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While loop flow diagram

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.

Contents

OverviewEdit

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 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 C programming language (as well as 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.

Note that 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
}

Equivalent constructsEdit

In the C programming language,

while (condition) {
   statements;
}

is equivalent to

if (condition) {
    do {
        statements;
    } while (condition);
}

or

while (true) {
    if (!condition)
        break;
    
    statements;
}

or

    goto TEST;
LOOPSTART:
    statements;
TEST:
    if (condition)
        goto LOOPSTART;

or

TEST:
    if (!condition)
        goto LOOPEND;
    
    statements;
    goto TEST;
LOOPEND:

Those last two are not recommended because the use of "goto" statements makes it hard for a programmer to understand the flow of control, and is generally regarded as a last resort.

Also, in C and its descendants, a while loop is a for loop with no initialization or counting expressions, i.e.,

for (; condition;) {
    statements;
}

In addition, it has been shown that a loop with an inner for loop performs more computations per unit time than a loop without it or one with an inner while loop. This means the nested while loop will finish slower, given the same number of computations to perform, compared to the nested for loop.[3]

Demonstrating while loopsEdit

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

ActionScript 3Edit

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

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

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

AdaEdit

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;

AutoHotkeyEdit

counter := 5
factorial := 1

While counter > 0
    factorial *= counter--    ; Curly braces are optional for single-line blocks.

MsgBox % factorial

Microsoft Small BasicEdit

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 BasicEdit

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) shellEdit

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

echo $factorial

C or C++Edit

int main() {
    int counter = 5;
    long factorial = 1;
 
    while (counter > 1)
        factorial *= counter--;
    
    printf("%d.01", factorial);
}

Script syntaxEdit

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

writeOutput(factorial);

Tag syntaxEdit

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

FortranEdit

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

Java, C#, DEdit

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

int counter = 5;
long factorial = 1;

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

JavaScriptEdit

var counter = 5;
var factorial = 1;

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

console.log(factorial);

LuaEdit

counter = 5
factorial = 1

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

print(factorial)

MATLABEdit

counter = 5;
factorial = 1;

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

factorial

MathematicaEdit

Block[{counter=5,factorial=1},  (*localize counter and factorial*)
    While[counter>0,            (*While loop*)
        factorial*=counter;     (*Multiply*)
        counter--;              (*Decrement*)
    ];

    factorial
]

Oberon, Oberon-2 (programming language), Oberon-07, or Component PascalEdit

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 LanguageEdit

int $counter = 5;
int $factorial = 1;

int $multiplication;

while ($counter > 0) {
    $multiplication = $factorial * $counter;

    $counter -= 1;
    
    print("Counter is: " + $counter + ", multiplication is: " + $multiplication + "\n");
}

PascalEdit

program Factorial1;
var
    Counter, Factorial: integer;
begin
    Counter := 5;
    Factorial := 1;
    
    while Counter > 0 do
    begin
        Factorial := Factorial * Counter;
        Counter := Counter - 1
    end;

    WriteLn(Factorial)
end.

PerlEdit

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;

PHPEdit

$counter = 5;
$factorial = 1;

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

print $factorial;

PL/IEdit

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

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

PythonEdit

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!")

RacketEdit

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)

But note that an imperative programming style is often discouraged in Racket (as in Scheme).

RubyEdit

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

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

puts factorial

SmalltalkEdit

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

SwiftEdit

var counter = 5                 // Set the value to 5 
var 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 factorial x counter.
    counter -= 1                // Set the new value of counter to  counter - 1.
}

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

TclEdit

set counter 5
set factorial 1

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

puts $factorial

VEXEdit

int counter = 5;
int factorial = 1;

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

printf("%d", factorial);

Windows PowerShellEdit

$number = 5
$counter = $number
$factorial = 1

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

$factorial

While programming languageEdit

The While programming language[4] is a simple programming language constructed from assignments, sequential composition, conditionals and while statements, used in the theoretical analysis of imperative programming language semantics.[5][6]

C := 5;
F := 1;

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

See alsoEdit

ReferencesEdit

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