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While loop

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Control flow statement for repeating execution until a condition is met

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While loop flow diagram

Loop constructs
Do while loop While loop For loop Foreach loop Infinite loop Control flow
v t e

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 all of their following in 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 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 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.

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
}

Demonstrating while loops

These while loops will calculate the factorial of the number 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)

The Wikibook Ada_Programming has a page on the topic of: Control

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;

APL

Further information: APL (programming language)

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

or simply

!5

AutoHotkey

Further information: AutoHotkey

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

Small Basic

Further information: Microsoft Small Basic

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

Visual Basic

Further information: Visual Basic

DimcounterAsInteger=5' init variable and set value
DimfactorialAsInteger=1' initialize factorial variable
DoWhilecounter>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

Further information: Bourne shell

counter=5
factorial=1
while[$counter-gt0];do
factorial=$((factorial*counter))
counter=$((counter-1))
done
echo$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)

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

Fortran

Further information: 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

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

MATLAB, Octave

Further information: MATLAB and GNU Octave

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

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;
VAR
Counter,Factorial:INTEGER;
BEGIN
Counter:=5;
Factorial:=1;
WHILECounter>0DO
Factorial:=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)

var
counter=5# Set counter value to 5
factorial=1# Set factorial value to 1
whilecounter>0:# While counter is greater than 0
factorial*=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. While repeats one statement (unless enclosed in a begin-end block) as long as the condition is true. The repeat statement repetitively executes a block of one or more statements through an until statement and continues repeating unless the condition is false. The main difference between the two is the while loop may execute zero times if the condition is initially false, the repeat-until loop always executes at least once.

programFactorial1;
var
Fv:integer;
procedurefact(counter:integer);
var
Factorial:integer;
begin
Factorial:=1;
whileCounter>0do
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): ');
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

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 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 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

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 5
i=1
factorial=1
whilei<=5
factorial*=i
i+=1
end
putsfactorial

Rust

Further information: Rust (programming language)

fn main(){
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].
Transcript show: factorial

Swift

Further information: Swift (programming language)

var counter = 5 // Set the initial counter value to 5
var factorial = 1 // Set the initial factorial 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

Further information: Tcl

setcounter5
setfactorial1
while{$counter>0}{
setfactorial[expr$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
$factorial = 1
while ($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) do
 F := 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

^ ^a ^b "The while and do-while Statements (The Java Tutorials > Learning the Java Language > Language Basics)". Dosc.oracle.com. Retrieved 2016年10月21日.
^ "while (C# reference)". Msdn.microsoft.com. Retrieved 2016年10月21日.
^ "Chapter 3: The While programming language" (PDF). Profs.sci.univr.it. Retrieved 2016年10月21日.
^ Flemming Nielson; Hanne R. Nielson; Chris Hankin (1999). Principles of Program Analysis. Springer. ISBN 978-3-540-65410-0 . Retrieved 29 May 2013.
^ Illingworth, Valerie (11 December 1997). Dictionary of Computing . Oxford Paperback Reference (4th ed.). Oxford University Press. ISBN 9780192800466.

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