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Loop constructs |
---|

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.

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
}
```

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

Further information: ActionScript 3.0 |

```
var counter: int = 5;
var factorial: int = 1;
while (counter > 1) {
factorial *= counter;
counter--;
}
Printf("Factorial = %d", factorial);
```

Further information: Ada (programming language) |

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;
```

Further information: APL (programming language) |

```
counter ← 5
factorial ← 1
:While counter > 0
factorial ×← counter
counter -← 1
:EndWhile
⎕ ← factorial
```

or simply

```
!5
```

Further information: AutoHotkey |

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

Further information: 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
```

Further information: 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
```

Further information: Bourne shell |

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

Further information: C (programming language) and C++ |

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

Further information: ColdFusion Markup Language |

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

Further information: Tag (programming) |

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

Further information: 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
```

Further information: Go (programming language) |

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
}
```

Further information: Java (programming language), C Sharp (programming language), and 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--;
```

Further information: JavaScript |

```
let counter = 5;
let factorial = 1;
while (counter > 1)
factorial *= counter--;
console.log(factorial);
```

Further information: Lua (programming language) |

```
counter = 5
factorial = 1
while counter > 0 do
factorial = factorial * counter
counter = counter - 1
end
print(factorial)
```

Further information: MATLAB and GNU Octave |

```
counter = 5;
factorial = 1;
while (counter > 0)
factorial = factorial * counter; %Multiply
counter = counter - 1; %Decrement
end
factorial
```

Further information: Wolfram Mathematica and Wolfram Language |

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

Further information: Oberon (programming language), Oberon-2, Oberon-07, and 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.
```

Further information: 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");
}
```

Further information: Nim (programming language) |

```
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!"
```

Further information: Pascal (programming language) |

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.
```

Further information: 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;
```

Further information: PHP |

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

Further information: PL/I |

```
declare counter fixed initial(5);
declare factorial fixed initial(1);
do while(counter > 0)
factorial = factorial * counter;
counter = counter - 1;
end;
```

Further information: Python (programming language) |

```
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!")
```

Further information: Racket (programming language) and 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.

Further information: Ruby (programming language) |

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

Further information: Rust (programming language) |

```
fn main() {
let mut counter = 5;
let mut factorial = 1;
while counter > 1 {
factorial *= counter;
counter -= 1;
}
println!("{}", factorial);
}
```

Further information: 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
```

Further information: Swift (programming language) |

```
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.
```

Further information: Tcl |

```
set counter 5
set factorial 1
while {$counter > 0} {
set factorial [expr $factorial * $counter]
incr counter -1
}
puts $factorial
```

Further information: VEX prefix |

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

Further information: PowerShell |

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

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;
```