While loop
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 executed and examined - if the condition is false the code within the block is then executed. 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 = x + 1;
}
firstly checks whether x is less than 3, which it does not, so it executes the instruction x = x + 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;
}
See also
Referenced By
Control flow | Control structure | Program loop | While
|
