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 has a "repeat until" loop, which continues to run until the control expression is true (and then terminates) — whereas a "while" loop runs while the control expression is true (and terminates once the expression becomes false).
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do { do_work(); } while (condition);
is equivalent to
do_work(); while (condition) { do_work(); }
which (as long as the continue statement is not used) is technically equivalent to the following (though these examples are not typical or modern style):
while (true) { do_work(); if (!condition) break; }
or
LOOPSTART: do_work(); if (condition) goto LOOPSTART;
These example programs calculate the factorial of 5 using their respective languages' syntax for a do-while loop.
var counter:int = 5; var factorial:int = 1; do { factorial *= counter--; /* Multiply, then decrement. */ } while (counter > 0); trace(factorial);
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 Factorial;
#! /bin/bash counter=5 factorial=1 factorial=$(($factorial*$counter)) counter=$(($counter-1)) while [ $counter -gt 0 ]; do factorial=$(($factorial*$counter)) counter=$(($counter-1)) done echo $factorial
Note: Bash only does integer mathematics. For real numbers, try piping output through awk or bc.
unsigned int counter = 5; unsigned long factorial = 1; do { factorial *= counter--; /* Multiply, then decrement. */ } while (counter > 0); printf("%lu\n", factorial);
A do while loop may be employed in macros since they shouldn't provide the final semicolon (;).[1] Thus a multi-statement macro such as swap
where the first parameter is the type can be executed only once using a do while loop where the condition is 0:
#define SWAP(type, x, y) do {type tmp=x; x=y; y=tmp;} while (0)
int counter = 5; int factorial = 1; do { factorial *= counter--; /* Multiply, then decrement. */ } while (counter > 0); System.Console.WriteLine(factorial);
program FactorialProg integer :: counter = 5 integer :: factorial = 1 do factorial = factorial * counter counter = counter - 1 if (counter == 0) exit end do print *, factorial end program FactorialProg
With Fortran 90 and later, we use the equivalent construct mentioned above. Using it is better practice, since it is immediately apparent that this is a while-loop.
program FactorialProg integer :: counter = 5 integer :: factorial = 1 factorial = factorial * counter counter = counter - 1 do while (counter > 0) factorial = factorial * counter counter = counter - 1 end do print *, factorial end program FactorialProg
int counter = 5; int factorial = 1; do { factorial *= counter--; /* Multiply, then decrement. */ } while (counter > 0); System.out.println(factorial);
var counter = 5; var factorial = 1; do { factorial *= counter--; } while (counter > 0); document.body.appendChild(document.createTextNode(factorial));
$counter = 5; $factorial = 1; do { $factorial *= $counter--; } while ($counter > 0); print $factorial;
<?php $counter = 5; $factorial = 1; do { $factorial *= $counter--; } while ($counter > 0); echo $factorial; ?>
In Racket, as in other Scheme implementations, a "named-let" is a popular way to implement loops:
Compare this with the first example of the while loop example for Racket.
counter = 5 factorial = 1 begin factorial *= counter counter -= 1 end while counter > 0 puts factorial
| counter factorial | counter := 5. factorial := 1. [counter > 0] whileTrue: [factorial := factorial * counter. counter := counter - 1]. Transcript show: factorial printString
Dim counter As Integer = 5 Dim factorial As Integer = 1 Do factorial *= counter counter -= 1 Loop While counter > 0 Console.WriteLine(factorial)