While loop

While loop flow diagram

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. ^[1] The conditional is evaluated, and if true, ^[1] 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#, ^[2] Objective-C, and C++, (which use the same syntax in this case), the code fragment

intx=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 stuffif(someCondition)break;// more stuff}

Examples

These while loops calculate the factorial of 5:

ActionScript 3

Further information: ActionScript 3.0

varcounter:int=5;varfactorial:int=1;while(counter>1){factorial*=counter;counter--;}Printf("Factorial = %d",factorial);

Ada

Further information: Ada (programming language)

withAda.Integer_Text_IO;procedureFactorialisCounter:Integer:=5;Factorial:Integer:=1;beginwhileCounter>0loopFactorial:=Factorial*Counter;Counter:=Counter-1;endloop;Ada.Integer_Text_IO.Put(Factorial);endFactorial;

APL

Further information: APL (programming language)

counter←5factorial←1:Whilecounter>0factorial×←countercounter-←1:EndWhile⎕←factorial

or simply

!5

AutoHotkey

Further information: AutoHotkey

counter:=5factorial:=1Whilecounter>0factorial*=counter--MsgBox%factorial

Small Basic

Further information: Microsoft Small Basic

counter=5' Counter = 5factorial=1' initial value of variable "factorial"Whilecounter>0factorial=factorial*countercounter=counter-1TextWindow.WriteLine(counter)EndWhile

Visual Basic

Further information: Visual Basic

DimcounterAsInteger=5' init variable and set valueDimfactorialAsInteger=1' initialize factorial variableDoWhilecounter>0factorial=factorial*countercounter=counter-1Loop' program goes here, until counter = 0'Debug.Print factorial         ' Console.WriteLine(factorial) in Visual Basic .NET

Bourne (Unix) shell

Further information: Bourne shell

counter=5factorial=1while[$counter-gt0];dofactorial=$((factorial*counter))counter=$((counter-1))doneecho$factorial

C, C++

Further information: C (programming language) and C++

intmain(){intcount=5;intfactorial=1;while(count>1){factorial*=count--;}printf("%d",factorial);}

ColdFusion Markup Language (CFML)

Further information: ColdFusion Markup Language

Script syntax

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

Tag syntax

Further information: Tag (programming)

<cfsetcounter=5><cfsetfactorial=1><cfloopcondition="counter GT 1"><cfsetfactorial*=counter--></cfloop><cfoutput>#factorial#</cfoutput>

Fortran

Further information: Fortran

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

Go

Further information: Go (programming language)

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

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

Java, C#, D

Further information: Java (programming language), C Sharp (programming language), and D (programming language)

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

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

JavaScript

Further information: JavaScript

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

Lua

Further information: Lua (programming language)

counter=5factorial=1whilecounter>0dofactorial=factorial*countercounter=counter-1endprint(factorial)

MATLAB, Octave

Further information: MATLAB and GNU Octave

counter=5;factorial=1;while(counter>0)factorial=factorial*counter;%Multiplycounter=counter-1;%Decrementendfactorial

Mathematica

Further information: Wolfram Mathematica and Wolfram Language

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, Component Pascal

Further information: Oberon (programming language), Oberon-2, Oberon-07, and Component Pascal

MODULEFactorial;IMPORTOut;VARCounter,Factorial:INTEGER;BEGINCounter:=5;Factorial:=1;WHILECounter>0DOFactorial:=Factorial*Counter;DEC(Counter)END;Out.Int(Factorial,0)ENDFactorial.

Maya Embedded Language

Further information: 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");}

Nim

Further information: Nim (programming language)

varcounter=5# Set counter value to 5factorial=1# Set factorial value to 1whilecounter>0:# While counter is greater than 0factorial*=counter# Set new value of factorial to counter.deccounter# Set the counter to counter - 1.echofactorial

Non-terminating while loop:

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

Pascal

Further information: Pascal (programming language)

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.

programFactorial1;varFv:integer;procedurefact(counter:integer);varFactorial:integer;beginFactorial:=1;whileCounter>0dobeginFactorial:=Factorial*Counter;Counter:=Counter-1end;WriteLn(Factorial)end;beginWrite('Enter a number to return its factorial: ');readln(fv);repeatfact(fv);Write('Enter another number to return its factorial (or 0 to quit): ');untilfv=0;end.

Perl

Further information: 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:

openIN,"<test.txt";while(<IN>){print;}closeIN;

PHP

Further information: PHP

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

PL/I

Further information: PL/I

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

declarecounterfixedinitial(5); declarefactorialfixedinitial(1);dowhile(counter>0)factorial=factorial*counter;counter=counter-1;end;

Python

Further information: Python (programming language)

counter=5# Set the value to 5factorial=1# Set the value to 1whilecounter>0:# While counter(5) is greater than 0factorial*=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:

whileTrue:print("Help! I'm stuck in a loop!")

Racket

Further information: Racket (programming language) and Scheme (programming language)

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

#lang racket(definecounter5)(definefactorial1)(letloop()(when(>counter0)(set!factorial(*factorialcounter))(set!counter(sub1counter))(loop)))(displaylnfactorial)

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

#lang racket(define-syntax-rule(whiletestbody...); implements a while loop(letloop()(whentestbody...(loop))))(definecounter5)(definefactorial1)(while(>counter0)(set!factorial(*factorialcounter))(set!counter(sub1counter)))(displaylnfactorial)

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

Ruby

Further information: Ruby (programming language)

# Calculate the factorial of 5i=1factorial=1whilei<=5factorial*=ii+=1endputsfactorial

Rust

Further information: Rust (programming language)

fnmain(){letmutcounter=5;letmutfactorial=1;whilecounter>1{factorial*=counter;counter-=1;}println!("{}",factorial);}

Smalltalk

Further information: 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].Transcriptshow:factorial

Swift

Further information: Swift (programming language)

varcounter=5// Set the initial counter value to 5varfactorial=1// Set the initial factorial value to 1whilecounter>0{// While counter(5) is greater than 0factorial*=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

Further information: Tcl

setcounter5setfactorial1while{$counter>0}{setfactorial[expr$factorial*$counter]incrcounter-1}puts$factorial

VEX

Further information: VEX prefix

intcounter=5;intfactorial=1;while(counter>1)factorial*=counter--;printf("%d",factorial);

PowerShell

Further information: PowerShell

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

While (language)

While ^[3] is a simple programming language constructed from assignments, sequential composition, conditionals, and while statements, used in the theoretical analysis of imperative programming language semantics. ^{[4] [5]}

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

See also

• Do while loop
• For loop
• Foreach
• Primitive recursive function
• General recursive function
• LOOP (programming language) – a programming language with the property that the functions it can compute are exactly the primitive recursive functions

References

1. "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. "Chapter 3: The While programming language" (PDF). Profs.sci.univr.it. Retrieved 2016-10-21.
4. Flemming Nielson; Hanne R. Nielson; Chris Hankin (1999). Principles of Program Analysis. Springer. ISBN   978-3-540-65410-0 . Retrieved 29 May 2013.
5. Illingworth, Valerie (11 December 1997). Dictionary of Computing . Oxford Paperback Reference (4th ed.). Oxford University Press. ISBN   9780192800466.