Do while loop

do {
    do_work();
} while (condition);

is equivalent to

do_work();

while (condition) {
    do_work();
}

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) {
   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.

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;

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

int counter = 5;
int factorial = 1;

do {
    factorial *= counter--; /* Multiply, then decrement. */
} while (counter > 0);

printf("factorial of 5 is %d\n", 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 if. It is recommended in CERT C Coding Standard rule PRE10-C.^[1]

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 does not have a do-while construct either, but it does have a while loop construct which uses the keywords "do while" and is thus actually the same as the for loop.^[2]

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

    factorial = factorial * counter
    counter = counter - 1

    do while (counter > 0) ! Truth value is tested before the loop
        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("The factorial of 5 is " + factorial);

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;

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 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)

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.
    ))

| counter factorial |
counter := 5.
factorial := 1.

[counter > 0] whileTrue:
    [factorial := factorial * counter.
    counter := counter - 1].

Transcript show: factorial printString

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

 

The Wikibook Ada Programming has a page on the topic of: Control

• do {...} while (0) in C macros