Do while loop

In many computer programming languages, a do while loop is a control flow statement that executes a block of code and then either repeats the block or exits the loop depending on a given boolean condition.

The ''do while'' construct consists of a process symbol and a condition. First the code within the block is executed. 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.

Do while loops check the condition after the block of code is executed. This control structure can be known as a post-test loop. This means the do-while loop is an exit-condition loop. However a while loop will test the condition before the code within the block is executed.

This means that the code is always executed first and then the expression or test condition is evaluated. This process is repeated as long as the expression evaluates to true. If the expression is false the loop terminates. A while loop sets the truth of a statement as a necessary condition for the code's execution. A do-while loop provides for the action's ongoing execution until the condition is no longer true.

It is possible and sometimes desirable for the condition to always evaluate to be true. This creates an infinite loop. When an infinite loop is created intentionally there is usually another control structure that allows termination of the loop. For example, a break statement would allow termination of an infinite loop.

Some languages may use a different naming convention for this type of loop. For example, the Pascal and Lua languages have a "''repeat until''" loop, which continues to run ''until'' the control expression is true and then terminates. In contrast a "while" loop runs ''while'' the control expression is true and terminates once the expression becomes false.

Equivalent constructs

do while (condition);

is equivalent to

do_work();

while (condition)

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 (though these examples are not typical or modern style used in everyday computers):

while (true)

or

LOOPSTART:
do_work();
if (condition) goto LOOPSTART;

Demonstrating do while loops

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

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;

BASIC

Early BASICs (such as GW-BASIC) 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 (without outer testing, but with a conditional EXIT LOOP somewhere inside the 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

C, C++, D

int counter = 5;
int factorial = 1;

do while (counter > 0);

std::println("factorial of 5 is ", factorial);

Do-while(0) statements are also commonly used in C macros as a way to wrap multiple statements into a regular (as opposed to compound) 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.

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 (CNT.GT.0) GOTO 1
PRINT*,FACT
END

Fortran 90 and later supports a DO-While construct:

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

Java

int counter = 5;
int factorial = 1;

do while (counter > 0);

System.out.println("The factorial of 5 is " + factorial);

Pascal

Pascal uses repeat/until syntax instead of do while.

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

PL/I

The PL/I DO statement subsumes the functions of the post-test loop (''do until''), the pre-test loop (''do while''), 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(5);
declare factorial fixed initial(1);

do until(counter <= 0);
factorial = factorial * counter;
counter = counter - 1;
end;

put(factorial);

Python

Python does not have a DO-WHILE loop, but its effect can be achieved by an infinite loop with a breaking condition at the end.
factorial = 1
counter = 5

while True:
factorial *= counter
counter -= 1
if counter < 1:
break

print(factorial)

Racket

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

#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. Be aware that a named let can also take arguments.

Racket and Scheme also provide a proper do loop.

(define (factorial n)
(do ((counter n (- counter 1))
(result 1 (* result counter)))
((= counter 0) result) ; Stop condition and return value.
; The body of the do-loop is empty.
))

Smalltalk

, counter factorial ,
counter := 5.
factorial := 1.

ounter > 0whileTrue:
actorial := factorial * counter.
counter := counter - 1

Transcript show: factorial printString

See also

* Control flow
* For loop
* Foreach loop
* Repeat loop (disambiguation)
* While loop

References

External links

do while (0) in C macros

{{DEFAULTSORT:Do While Loop
Control flow
Iteration in programming
Programming language comparisons

Articles with example Ada code
Articles with example BASIC code
Articles with example C code
Articles with example C++ code
Articles with example D code
Articles with example Fortran code
Articles with example Java code
Articles with example Pascal code
Articles with example Racket code
Articles with example Smalltalk code

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