Do while loop

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In most computer programming languages, a do while loop, sometimes just called a do loop, is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. Note though that unlike most languages, Fortran's do loop is actually analogous to the for loop.

The do while construct consists of a block of code and a condition. First, the code within the block is executed, and then the condition is evaluated. If the condition is true the code within the block is executed again. This repeats until the condition becomes false. Because do while loops check the condition after the block is executed, the control structure is often also known as a post-test loop. Contrast with the while loop, which tests the condition before the code within the block is executed.

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 allows termination of the loop.

Some languages may use a different naming convention for this type of loop. For example, the Pascal language refers to it as a "repeat until" loop. Note that in Pascal, the loop will continue to run until the control expression is "true" as opposed to other languages where the loop terminates when the control expression is "false".

For example, in the C programming language, the code fragment:

x = 0;
do {
   x = x + 1;
} while (x < 3);

first executes the instruction x = x + 1, which results in x = 1. It then checks the condition x < 3, which is true, so it executes the code block again. It repeats this execute and check process until the variable x has the desired value, 3.

Contents

[edit] Equivalent Constructs

do {
   statements;
} while (condition);

is equivalent to

while (true) {
   statements;
   if (!condition) break;
}

or

LOOPSTART:
   statements;
   if (condition) goto LOOPSTART;

[edit] Demonstrating do while loops

These do while loops will calculate the factorial of a number:

[edit] Ada

Wikibooks
Wikibooks' Ada_Programming has more about this subject: 
with Ada.Integer_Text_IO;
 
procedure Factorial is
  Counter   : Integer := 5;
  Factorial : Integer := 1;
begin
  loop
    Factorial := Factorial * Counter;
    Counter   := Counter - 1;
    exit when Counter = 0;
  end loop;
 
  Ada.Integer_Text_IO.Put (Factorial);
end Main;

[edit] ActionScript

var counter:Number = 5;
var factorial:Number = 1;
do {
   factorial *= counter;
   counter --;
} while (counter > 0)
trace(factorial);

[edit] ALGOL 68

ALGOL 68 is unusual as typically the "DO ~ WHILE ~" is an adaptation of the "WHILE ~ DO ~ OD" loop clause.

PRAGMAT precision 101 PRAGMAT
INT counter:= 70;
  LONG LONG INT factorial := 1;
  WHILE # Multiply, then decrement.#
    factorial *:= counter;
    counter-:=1;
    counter > 0 # the result becomes the test condition #
  DO SKIP OD;
  printf(($" one googol:"101dl$, factorial))

Output:

one googol:11978571669969891796072783721689098736458938142546425857555362864628009582789845319680000000000000000

[edit] QBasic or Visual Basic

Dim counter As Integer : counter = 5
Dim factorial as Long : factorial = 1
Do
  factorial = factorial * counter 'multiply
  counter = counter - 1 'decrement
Loop While (counter > 0)
Print factorial


[edit] Visual Basic .NET

Dim counter As UInteger = 5
Dim factorial as ULong = 1
Do
  factorial *= counter 'multiply
  counter -= 1 'decrement
Loop While counter > 0
System.Console.WriteLine(factorial)

[edit] C or C++

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

[edit] C#

uint counter = 5;
ulong factorial = 1;
do
{
      factorial *= counter--;
}
while (counter > 0);
System.Console.WriteLine(factorial);

[edit] D

uint i = 0;
do {
    ++i;
    writefln(i);
} while(i < 5);

[edit] Java

int counter = 5;
long factorial = 1;
do {
  factorial *= counter--; //Multiply, then decrement.
} while (counter > 0);
System.out.println(factorial);

[edit] JavaScript

var counter = 5;
var factorial = 1;
do {
  factorial *= counter--; //Multiply, then decrement.
}
while (counter)
document.body.appendChild(document.createTextNode(factorial));

[edit] REALbasic

Dim counter As Integer = 5
Dim factorial as Integer = 1
Do
  factorial = factorial * counter // multiply
  counter = counter - 1 // decrement
While counter > 0
MsgBox Str( factorial )
Loop

[edit] FORTRAN 77

Program FACTORIALPROG
      Integer start
      Integer factorial,counter
      Data Start /5/
      Factorial = 1      
      Counter = Start
      do while(counter>0)
        Factorial = Factorial*counter
        Counter = Counter-1
      end do
      print *,Factorial
      End Program

[edit] Pascal

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

[edit] PHP

<?php
do {
    if ($i < 5) {
        echo "i is not big enough";
        break;
    }
    $i *= $factor;
    if ($i < $minimum_limit) {
        break;
    }
   echo "i is ok";
 
    /* process i */
 
} while (0);
?>

[edit] See also