Question

Assume I’m given two unsigned integers:

size_t A, B;

They’re loaded out with some random numbers, and A may be larger, equal, or smaller than B. I want to loop from A to B. However, the comparison and increment both depend on which is larger.

for (size_t i = A; i <= B; ++i) //A <= B
for (size_t i = A; i >= B; --i) //A >= B

The obvious brute force solution is to embed these in if statements:

if (A <= B)
{
 for (size_t i = A; i <= B; ++i) ...
}
else
{
 for (size_t i = A; i >= B; --i) ...
}

Note that I must loop from A to B, so I can’t have two intermediate integers and toss A and B into the right slots then have the same comparison and increment. In the “A is larger” case I must decrement, and the opposite must increment.

I’m going to have potentially many nested loops that require this same setup, which means every if/else will have a function call, which I have to pass lots of variables through, or another if/else with another if/else etc.

Is there any tricky shortcut to avoid this without sacrificing much speed? Function pointers and stuff in a tight, often repeated loop sound extremely painful to me. Is there some crazy templates solution?

Answer 1

size_t const delta = size_t(A < B? 1 : -1);
size_t i = A;
for( ;; )
{
    // blah

    if( i == B ) { break; }
    i += delta;
}