In most computer programming languages, a while loop is a control structure that allows code to be executed repeatedly based on a given boolean 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 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:

In QBasic or Visual Basic:

 Dim Counter as Byte, Factorial as Long
 Counter = 5 'Value to take factorial of.
 Factorial = 1
 While (Counter > 0)
   Factorial = Factorial * Counter
   Counter = Counter - 1
 Wend
 Print Factorial 'Prints out the result.

In C or C++:

 int main() {
   unsigned int Counter = 5;
   unsigned long Factorial = 1;
   while (Counter > 0)
     Factorial *= Counter--; /*Multiply, then decrement.*/
   printf("%i", Factorial);
   return 0;
 }

In Tcl (Tool command language)

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

In Java programming language:

 int counter = 5;
 int factorial = 0;
 while (counter > 0){
    factorial *= counter--;
 }
 System.out.println(factorial);