Counting down in for-loops

Question

I believe (from some research reading) that counting down in for-loops is actually more efficient and faster in runtime. My full software code is C++

I currently have this:

for (i=0; i<domain; ++i) {

my 'i' is unsigned resgister int, also 'domain' is unsigned int

in the for-loop i is used for going through an array, e.g.

array[i] = do stuff

converting this to count down messes up the expected/correct output of my routine.

I can imagine the answer being quite trivial, but I can't get my head round it.

UPDATE: 'do stuff' does not depend on previous or later iteration. The calculations within the for-loop are independant for that iteration of i. (I hope that makes sense).

UPDATE: To achieve a runtime speedup with my for-loop, do I count down and if so remove the unsigned part when delcaring my int, or what other method?

Please help.

Answer 1

There is only one correct method of looping backwards using an unsigned counter:

for( i = n; i-- > 0; )
{
    // Use i as normal here
}

There's a trick here, for the last loop iteration you will have i = 1 at the top of the loop, i-- > 0 passes because 1 > 0, then i = 0 in the loop body. On the next iteration i-- > 0 fails because i == 0, so it doesn't matter that the postfix decrement rolled over the counter.

Very non obvious I know.