While loop

While loop flow 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.

Overview

The while construct consists of a block of code and a condition/expression.[1] The condition/expression is evaluated, and if the condition/expression is true,[1] the code within 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#,[2] 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
}

Equivalent constructs

In the C programming language,

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:

Those last two are not recommended because the use of "goto" statements makes it hard for a programmer to understand the flow of control, and is generally regarded as a last resort.

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

for ( ; condition; )
{
   statements;
}

In addition, it has been shown that a loop with an inner for loop performs more computations per unit time than a loop without it or one with an inner while loop. This means the nested while loop will finish slower, given the same number of computations to perform, compared to the nested for loop.[3]

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--;
}
Printf ("Factorial =%d", 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;

AutoHotkey

counter := 5
factorial := 1

While counter > 0
    factorial *= counter--    ; Curly braces are optional for single-line blocks.

MsgBox % factorial

Microsoft Small Basic

counter = 5    ' Counter = 5
factorial = 1  ' initial value of variable "factorial"

While counter > 0
   factorial = factorial * counter
   counter = counter - 1
   TextWindow.WriteLine(counter)
EndWhile

sHaHid KHan...

Visual Basic

Dim counter As Integer = 5    ' init variable and set value
Dim factorial As Integer = 1  ' initialize factorial variable

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

'Debug.Print factorial         ' Console.WriteLine(factorial) in Visual Basic .NET

Bourne (Unix) shell

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

echo $factorial

C or C++

int main(void)
{
    int counter = 5;
    long factorial = 1;
 
    while (counter > 1)
    {
        factorial *= counter--;
    }
    printf("%d", factorial);
    return 0;
}

Script syntax

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

Tag syntax

<cfset counter = 5>
<cfset factorial = 1>
<cfloop condition="counter GT 1">
    <cfset factorial *= counter-->
</cfloop>
<cfoutput>#factorial#</cfoutput>

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--;
}

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 (programming language), 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.

Maya Embedded Language

int $counter = 5;
int $factorial = 1;

int $multiplication;

while ($counter > 0)
{
    $multiplication = ($factorial * $counter);
     
    $counter -= 1;
    
    print ("Counter is: " + $counter + ", multiplication is: " + $multiplication + "\n");
}

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;

PL/I

declare counter   fixed initial(5);
declare factorial fixed initial(1);

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

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'm stuck in a 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).

Ruby

# Calculate the factorial of 5
i = 1
factorial = 1
while i < 5
  factorial *= i
  i += 1
end
puts factorial

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

Swift

var counter = 5                 // Set the value to 5 
var 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.

Tcl

set counter 5
set factorial 1

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

puts $factorial

VEX

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

printf("%d", factorial);

Windows PowerShell

$number = 5
$counter = $number
$factorial = 1
while ($counter) {
    $factorial *= $counter--
}
$factorial

While programming language

The While programming language[4] is a simple programming language constructed from assignments, sequential composition, conditionals and while statements, used in the theoretical analysis of imperative programming language semantics.[5][6]

C := 5;
F := 1;
while (C > 1) do
    F := F * C;
    C := C - 1;

See also

References

  1. 1 2 "The while and do-while Statements (The Java™ Tutorials > Learning the Java Language > Language Basics)". Dosc.oracle.com. Retrieved 2016-10-21.
  2. ↑ "while (C# reference)". Msdn.microsoft.com. Retrieved 2016-10-21.
  3. ↑ Inner loop program construct: A faster way for program execution
  4. ↑ "Chapter 3 : The While programming language" (PDF). Profs.sci.univr.it. Retrieved 2016-10-21.
  5. ↑ Flemming Nielson; Hanne R. Nielson; Chris Hankin (1999). Principles of Program Analysis. Springer. ISBN 978-3-540-65410-0. Retrieved 29 May 2013.
  6. ↑ Illingworth, Valerie (11 December 1997). Dictionary of Computing. Oxford Paperback Reference (4th ed.). Oxford University Press. ISBN 9780192800466.
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