This article needs additional citations for verification. (October 2016) |
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
editThe 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
editThese while loops will calculate the factorial of the number 5:
ActionScript 3
editvar 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
editcounter ← 5
factorial ← 1
:While counter > 0
factorial ×← counter
counter -← 1
:EndWhile
? ← factorial
or simply
!5
AutoHotkey
editcounter := 5
factorial := 1
While counter > 0
factorial *= counter--
MsgBox % factorial
Small Basic
editcounter = 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
editDim 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
editcounter=5
factorial=1
while [ $counter -gt 0 ]; do
factorial=$((factorial * counter))
counter=$((counter - 1))
done
echo $factorial
C, C++
editint main() {
int count = 5;
int factorial = 1;
while (count > 1)
factorial *= count--;
printf("%d", factorial);
}
ColdFusion Markup Language (CFML)
editScript syntax
editcounter = 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
editprogram 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
editGo 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
editThe code for the loop is the same for Java, C# and D:
int counter = 5;
int factorial = 1;
while (counter > 1)
factorial *= counter--;
JavaScript
editlet counter = 5;
let factorial = 1;
while (counter > 1)
factorial *= counter--;
console.log(factorial);
Lua
editcounter = 5
factorial = 1
while counter > 0 do
factorial = factorial * counter
counter = counter - 1
end
print(factorial)
MATLAB, Octave
editcounter = 5;
factorial = 1;
while (counter > 0)
factorial = factorial * counter; %Multiply
counter = counter - 1; %Decrement
end
factorial
Mathematica
editBlock[{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
editMODULE 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
editint $counter = 5;
int $factorial = 1;
int $multiplication;
while ($counter > 0) {
$multiplication = $factorial * $counter;
$counter -= 1;
print("Counter is: " + $counter + ", multiplication is: " + $multiplication + "\n");
}
Nim
editvar
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
editPascal 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
editmy $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
editdeclare counter fixed initial(5);
declare factorial fixed initial(1);
do while(counter > 0)
factorial = factorial * counter;
counter = counter - 1;
end;
Python
editcounter = 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
editIn 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
editfn main() {
let mut counter = 5;
let mut factorial = 1;
while counter > 1 {
factorial *= counter;
counter -= 1;
}
println!("{}", factorial);
}
Smalltalk
editContrary 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
editvar 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
editset counter 5
set factorial 1
while {$counter > 0} {
set factorial [expr $factorial * $counter]
incr counter -1
}
puts $factorial
VEX
editint counter = 5;
int factorial = 1;
while (counter > 1)
factorial *= counter--;
printf("%d", factorial);
PowerShell
edit$counter = 5
$factorial = 1
while ($counter) {
$factorial *= $counter--
}
$factorial
While (language)
editWhile[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
edit- 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
edit- ^ a b "The while and do-while Statements (The Java Tutorials > Learning the Java Language > Language Basics)". Dosc.oracle.com. Retrieved 2025-08-07.
- ^ "while (C# reference)". Msdn.microsoft.com. Retrieved 2025-08-07.
- ^ "Chapter 3: The While programming language" (PDF). Profs.sci.univr.it. Retrieved 2025-08-07.
- ^ 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.