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

I’m guessing your backward for loop looks like this:

for (i = domain - 1; i >= 0; --i) {

In that case, because i is unsigned, it will always be greater than or equal to zero. When you decrement an unsigned variable that is equal to zero, it will wrap around to a very large number. The solution is either to make i signed, or change the condition in the for loop like this:

for (i = domain - 1; i >= 0 && i < domain; --i) {

Or count from domain to 1 rather than from domain - 1 to 0:

for (i = domain; i >= 1; --i) {
    array[i - 1] = ...; // notice you have to subtract 1 from i inside the loop now
}