Is there ever a need for a “do {…} while ( )” loop?

Question

Bjarne Stroustrup (C++ creator) once said that he avoids \"do/while\" loops, and prefers to write the code in terms of a \"while\" loop instead. [See quote below.]

S

Answer 1

A do-while loop can always be rewritten as a while loop.

Whether to use only while loops, or while, do-while, and for-loops (or any combination thereof) depends largely on your taste for aesthetics and the conventions of the project you are working on.

Personally, I prefer while-loops because it simplifies reasoning about loop invariants IMHO.

As to whether there are situations where you do need do-while loops: Instead of

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

you can always

loopBody();
while(condition())
{
  loopBody();
}

so, no, you never need to use do-while if you cannot for some reason. (Of course this example violates DRY, but it's only a proof-of-concept. In my experience there is usually a way of transforming a do-while loop to a while loop and not to violate DRY in any concrete use case.)

"When in Rome, do as the Romans."

BTW: The quote you are looking for is maybe this one ([1], last paragraph of section 6.3.3):

From my experience, the do-statement is a source of error and confusion. The reason is that its body is always executed once before the condition is tested. For the correct functioning of the body, however, a similar condition to the final condition has to hold in the first run. More often than I expected I have found these conditions not to be true. This was the case both when I wrote the program in question from scratch and then tested it as well as after a change of the code. Additionally, I prefer the condition "up-front, where I can see it". I therefore tend to avoid do-statements.

(Note: This is my translation of the German edition. If you happen to own the English edition, feel free to edit the quote to match his original wording. Unfortunately, Addison-Wesley hates Google.)

[1] B. Stroustrup: The C++ programming language. 3rd Edition. Addison-Wessley, Reading, 1997.