While loop: Wikis

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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 pre-test 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

Equivalent constructs

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;
}

Demonstrating while loops

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

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Ada

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;

Bash

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

QBasic or Visual Basic

' 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.

C or C++

unsigned int counter = 5;
unsigned long factorial = 1;
 
while (counter > 0)
{
  factorial *= counter--;    /* Multiply and decrement */
} 
 
printf("%lu", factorial);

Java, C#, D

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);

JavaScript

var counter = 5;
var factorial = 1;
 
while ( --counter > 1 )
{
  factorial *= counter;
}
 
document.body.appendChild(document.createTextNode(factorial));

Lua

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

MATLAB

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

Mathematica

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

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.

Oberon, Oberon 2, 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.

Perl

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;

PHP

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

Python

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

| count factorial |
count := 5.
factorial := 1.
[ count > 0 ] whileTrue: 
    [ factorial := factorial * (count := count - 1) ]
Transcript show: factorial

Tcl (Tool command language)

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

Windows PowerShell

$counter = 5
$factorial = 1
while ($counter -gt 0) {
  $factorial *= $counter-- # Multiply, then decrement.
}
Write-Output $factorial

See also


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