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 edit

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

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 edit

These while loops will calculate the factorial of the number 5:

ActionScript 3 edit

var counter: int = 5;
var factorial: int = 1;

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

Printf("Factorial = %d", factorial);

Ada edit

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 edit

counter ← 5
factorial ← 1

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

⎕ ← factorial

or simply

!5

AutoHotkey edit

counter := 5
factorial := 1

While counter > 0
    factorial *= counter--

MsgBox % factorial

Small Basic edit

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 edit

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 edit

counter=5
factorial=1
while [ $counter -gt 0 ]; do
    factorial=$((factorial * counter))
    counter=$((counter - 1))
done

echo $factorial

C, C++ edit

int main() {
    int count = 5;
    int factorial = 1;

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

    printf("%d", factorial);
}

ColdFusion Markup Language (CFML) edit

Script syntax edit

counter = 5;
factorial = 1;

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

writeOutput(factorial);

Tag syntax edit

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

Fortran edit

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 edit

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 edit

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 edit

let counter = 5;
let factorial = 1;

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

console.log(factorial);

Lua edit

counter = 5
factorial = 1

while counter > 0 do
  factorial = factorial * counter
  counter = counter - 1
end

print(factorial)

MATLAB, Octave edit

counter = 5;
factorial = 1;

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

factorial

Mathematica edit

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 edit

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 edit

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 edit

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 edit

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 edit

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, "<test.txt";

while (<IN>) {
    print;
}

close IN;

PHP edit

$counter = 5;
$factorial = 1;

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

echo $factorial;

PL/I edit

declare counter   fixed initial(5);
declare factorial fixed initial(1);

do while(counter > 0)
    factorial = factorial * counter;
    counter = counter - 1;
end;

Python edit

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 edit

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 edit

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

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

puts factorial

Rust edit

fn main() {
    let mut counter = 5;
    let mut factorial = 1;

    while counter > 1 {
        factorial *= counter;
        counter -= 1;
    }

    println!("{}", factorial);
}

Smalltalk edit

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 edit

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 edit

set counter 5
set factorial 1

while {$counter > 0} {
    set factorial [expr $factorial * $counter]
    incr counter -1
}

puts $factorial

VEX edit

int counter = 5;
int factorial = 1;

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

printf("%d", factorial);

PowerShell edit

$counter = 5
$factorial = 1

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

$factorial

While (language) edit

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;

References edit

^ ^a ^b "The while and do-while Statements (The Java Tutorials > Learning the Java Language > Language Basics)". Dosc.oracle.com. Retrieved 2016-10-21.
^ "while (C# reference)". Msdn.microsoft.com. Retrieved 2016-10-21.
^ "Chapter 3: The While programming language" (PDF). Profs.sci.univr.it. Retrieved 2016-10-21.
^ 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.

[:0-1] "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.

[NielsonNielson1999-4] 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.

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[5]

While loop

Overview edit

Demonstrating while loops edit

ActionScript 3 edit

Ada edit

APL edit

AutoHotkey edit

Small Basic edit

Visual Basic edit

Bourne (Unix) shell edit

C, C++ edit

ColdFusion Markup Language (CFML) edit

Script syntax edit

Tag syntax edit

Fortran edit

Go edit

Java, C#, D edit

JavaScript edit

Lua edit

MATLAB, Octave edit

Mathematica edit

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

Maya Embedded Language edit

Nim edit

Pascal edit

Perl edit

PHP edit

PL/I edit

Python edit

Racket edit

Ruby edit

Rust edit

Smalltalk edit

Swift edit

Tcl edit

VEX edit

PowerShell edit

While (language) edit

See also edit

References edit