From Wikipedia, the free encyclopedia

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 first evaluated - if the condition is true the code within the block is then 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 pre-test loop. Compare with the do while loop, which tests the condition after the loop has executed.

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, in the C programming language, the code fragment

x = 0;
while (x < 3)
{
   x++;
}

first checks whether x is larger than 3, which it is not, so it increments x by 1. It then checks the condition again, and executes again, repeating this process until the variable x has the value 3.

Contents 1 Demonstrating while loops 1.1 QBasic or Visual Basic 1.2 REALbasic 1.3 C or C++ 1.4 Perl 1.5 PHP 1.6 Tcl (Tool command language) 1.7 Java, Csharp 1.8 Pascal 1.9 Smalltalk 1.10 Python 1.11 AutoIt 2 See also

These while loops will calculate the factorial of a number:

 'Initialise the variables
 Dim counter as Integer : counter = 5
 Dim factorial as Long : factorial = 1
 Do While counter > 0
   factorial = factorial * counter     'Multiplei
   counter = counter - 1               'Decrement
 Loop
 Print factorial                       'Prints out the result.

 Dim counter as Integer = 5
 Dim factorial as Integer = 1
 While counter > 0
   factorial = factorial * counter     //Multiply
   counter = counter - 1               //Decrement
 Wend
 MsgBox Str( factorial )               // Prints out the result.

 unsigned int counter = 5;
 unsigned long factorial = 1;
 while (counter > 0)
 {
   factorial *= counter--; /*Multiply, then decrement.*/
 } 
 printf("%i", 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, " )
 {
   print;
 }
 close IN;

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

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

The code for the loop is the same for Java and Csharp:

 int counter = 5;
 long factorial = 1;
 while (counter > 0)
    factorial *= counter--; // Multiply, then decrement.

For Java the result is printed as follows:

 System.out.println(factorial);

The same in C#

 System.Console.WriteLine(factorial);

 program Factorial
 var
   Counter, Factorial: integer;
 begin
   Counter := 5;
   Factorial := 1;
   while Counter > 0 do begin
     Factorial := Factorial * Counter;
     Counter := Counter - 1;
   end;
   Write(Factorial);
 end.

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) ]

 counter = 5
 factorial = 1
 while counter>0:
       factorial *= counter
       counter -= 1
 print factorial

 Dim $counter = 5, $factorial = 1
 While $counter > 0
    $factorial *= $counter
    $counter -= 1
 WEnd
 MsgBox(0,"Factorial", $factorial)

• For loop