While loop

In computer programming, a while loop is a control flow statement that allows code to be executed repeatedly based on a Boolean condition. The while loop can be thought of as a repeating if statement.

Overview

A while consists of a block of code and a conditional expression. The conditional is evaluated, and if true, the block of code is executed. This repeats until the conditional becomes false. Because the while loop checks the conditional before the block is executed, the control structure is also known as a pre-test loop. In contrast, do-while loop tests the conditional after the block.

For example, in the languages C, 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 the loop body is entered, where printf() is called and x is incremented by 1. After completing the statements in the loop body, the condition, (x < 5), is checked again, and the loop is executed again. This process repeats until x has the value 5.

The condition can always valuate as true to create an infinite loop. In this case, there may be a early-exit 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 }

Examples

These while loops calculate the factorial of 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 > 1 loop Factorial := Factorial * Counter; Counter := Counter - 1; end loop; Ada.Integer_Text_IO.Put (Factorial); end Factorial;

APL

counter ← 5 factorial ← 1 :While counter > 1  factorial ×← counter  counter -← 1 :EndWhile ⎕ ← factorial

or simply

!5

AutoHotkey

counter := 5 factorial := 1 While counter > 1 factorial *= counter-- MsgBox % factorial

Small Basic

counter = 5 ' Counter = 5 factorial = 1 ' initial value of variable "factorial" While counter > 1  factorial = factorial * counter  counter = counter - 1 EndWhile TextWindow.WriteLine("Factorial = " + factorial)

Visual Basic

Dim counter As Integer = 5 ' init variable and set value Dim factorial As Integer = 1 ' initialize factorial variable Do While counter > 1  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 [ $counter -gt 1 ]; do  factorial=$((factorial * counter))  counter=$((counter - 1)) done echo $factorial

C, C++

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

ColdFusion Markup Language (CFML)

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 > 1)  factorial = factorial * counter  counter = counter - 1  end do  print *, factorial end program FactorialProg

Go

Go has no while statement, but it has the function of a for statement when omitting some elements of the for statement.

counter, factorial := 5, 1 for counter > 1 { counter, factorial = counter-1, factorial*counter }

Java, C#, D

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

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

JavaScript

let counter = 5; let factorial = 1; while (counter > 1)  factorial *= counter--; console.log(factorial);

Kotlin

Source:

var counter = 5 var factorial = 1 while (counter > 1) {  factorial = factorial * counter  counter = counter - 1 } println(factorial)

Lua

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

MATLAB, Octave

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>1, (*While loop*)  factorial*=counter; (*Multiply*)  counter--; (*Decrement*)  ];  factorial ]

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

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

Maya Embedded Language

int $counter = 5; int $factorial = 1; while ($counter > 1) {  $factorial *= $counter;  $counter -= 1; } print("Factorial = " + $factorial + "\n");

Nim

var  counter = 5 # Set counter value to 5  factorial = 1 # Set factorial value to 1 while counter > 1: # While counter is greater than 0  factorial *= counter # Set new value of factorial to counter.  dec counter # Set the counter to counter - 1. echo factorial

Non-terminating while loop:

while true:  echo "Help! I'm stuck in a loop!"

Pascal

Pascal has two forms of the while loop, while and repeat-until. while repeats one statement (unless enclosed in a begin-end block) as long as the condition is true. repeat-until repetitively executes a block of one or more statements until the a condition is false. The main difference between the two is that while executes zero times if the condition is initially false, whereas repeat-until executes at least once.

program Factorial1; var  Fv: integer;  procedure fact(counter:integer);  var  Factorial: integer;  begin  Factorial := 1;  while Counter > 1 do  begin  Factorial := Factorial * Counter;  Counter := Counter - 1  end;  WriteLn(Factorial)  end; begin  Write('Enter a number to return its factorial: ');  readln(fv);  repeat  fact(fv);  Write('Enter another number to return its factorial (or 0 to quit): ');  until fv=0; end.

Perl

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

PL/I

The PL/I DO statement can act as either a for loop, a while loop, or a do until loop.

declare counter fixed initial(5); declare factorial fixed initial(1); do while(counter > 1)  factorial = factorial * counter;  counter = counter - 1; end;

Python

counter = 5 # Set the value to 5 factorial = 1 # Set the value to 1 while counter > 1: # While counter(5) is greater than 0 factorial *= counter # Set new value of factorial to counter. counter -= 1 # Set the 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 1)  (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 1)  (set! factorial (* factorial counter))  (set! counter (sub1 counter))) (displayln factorial)

However, an imperative programming style is often discouraged in Scheme and Racket.

Ruby

# Count Down Variant counter = 5 factorial = 1 while counter > 1  factorial *= counter  counter -= 1 end puts factorial

# Count Up Variant counter = 2 factorial = 1 while counter <= 5  factorial *= counter  counter += 1 end puts factorial

Rust

fn main() {  let mut counter = 5;  let mut factorial = 1;  while counter > 1 {  factorial *= counter;  counter -= 1;  }  println!("{}", 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 initial counter value to 5 var factorial = 1 // Set the initial factorial value to 1 while counter > 1 { // 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 > 1} {  set factorial [expr $factorial * $counter]  incr counter -1 } puts $factorial

PowerShell

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

While (language)

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

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