sequence 10..1
    while (i > 0) {
        std::cout << i << ' ';
        i--;
    }
    std::cout << '\n';

    // same as above, i is decremented before being printed
    i = 10;
    while (i > 0) {
        i--;
        std::cout << i << ' ';
    }
    std::cout << '\n';

    // same as above, with i-- integrated into the condition
    i = 10;
    while (i-- > 0) { // check i > 0 and then decrement i
        std::cout << i << ' ';
    }
    std::cout << '\n';

    // the difference between --i and i--
    i = 10;
    while (--i > 0) { // decrement i and check i > 0
        std::cout << i << ' ';
    }
    std::cout << '\n';

    // Since i is going towards zero, we may just check
    // whether it is equal to zero
    i = 10;
    while (--i != 0) {
        std::cout << i << ' ';
    }
    std::cout << '\n';
}
|output=
10 9 8 7 6 5 4 3 2 1
9 8 7 6 5 4 3 2 1 0
9 8 7 6 5 4 3 2 1
9 8 7 6 5 4 3 2 1
9 8 7 6 5 4 3 2 1
}}