'do...while' vs. 'while'

Possible Duplicates:
While vs. Do While
When should I use do-while instead of while loops?

I've been programming for a while now (2 years work + 4.5 years degree + 1 year pre-college), and I've never used a do-while loop short of being forced to in the Introduction to Programming course. I have a growing feeling that I'm doing programming wrong if I never run into something so fundamental.

Could it be that I just haven't run into the correct circumstances?

What are some examples where it would be necessary to use a do-while instead of a while?

(My schooling was almost all in C/C++ and my work is in C#, so if there is another language where it absolutely makes sense because do-whiles work differently, then these questions don't really apply.)

To clarify...I know the difference between a while and a do-while. While checks the exit condition and then performs tasks. do-while performs tasks and then checks exit condition.

Solution 1:

If you always want the loop to execute at least once. It's not common, but I do use it from time to time. One case where you might want to use it is trying to access a resource that could require a retry, e.g.

do
{
   try to access resource...
   put up message box with retry option

} while (user says retry);

Solution 2:

do-while is better if the compiler isn't competent at optimization. do-while has only a single conditional jump, as opposed to for and while which have a conditional jump and an unconditional jump. For CPUs which are pipelined and don't do branch prediction, this can make a big difference in the performance of a tight loop.

Also, since most compilers are smart enough to perform this optimization, all loops found in decompiled code will usually be do-while (if the decompiler even bothers to reconstruct loops from backward local gotos at all).

Solution 3:

I have used this in a TryDeleteDirectory function. It was something like this

do
{
    try
    {
        DisableReadOnly(directory);
        directory.Delete(true);
    }
    catch (Exception)
    {
        retryDeleteDirectoryCount++;
    }
} while (Directory.Exists(fullPath) && retryDeleteDirectoryCount < 4);

Solution 4:

Do while is useful for when you want to execute something at least once. As for a good example for using do while vs. while, lets say you want to make the following: A calculator.

You could approach this by using a loop and checking after each calculation if the person wants to exit the program. Now you can probably assume that once the program is opened the person wants to do this at least once so you could do the following:

do
{
    //do calculator logic here
    //prompt user for continue here
} while(cont==true);//cont is short for continue