While loop

{{Short description|Control flow statement for repeating execution until a condition is met}}

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.{{cite web|url=http://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html|title=The while and do-while Statements (The Java Tutorials > Learning the Java Language > Language Basics)|website=Dosc.oracle.com|access-date=2016-10-21}} 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#,{{cite web |url=http://msdn.microsoft.com/en-us/library/2aeyhxcd.aspx |title=while (C# reference)|website=Msdn.microsoft.com|access-date=2016-10-21}} 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.

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=

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;

=APL=

counter ← 5

factorial ← 1

:While counter > 0

factorial ×← counter

counter -← 1

:EndWhile

⎕ ← factorial

or simply

=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() {

int count = 5;

int factorial = 1;

while (count > 1)

factorial *= count--;

printf("%d", factorial);

}

=ColdFusion Markup Language (CFML)=

==Script syntax==

counter = 5;

factorial = 1;

while (counter > 1) {

factorial *= counter--;

}

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 {

counter, factorial = counter-1, factorial*counter

}

=Java, C#, D=

{{Further|Java (programming language)|C Sharp (programming language)|D (programming language)}}

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[{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|Oberon (programming language)|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) {

$multiplication = $factorial * $counter;

$counter -= 1;

print("Counter is: " + $counter + ", multiplication is: " + $multiplication + "\n");

}

=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) {

$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, "

while () {

print;

}

close IN;

=PHP=

$counter = 5;

$factorial = 1;

while ($counter > 0) {

$factorial *= $counter--; // Multiply, then decrement.

}

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=

{{Further|Racket (programming language)|Scheme (programming language)}}

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() {

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 > 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);

=PowerShell=

$counter = 5

$factorial = 1

while ($counter) {

$factorial *= $counter--

}

$factorial

=While (language)=

While{{cite web|url=http://profs.sci.univr.it/~merro/files/WhileExtra_l.pdf |title=Chapter 3: The While programming language |website=Profs.sci.univr.it |access-date=2016-10-21}} is a simple programming language constructed from assignments, sequential composition, conditionals, and while statements, used in the theoretical analysis of imperative programming language semantics.{{cite book|author1=Flemming Nielson|author2=Hanne R. Nielson|author3=Chris Hankin|title=Principles of Program Analysis|url=https://books.google.com/books?id=RLjt0xSj8DcC|access-date=29 May 2013|year=1999|publisher=Springer|isbn=978-3-540-65410-0}}{{cite book |first=Valerie |last=Illingworth |date=11 December 1997 |title=Dictionary of Computing |edition=4th |series=Oxford Paperback Reference |publisher=Oxford University Press |isbn=9780192800466 |url-access=registration |url=https://archive.org/details/dictionaryofcomp00illi}}

C := 5;

F := 1;

while (C > 1) do

F := F * C;

C := C - 1;