While loop

From Wikipedia, the free encyclopedia

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 lesser than 3, which it is, 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.

Contents

[edit] Equivalent constructs

while (condition) {
   statements;
}

is equivalent to

while (true) {
   if (!condition) break;
   statements;
}

or

goto TEST;
LOOPSTART:
   statements;
TEST:
   if (condition) goto LOOPSTART;

Also, in C and its descendants, a while loop is a for loop with no initialization or counting expressions, i.e.,

for ( ; condition; )
{
   statements;
}

[edit] Demonstrating while loops

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

[edit] Ada

Wikibooks
Wikibooks' Ada_Programming has more about this subject: 
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.Integer_Text_IO.Put (Factorial);
end Main;

[edit] AutoIt

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

[edit] 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.

[edit] Visual Basic .NET

' Initialize the variables
Dim counter As UInteger = 5
Dim factorial As UInteger = 1
 
Do While counter > 0
  factorial = factorial * counter     ' Multiply
  counter -= 1                        ' Decrement
Loop
 
System.Console.WriteLine(factorial)   ' Prints out the result.

[edit] C or C++

unsigned int counter = 5;
unsigned long factorial = 1;
 
while (counter > 0)
{
  factorial *= counter--;    //Multiply and decrement
} 
 
printf("%i", factorial);

[edit] Java, C#

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

int counter = 5;
long factorial = counter;
 
while (--counter > 1)
{
   factorial *= counter;
}

For Java the result is printed as follows:

System.out.println(factorial);

The same in C#

System.Console.WriteLine(factorial);

[edit] JavaScript

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

[edit] Matlab

counter = 5;
factorial = 1;
 
while (counter > 0)
  factorial = factorial * counter;      %Multiply
  counter = counter - 1;                %Decrement
end
 
factorial

[edit] 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.

[edit] 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;

[edit] PHP

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

[edit] Python

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

[edit] 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) ]
Transcript show: factorial

[edit] Tcl (Tool command language)

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

[edit] Windows PowerShell

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

[edit] See also