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.
| Loop constructs |
|---|
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 (i.e., the truth of the condition's negation) 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 (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();
}
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;
Demonstrating do while loops
These example programs calculate the factorial 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
 |
The Wikibook Ada_Programming has a page on the topic of: Control |
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#
int counter = 5;
int factorial = 1;
do {
factorial *= counter--; /* Multiply, then decrement. */
} while (counter > 0);
System.Console.WriteLine(factorial);
C
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]
C++
int counter = 5;
int factorial = 1;
do {
factorial *= counter--;
} while (counter > 0);
std::cout << "factorial of 5 is "<< factorial << std::endl;
CFScript
factorial = 1;
count = 10;
do {
factorial *= count--;
} while (count > 1);
writeOutput(factorial);
D
int counter = 5;
int factorial = 1;
do {
factorial *= counter--; // Multiply, then decrement.
} while (counter > 0);
writeln("factorial of 5 is ", factorial);
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 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
Java
int counter = 5;
int factorial = 1;
do {
factorial *= counter--; /* Multiply, then decrement. */
} while (counter > 0);
System.out.println("The factorial of 5 is " + factorial);
//============================================//
// The below function does the same as above. //
//============================================//
int counter = 5;
int factorial = 1;
while (counter > 0){
factorial *= counter--; /* Multiply, then decrement. */
}
System.out.println("The factorial of 5 is " + factorial);
JavaScript
let counter = 5; // Declaring two variables, counter and factorial
let factorial = 1;
do {
factorial *= counter--; //What will be looped
} while (counter > 0); //The looping conditions
console.log(factorial); //Showing the result
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 {
factorial *= counter--
} while (counter > 0)
println("Factorial of 5 is $factorial")
Pascal
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 (counter);
until counter = 0;
PHP
$counter = 5;
$factorial = 1;
do {
$factorial *= $counter--;
} while ($counter > 0);
echo $factorial;
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 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(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.
))
Ruby
counter = 10
factorial = 2
begin
factorial *= counter
counter -= 2
end while counter > 1
puts factorial
Smalltalk
| counter factorial |
counter := 5.
factorial := 1.
[counter > 0] whileTrue:
[factorial := factorial * counter.
counter := counter - 1].
Transcript show: factorial printString
Swift
Swift 2.x and later:[5]
var counter = 5
var factorial = 1
repeat {
factorial *= counter
counter -= 1
} while counter > 0
print(factorial)
Swift 1.x:
var counter = 5
var factorial = 1
do {
factorial *= counter
counter -= 1
} while counter > 0
println(factorial)
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)
References
- "C multi-line macro: do/while(0) vs scope block". Stack Overflow.
- "Microsoft visual basic". msdn.microsoft.com. Retrieved 21 January 2016.
- "do...while". MDN Web Docs.
- "Control Flow: if, when, for, while - Kotlin Programming Language". Kotlin.
- "Control Flow — The Swift Programming Language (Swift 5.3)". docs.swift.org.