# 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.

Loop constructs |
---|

Do while loop |

While loop |

For loop |

Foreach loop |

Infinite loop |

Control flow |

## Overview

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 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 constructs

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* loops

*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

The Wikibook Ada_Programming has a page on the topic of: Control |

```
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;
```

### AutoHotkey

```
counter := 5
factorial := 1
While counter > 0
factorial *= counter--
MsgBox % factorial
```

### Microsoft 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 or C++

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

#### Script syntax

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

#### Tag syntax

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

### 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
```

### 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 & GNU 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 (programming language), Oberon-07, or 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");
}
```

### Pascal

```
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.
```

### 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, "<test.txt";
while (<IN>) {
print;
}
close IN;
```

### PHP

```
$counter = 5;
$factorial = 1;
while ($counter > 0) {
$factorial *= $counter; // Multiply first.
$counter--; // then decrement.
}
print $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)
```

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

### Ruby

```
# Calculate the factorial of 5
i = 1
factorial = 1
while i <= 5
factorial *= i
i += 1
end
puts 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);
```

### Windows PowerShell

```
$counter = 5
$factorial = 1
while ($counter) {
$factorial *= $counter--
}
$factorial
```

### While programming language

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 also

- Do while loop
- For loop
- Foreach
- LOOP (programming language) – a programming language with the property that the functions it can compute are exactly the primitive recursive functions

## References

- "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. - Inner loop program construct: A faster way for program execution
- "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.