While loop

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. The condition/expression is evaluated, and if the condition/expression is ''true'', 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#, Objective-C, and C++, (which use the same syntax in this case), the code fragment

int x = 0;

while (x < 5)

first checks whether x is less than 5, which it is, so then the 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)

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)

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;

APL

counter ← 5
factorial ← 1

:While counter > 0
factorial ×← counter
counter -← 1
:EndWhile

⎕ ← factorial

or simply

!5

AutoHotkey

counter := 5
factorial := 1

While counter > 0
factorial *= counter--

MsgBox % factorial

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

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, C++

int main()

ColdFusion Markup Language (CFML)

Script syntax

counter = 5;
factorial = 1;

while (counter > 1)

writeOutput(factorial);

Tag syntax

#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

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

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);

Lua

counter = 5
factorial = 1

while counter > 0 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 (*localize counter and factorial*)
While[counter>0, (*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 > 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)

Nim

var
counter = 5 # Set counter value to 5
factorial = 1 # Set factorial value to 1

while counter > 0: # 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. 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.

program Factorial1;
var
Fv: integer;

procedure fact(counter:integer);
var
Factorial: integer;

begin
Factorial := 1;

while Counter > 0 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 > 0)

print $factorial;

''While'' loops are frequently used for reading data line by line (as defined by the $/ line separator) from open filehandles:

open IN, ")

close IN;

PHP

$counter = 5;
$factorial = 1;

while ($counter > 0)

echo $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 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 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)

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

Ruby

# Calculate the factorial of 5
i = 1
factorial = 1

while i <= 5
factorial *= i
i += 1
end

puts factorial

Rust

fn main()

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.
ount > 0whileTrue:
actorial := 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 > 0

print(factorial) // Print the value of factorial.

Tcl

set counter 5
set factorial 1

while

puts $factorial

VEX

int counter = 5;
int factorial = 1;

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

printf("%d", factorial);

PowerShell

$counter = 5
$factorial = 1

while ($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;

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

{{DEFAULTSORT:While Loop
Control flow
Iteration in programming
Programming language comparisons

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