Information about While Loop

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. 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.

For example, in the C programming language (as well as Java and C++, which use the same syntax in this case), the code fragment

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

first checks whether x is larger 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 value 3.

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.

Demonstrating while loops

These while loops will calculate the factorial of the number 5:

QBasic or Visual Basic

'Initialize the variables Dim counter as Integer : counter = 5 Dim factorial as Long : factorial = 1 Do While counter > 0 factorial = factorial * counter 'Multiply counter = counter - 1 'Decrement Loop Print factorial 'Prints out the result.

REALbasic

Dim counter as Integer = 5 Dim factorial as Integer = 1 While counter > 0 factorial = factorial * counter //Multiply counter = counter - 1 //Decrement Wend MsgBox Str( factorial ) // Prints out the result.

C or C++

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 and C++, but the while loop could also have been written on one line:

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

While loops are frequently used for reading data line by line (as defined by the $/ line separator) from open filehandles:

open IN, "<test.txt"; while ( <IN> ) { print; } close IN;

PHP

$counter = 5; $factorial = 1; while($counter > 0) { $factorial *= $counter--; // Multiply, then decrement. } echo $factorial;

Tcl (Tool command language)

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

Java, C#

The code for the loop is the same for Java and C#:

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);

JavaScript

>
var counter = 5;
var factorial = 1;
while (counter)
{
  factorial *= counter--; //Multiply, then decrement.
}
document.body.appendChild(document.createTextNode(factorial));

Pascal

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.

Smalltalk

Contrary to other languages, in Smalltalk a while loop is not a language construct but defined in the class BlockClosure as a method with one parameter, the body as a closure, using self as the condition.

| count factorial | count := 5. factorial := 1. [ count > 0 ] whileTrue: [ factorial := factorial * (count := count - 1) ]

Python

counter = 5 factorial = 1 while counter > 0: factorial *= counter counter -= 1 print factorial

AutoIt

Dim $counter = 5, $factorial = 1 While $counter > 0 $factorial *= $counter $counter -= 1 WEnd MsgBox(0,"Factorial", $factorial)

Windows PowerShell

$counter = 5 $factorial = 1 while ($counter -gt 0) { $factorial *= $counter-- # Multiply, then decrement. } Write-Host $factorial

See also

• Do while loop
• For loop
• Foreach