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 repeatedly executes the block, or not, depending on a given boolean condition at the end of the block.
Contents
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
is equivalent to
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):
or
Demonstrating do while loops
These example programs calculate the factorial of 5 using their respective languages' syntax for a do-while loop.
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:
Fortran
With legacy FORTRAN 77 there is no DO-WHILE construct but the same effect can be achieved with GOTO:
With Fortran 90 and later, the do-while loop is actually the same as the for loop.
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.
Racket
In Racket, as in other Scheme implementations, a "named-let" is a popular way to implement loops:
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.
Swift
Swift 2.x:
Swift 1.x: