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. The condition is evaluated, and if the condition is true, the code within the block is executed. This repeats until the condition becomes false. Because while loops check the condition before the block is executed, the control structure is often also known as a pretest loop. Compare with the do while loop, which tests the condition after the loop has executed.
For example, in the C programming language (as well as Java and C++, which use the same syntax in this case), the code fragment
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.
Contents 
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;
Also, in C and its descendants, a while loop is a for loop with no initialization or counting expressions, i.e.,
for ( ; condition; ) { statements; }
These while loops will calculate the factorial of the number 5:
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 Main;
counter=5 factorial=1 while [ $counter gt 0 ]; do factorial=$((factorial * counter)) counter=$((counter  1)) done echo $factorial
' Initialize the variables Dim counter As Integer : counter = 5 Dim factorial As Long : factorial = 1 Do While counter > 0 factorial = factorial * counter ' Multiply counter = counter  1 ' Decrement Loop Print factorial ' Prints out the result.
unsigned int counter = 5; unsigned long factorial = 1; while (counter > 0) { factorial *= counter; /* Multiply and decrement */ } printf("%lu", factorial);
The code for the loop is the same for Java, C# and D:
int counter = 5; long factorial = 1; while (counter > 1) { factorial *= counter; }
For Java the result is printed as follows:
System.out.println(factorial);
The same in C#
System.Console.WriteLine(factorial);
And finally in D
writefln(factorial);
var counter = 5; var factorial = 1; while ( counter > 1 ) { factorial *= counter; } document.body.appendChild(document.createTextNode(factorial));
counter = 5 factorial = 1 while counter > 0 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 factorial
Block[{counter=5,factorial=1}, (*localize counter and factorial*) While[counter>0, (*While loop*) factorial*=counter; (*Multiply*) counter; (*Decrement*) ]; factorial ]
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.
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.
my $counter = 5; my $factorial = 1; while ( $counter > 0 ) { $factorial *= $counter; # Multiply, then decrement } print $factorial;
Very similar to C and C++, but the while loop could also have been written on one line:
$factorial *= $counter while $counter > 0;
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 > 0) { $factorial *= $counter; // Multiply first. $counter; // then decrement. } print $factorial;
counter = 5 factorial = 1 while counter > 0: factorial *= counter counter = 1 print factorial
Contrary to other languages, in Smalltalk a while loop is not a language construct but defined in the class BlockClosure
as a method with one parameter, the body as a closure, using self as the condition.
 count factorial  count := 5. factorial := 1. [ count > 0 ] whileTrue: [ factorial := factorial * (count := count  1) ] Transcript show: factorial
set counter 5 set factorial 1 while {$counter > 0} { set factorial [expr $factorial * $counter] incr counter 1 } puts $factorial
$counter = 5 $factorial = 1 while ($counter gt 0) { $factorial *= $counter # Multiply, then decrement. } WriteOutput $factorial
