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Concept
While loop
Summary
In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The while loop can be thought of as a repeating if statement.
The while construct consists of a block of code and a condition/expression. The condition/expression is evaluated, and if the condition/expression is true, the code within all of their following in the block is executed. This repeats until the condition/expression becomes false. Because the while loop checks the condition/expression before the block is executed, the control structure is often also known as a pre-test loop. Compare this with the do while loop, which tests the condition/expression after the loop has executed.
For example, in the C programming language (as well as Java, C#, Objective-C, and C++, which use the same syntax in this case), the code fragment
int x = 0;
while (x < 5) {
printf ("x = %d\n", x);
x++;
}
first checks whether x is less than 5, which it is, so then the {loop body} is entered, where the printf function is run and x is incremented by 1. After completing all the statements in the loop body, the condition, (x < 5), is checked again, and the loop is executed again, this process repeating until the variable x has the value 5.
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:
while (true) {
// do complicated stuff
if (someCondition)
break;
// more stuff
}
These while loops will calculate the factorial of the number 5:
var counter: int = 5;
var factorial: int = 1;
while (counter > 1) {
factorial *= counter;
counter--;
}
Printf("Factorial = %d", factorial);
with Ada.Integer_Text_IO;
procedure Factorial is
Counter : Integer := 5;
Factorial : Integer := 1;
begin
while Counter > 0 loop
Factorial := Factorial * Counter;
Counter := Counter - 1;
end loop;
Ada.
Official source
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