In most computer programming languages, a while loop is a control structure that allows code to be executed repeatedly based on a given Boolean datatype condition.

The while construct consists of a block of code and a condition. The condition is first evaluated - if the condition is true the code within the block is then executed. This repeats until the condition becomes false. Because while loops check the condition before the block is executed, the control structure is often also known as a pre-test loop. Compare with the do while loop, which tests the condition after the loop has executed.

Note that it is possible, and in some cases desirable, for the condition to always evaluate to true, creating an infinite loop. When such a loop is created intentionally, there is usually another control structure (such as a break (computing) statement) that controls termination of the loop.

For example, in the C programming language, the code fragment

x = 0; while (x < 3) { x++; }

first checks whether x is less than 3, which it is not, so it increments x by 1. It then checks the condition again, and executes again, repeating this process until the Variable x has the desired value, 3.

= Demonstrating while loops =

These while loops will calculate the factorial of a number:

== QBasic or Visual Basic==

Dim counter as Integer : counter = 5 Dim factorial as Long : factorial = 1 While (counter > 0) factorial = factorial * counter Multiply counter = counter - 1 Decrement Wend Print factorial Prints out the result.

==C programming language or C plus plus==

unsigned int counter = 5; unsigned long factorial = 1; while (counter > 0) factorial *= counter--; /*Multiply, then decrement.*/ printf( %i , factorial);

== Perl ==

my $counter = 5; my $factorial = 1; while ( $counter > 0 ) { $factorial *= $counter--; # Multiply, then decrement } print $factorial;

Very similar to C programming language and C++, but the while loop could also have been written on one line:

$factorial *= $counter-- while ( $counter > 0 );

== Tcl (Tool command language) ==

set counter 5 set factorial 1 while {$counter > 0} { set factorial [expr $factorial * $counter] incr counter -1 } puts $factorial

== Java programming language, Csharp ==

The code for the loop is the same for Java and Csharp:

int counter = 5; long factorial = 1; while (counter > 0) factorial *= counter--; // Multiply, then decrement.

For Java the result is printed as follows: System.out.println(factorial);

The same in C# System.Console.WriteLine(factorial);

== Pascal programming language ==

program Factorial var Counter, Factorial: integer; begin Counter := 5; Factorial := 1; while Counter > 0 do begin Factorial := Factorial * Counter; Counter := Counter - 1; end; Write(Factorial); end.

= See also =