cfor-loop

What's the best way to do a reverse 'for' loop with an unsigned index?

My first attempt of reverse for loop that does something n times was something like:

for ( unsigned int i = n-1; i >= 0; i-- ) {
    ...     
}

This fails because in unsigned arithmetic i is guaranteed to be always greater or equal than zero, hence the loop condition will always be true. Fortunately, gcc compiler warned me about a 'pointless comparison' before I had to wonder why the loop was executing infinitely.

I'm looking for an elegant way of resolving this issue keeping in mind that:

  1. It should be a backwards for loop.
  2. The loop index should be unsigned.
  3. n is unsigned constant.
  4. It should not be based on the 'obscure' ring arithmetics of unsigned integers.
Solution

  • How about:

    for (unsigned i = n ; i-- > 0 ; )
{
  // do stuff with i
}