Question

lets say I want to iterate through an array of doubles and sum them. I have two ways to do this.

A)

double sum (double * series, int size) {
    double sum = 0.0;
    for (int i = 0; i < size; i++) {
        sum += *series++;
    }
    return sum;
}

B)

double sum (double * series, int size) {
    double sum = 0.0;
    for (int i = 0; i < size; i++) {
        sum += series[i];
    }
    return sum;
}

which is better and why / when should I use one over the other.

Answer 1

This is a question of readability, it should not affect performance. I think B is the most readable, and therefore preferable.

I could also propose a third variant, which is range-based (note the begin and end parameters):

double sum (double* begin, double* end) {
    double sum = 0.;
    for (double* it = begin; it != end; ++it) {
        sum += *it;
    }
    return sum;
}

This is idiomatic C++ in many cases and generalizes more easily. That is not to say that it is always preferable, it is just another variant in a question about readability and maintainability.