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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 = 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
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