Do while loop


In most computer programming languages, a do while loop is a control flow statement that executes a block of code at least once, and then either repeatedly executes the block, or stops executing it, depending on a given boolean condition at the end of the block.
The do while construct consists of a process symbol 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, the do-while loop is an exit-condition loop. This means that the code must always be executed first and then the expression or test condition is evaluated. If it is true, the code executes the body of the loop again. This process is repeated as long as the expression evaluates to true. If the expression is false, the loop terminates and control transfers to the statement following the do-while loop. In other words, whereas a while loop sets the truth of a statement as a condition precedent for the code's execution, a do-while loop provides for the action's ongoing execution subject to defeasance by the condition's falsity, which falsity is set as a condition subsequent.
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 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 — whereas a "while" loop runs while the control expression is true.

Equivalent constructs


do while ;

is equivalent to

do_work;
while

In this manner, the do... while loop saves the initial "loop priming" with do_work; on the line before the while loop.
As long as the continue statement is not used, the above is technically equivalent to the following :

while

or

LOOPSTART:
do_work;
if goto LOOPSTART;

Demonstrating do while loops

These example programs calculate the factorial of 5 using their respective languages' syntax for a do-while loop.

ActionScript 3">ActionScript">ActionScript 3


var counter: int = 5;
var factorial: int = 1;
do while ;
trace;

Ada">Ada (programming language)">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 ;
end Factorial;

[BASIC]

Early BASICs used the syntax WHILE/WEND. Modern BASICs such as PowerBASIC provide both WHILE/WEND and DO/LOOP structures, with syntax such as DO WHILE/LOOP, DO UNTIL/LOOP, DO/LOOP WHILE, DO/LOOP UNTIL, and DO/LOOP. Typical BASIC source code:

Dim factorial As Integer
Dim counter As Integer
factorial = 1
counter = 5
Do
factorial = factorial * counter
counter = counter - 1
Loop While counter > 0
Print factorial

[Bourne Again Shell]


until condition ; do
#statements
done
  1. does the same.
while condition ; do
#statements
done

C#">C Sharp (programming language)">C#


int counter = 5;
int factorial = 1;
do while ;
System.Console.WriteLine;

C">C (programming language)">C


int counter = 5;
int factorial = 1;
do while ;
printf;

Do-while statements are also commonly used in C macros as a way to wrap multiple statements into a regular statement. It makes a semicolon needed after the macro, providing a more function-like appearance for simple parsers and programmers as well as avoiding the scoping problem with. It is recommended in CERT C Coding Standard rule PRE10-C.

[C++]


int counter = 5;
int factorial = 1;
do while ;
std::cout << "factorial of 5 is "<< factorial << std::endl;

[CFScript]


factorial = 1;
count = 10;
do while ;
writeOutput;

D">D (programming language)">D


int counter = 5;
int factorial = 1;
do while ;
writeln;

[Fortran]

With legacy FORTRAN 77 there is no DO-WHILE construct but the same effect can be achieved with GOTO:

INTEGER CNT,FACT
CNT=5
FACT=1
1 CONTINUE
FACT=FACT*CNT
CNT=CNT-1
IF GOTO 1
PRINT*,FACT
END

With Fortran 90 and later, the do-while loop is actually the same as the for loop.

program FactorialProg
integer :: counter = 5
integer :: factorial = 1

factorial = factorial * counter
counter = counter - 1

do while
factorial = factorial * counter
counter = counter - 1
end do

print *, factorial
end program FactorialProg

Java">Java (programming language)">Java


int counter = 5;
int factorial = 1;
do while ;
System.out.println;
//

//
// The below function does the same as above. //
//

//
int counter = 5;
int factorial = 1;
while
System.out.println;

[JavaScript]


var counter = 5; // Declaring two variables, counter and factorial
var factorial = 1;
do while ; //The looping conditions
console.log; //Showing the result

Kotlin">Kotlin (programming language)">Kotlin


var counter = 5
var factorial = 1
//These line of code is almost the same as the above JavaScript codes, the only difference is the keyword that shows the results
do while
println

Pascal

does not have a do/while; instead, it has a repeat/until. As mentioned in the introduction, one can consider a repeat/until to be equivalent to a 'do code while not expression' construct.

factorial := 1;
counter := 5;
repeat
factorial := factorial * counter;
counter := counter - 1; // In Object Pascal one may use dec ;
until counter = 0;

[PHP]


$counter = 1;
$factorial = 1;
$loop = 5;
do
while ;

[PL/I]

The PL/I DO statement subsumes the functions of the post-test loop, the pre-test loop, and the for loop. All functions can be included in a single statement. The example shows only the "do until" syntax.

declare counter fixed initial;
declare factorial fixed initial;
do until;
 factorial = factorial * counter;
 counter = counter - 1;
end;
put;

Python">Python (programming language)">Python

Python lacks a specific do while flow control construct. However, the equivalent may be constructed out of a while loop with a break.

counter = 5
factorial = 1
while True:
factorial *= counter
counter -= 1

if counter 0:
break

print

Racket">Racket (programming language)">Racket

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

  1. lang racket


))

Compare this with the first example of the while loop example for Racket. Be aware that a named let can also take arguments.
Racket and Scheme also provide a proper do loop.


; Stop condition and return value.
; The body of the do-loop is empty.
))

Ruby">Ruby (programming language)">Ruby


counter = 10
factorial = 2
begin
factorial *= counter
counter -= 2
end while counter > 1
puts factorial

[Smalltalk]


counter := 5.
factorial := 1.
whileTrue:
.
Transcript show: factorial printString

Swift">Swift (programming language)">Swift

Swift 2.x:

var counter = 5
var factorial = 1
repeat while counter > 0
print

Swift 1.x:

var counter = 5
var factorial = 1
do while counter > 0
println

[Visual Basic .NET]


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