In most computer programming

Computer programming

Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...

 languages, a do while loop, sometimes just called a do loop, is a control flow

Control flow

In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....

 statement

Statement (programming)

In computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program written in such a language is formed by a sequence of one or more statements. A statement will have internal components .Many languages In computer programming...

 that allows code to be executed repeatedly based on a given Boolean

Boolean datatype

In computer science, the Boolean or logical data type is a data type, having two values , intended to represent the truth values of logic and Boolean algebra...

 condition. Note though that unlike most languages, Fortran

Fortran

Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...

's do loop is actually analogous to the for loop

For loop

In computer science a for loop is a programming language statement which allows code to be repeatedly executed. A for loop is classified as an iteration statement....

.

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

True

True may refer to:* Truth, the state of being in accord with fact or reality-Music:* True , 1996* True , 2002* True , 1983** "True"...

 the code within the block is executed again. This repeats until the condition becomes false

False

False or falsehood may refer to:*False *Lie or falsehood, a type of deception in the form of an untruthful statement*Falsity or falsehood, in law, deceitfulness by one party that results in damage to another...

. 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

While loop

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....

, 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

Infinite loop

An infinite loop is a sequence of instructions in a computer program which loops endlessly, either due to the loop having no terminating condition, having one that can never be met, or one that causes the loop to start over...

. 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).

Equivalent constructs

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;

Demonstrating do while loops

These example programs calculate the factorial

Factorial

In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n...

 of 5 using their respective languages' syntax for a do-while loop.

ActionScript 3

var counter:int = 5;
var factorial:int = 1;
do {
factorial *= counter--; /* Multiply, then decrement. */
} while (counter > 0);
trace(factorial);

Ada

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;

Bash

1. ! /bin/bash

counter=5
factorial=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

Bc programming language

bc, for bench calculator, is "an arbitrary precision calculator language" with syntax similar to the C programming language. bc is typically used as either a mathematical scripting language or as an interactive mathematical shell....

.

C or C++

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

C preprocessor

The C preprocessor is the preprocessor for the C and C++ computer programming languages. The preprocessor handles directives for source file inclusion , macro definitions , and conditional inclusion ....

 since they shouldn't provide the final semicolon . Thus a multi-statement macro such as swap where the first parameter is the type

Data type

In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...

 can be executed only once using a do while loop where the condition is 0:

1. define SWAP(type, x, y) do{type tmp=x; x=y; y=tmp;} while(0)

C#

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

Fortran

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, the following is also valid. Using it is better practice, since it is immediately apparent that this is a while-loop.

program FactorialProg
integer :: counter = 5
integer :: factorial = 1
do while (counter > 0)
factorial = factorial * counter
counter = counter - 1
end do
print *, factorial
end program FactorialProg

Java

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

JavaScript

var counter = 5;
var factorial = 1;
do {
factorial *= counter--; /* Multiply, then decrement. */
} while (counter > 0);
document.body.appendChild(document.createTextNode(factorial));
}

Perl

$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;
?>

Racket 

In Racket, as in other Scheme implementations, a "named-let" is a popular way to implement loops:

1. lang racket

(define counter 5)
(define factorial 1)
(let loop
(set! factorial (* factorial counter))
(set! counter (sub1 counter))
(when (> counter 0) (loop)))
(displayln factorial)

Compare this with the first example of the while loop example for Racket.

Ruby

counter = 5
factorial = 1
begin
factorial *= counter
counter -= 1
end while counter > 0
puts factorial

Smalltalk

| counter factorial |
counter := 5.
factorial := 1.
[counter > 0] whileTrue:
[factorial := factorial * counter.
counter := counter - 1].
Transcript show: factorial printString

Visual Basic.Net

Dim counter As Integer = 5
Dim factorial As Integer = 1
Do
factorial *= counter
counter -= 1
Loop While counter > 0
Console.WriteLine(factorial)