Question

I have a code that does something like this:

while (doorIsLocked()) {
    knockOnDoor();
}
openDoor();

but I want to be polite and always knock on the door before I open it.
I can write something like this:

knockOnDoor();
while (doorIsLocked()) {
    knockOnDoor();
}
openDoor();

but I’m just wondering if there’s a better idiom that doesn’t repeat a statement.

Answer 1

You can use a do-while instead of a while-do loop:

do {

  knockOnDoor();

} while (doorIsLocked());

openDoor();

Unlike a while-do loop, the do-while executes the body once before checking the termination condition.

See also

• Wikipedia/Do while loop

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Pre-test and post-test loops

The do-while loop — sometimes called just a do statement — in C/C++/C#/Java/some others is what is known as a “post-test” loop: the terminating condition is checked after each iteration. It loops while a condition is true, terminating immediately once it’s false.

Pascal has a repeat-until loop, which is also a “post-test” loop; it loops while a condition is false, terminating immediately once it’s true.

Fortran’s do-while loop on the other hand is a “pre-test” loop: the terminating condition is checked before each iteration. In C/C++/Java/C#/some others, while-do and for loops are also “pre-test” loops.

Always check language reference when in doubt.

References

• MSDN: C++ Language Reference: The do-while Statement
• MSDN: C# Language Reference: do
• Java Language Specification §14.13: The do Statement

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