While loop

A while consists of a block of code and a conditional expression.^[1] The conditional is evaluated, and if true,^[1] the block of code is executed. This repeats until the conditional becomes false. Because the while loop checks the conditional before the block is executed, the control structure is also known as a pre-test loop. In contrast, do-while loop tests the conditional after the block.

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 the loop body is entered, where printf() is called and x is incremented by 1. After completing the statements in the loop body, the condition, (x < 5), is checked again, and the loop is executed again. This process repeats until x has the value 5.

The condition can always valuate as true to create an infinite loop. In this case, there may be a early-exit 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 calculate the factorial of 5:

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

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

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

with Ada.Integer_Text_IO;

procedure Factorial is
    Counter   : Integer := 5;
    Factorial : Integer := 1;
begin
    while Counter > 1 loop
        Factorial := Factorial * Counter;
        Counter   := Counter - 1;
    end loop;

    Ada.Integer_Text_IO.Put (Factorial);
end Factorial;

counter ← 5
factorial ← 1

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

⎕ ← factorial

or simply

!5

counter := 5
factorial := 1

While counter > 1
    factorial *= counter--

MsgBox % factorial

counter = 5    ' Counter = 5
factorial = 1  ' initial value of variable "factorial"

While counter > 1
    factorial = factorial * counter
    counter = counter - 1
EndWhile

TextWindow.WriteLine("Factorial = " + factorial)

Dim counter As Integer = 5    ' init variable and set value
Dim factorial As Integer = 1  ' initialize factorial variable

Do While counter > 1
    factorial = factorial * counter
    counter = counter - 1
Loop     ' program goes here, until counter = 0

'Debug.Print factorial         ' Console.WriteLine(factorial) in Visual Basic .NET

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

echo $factorial

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

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

    printf("%d", factorial);
}

counter = 5;
factorial = 1;

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

writeOutput(factorial);

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

program FactorialProg
    integer :: counter = 5
    integer :: factorial = 1

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

    print *, factorial
end program FactorialProg

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
}

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

int counter = 5;
int factorial = 1;

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

let counter = 5;
let factorial = 1;

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

console.log(factorial);

counter = 5
factorial = 1

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

print(factorial)

counter = 5;
factorial = 1;

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

factorial

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

    factorial
]

MODULE Factorial;
IMPORT Out;
VAR
    Counter, Factorial: INTEGER;
BEGIN
    Counter := 5;
    Factorial := 1;

    WHILE Counter > 1 DO
        Factorial := Factorial * Counter;
        DEC(Counter)
    END;

    Out.Int(Factorial,0)
END Factorial.

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

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

print("Factorial = " + $factorial + "\n");

var
  counter = 5            # Set counter value to 5
  factorial = 1          # Set factorial value to 1

while counter > 1:       # 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 has two forms of the while loop, while and repeat-until. while repeats one statement (unless enclosed in a begin-end block) as long as the condition is true. repeat-until repetitively executes a block of one or more statements until the a condition is false. The main difference between the two is that while executes zero times if the condition is initially false, whereas repeat-until executes at least once.

program Factorial1;
var
    Fv: integer;

    procedure fact(counter:integer);
    var
        Factorial: integer;

    begin
         Factorial := 1;

         while Counter > 1 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.

my $counter   = 5;
my $factorial = 1;

while ($counter > 1) {
    $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;

$counter = 5;
$factorial = 1;

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

echo $factorial;

The PL/I DO statement can act as either a for loop, a while loop, or a do until loop.

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

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

counter = 5                           # Set the value to 5
factorial = 1                         # Set the value to 1

while counter > 1:                    # 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!")

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 1)
        (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 1)
    (set! factorial (* factorial counter))
    (set! counter (sub1 counter)))
(displayln factorial)

However, an imperative programming style is often discouraged in Scheme and Racket.

# Count Down Variant
counter = 5
factorial = 1

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

puts factorial

# Count Up Variant
counter = 2
factorial = 1

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

puts factorial

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

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

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

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

var counter = 5                 // Set the initial counter value to 5
var factorial = 1               // Set the initial factorial value to 1

while counter > 1 {             // 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.

set counter 5
set factorial 1

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

puts $factorial

int counter = 5;
int factorial = 1;

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

printf("%d", factorial);

$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;

• 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