While loop

While loop diagram

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 evaluated, and if the condition is true, the code within the block is executed. This repeats until the condition becomes false. Because while loop checks 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

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
}

Equivalent constructs

while (condition) {
   statements;
}

is equivalent to

if (condition) {
   do {
      statements;
   } while (condition);
}

or

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

or

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

or

TEST:
   if (!condition) goto LOOPEND;
   statements
   goto TEST;
LOOPEND:

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

for ( ; condition; )
{
   statements;
}

Demonstrating while loops

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

ActionScript 3

var counter:int = 5;
var factorial:int = 1;
 
while ( counter > 1 )
{
  factorial *= counter;
  counter--;
}
trace ("Factorial ", factorial);

Ada

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 Factorial;

Bash

counter=5
factorial=1
while [ $counter -gt 0 ]; do
    factorial=$((factorial * counter))
    counter=$((counter - 1))
done
 
echo $factorial

QBasic or Visual Basic

    Dim counter As Integer = 10    ' init variable and set value

    Do While counter > 0
          counter = counter - 1
    Loop     ' program goes here, until counter = 0

C or C++

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

Fortran

program FactorialProg
  integer :: counter = 5
  integer :: factorial = 1
  do while (counter > 0)
    factorial = factorial * counter
    counter = counter - 1
  end do
  print *, factorial
end program FactorialProg

Java, C#, D

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

int counter = 5;
long factorial = 1;
 
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);

And finally in D

writefln(factorial);

JavaScript

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

Lua

counter = 5
factorial = 1
 
while counter > 0 do
  factorial = factorial * counter
  counter = counter - 1
end
 
print(factorial)

MATLAB

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

Mathematica

Block[{counter=5,factorial=1},          (*localize counter and factorial*)
        While[counter>0,                (*While loop*)
               factorial*=counter;      (*Multiply*)
               counter--;               (*Decrement*)
             ];
     factorial
    ]

Oberon, Oberon-2, Oberon-07, or Component Pascal

MODULE Factorial;
IMPORT Out;
VAR
  Counter, Factorial: INTEGER;
BEGIN
  Counter := 5;
  Factorial := 1;
  WHILE Counter > 0 DO
    Factorial := Factorial * Counter;
    DEC(Counter)
  END;
  Out.Int(Factorial,0)
END Factorial.

Pascal

program Factorial1;
var
  Counter, Factorial: integer;
begin
  Counter := 5;
  Factorial := 1;
  while Counter > 0 do
  begin
    Factorial := Factorial * Counter;
    Counter := Counter - 1
  end;
  WriteLn(Factorial)
end.

Perl

my $counter   = 5;
my $factorial = 1;
 
while ( $counter > 0 ) {
    $factorial *= $counter--; # Multiply, then decrement
}
 
print $factorial;

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 first.
  $counter--; // then decrement.
}
print $factorial;

Python

counter = 5                            # Set the value to 5 
factorial = 1                          # Set the value to 1
 
while counter > 0:                      # While counter(5) is greater than 0  
      factorial *= counter              # Set new value of factorial to 
                                        # factorial x counter.
 
      counter -= 1                      # Set the new value of counter to
                                        # counter - 1.
 
print factorial                         # Print the value of factorial.

Non Terminating While Loop:-

while True:
    print "Help! I am Lost in this Loop."

Racket

In Racket, as in other Scheme implementations, a "named-let" is a popular way to implement loops:

#lang racket
(define counter 5)
(define factorial 1)
(let loop ()
  (when (> counter 0)
    (set! factorial (* factorial counter))
    (set! counter (sub1 counter))
    (loop)))
(displayln factorial)

Using a macro system, implementing a while-loop is a trivial exercise (commonly used to introduce macros):

#lang racket
(define-syntax-rule (while test body ...) ; implements a while loop
  (let loop () (when test body ...) (loop)))
(define counter 5)
(define factorial 1)
(while (> counter 0)
  (set! factorial (* factorial counter))
  (set! counter (sub1 counter)))
(displayln factorial)

But note that an imperative programming style is often discouraged in Racket (as in Scheme).

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.

Smalltalk also has a corresponding whileFalse: method.

| count factorial |
count := 5.
factorial := 1.
[ count > 0 ] whileTrue: 
    [ factorial := factorial * count
    count := count - 1 ].
Transcript show: factorial

Tcl (Tool command language)

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

Windows PowerShell

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

See also

• Do while loop
• For loop
• Foreach