Which kind of data organization using C arrays makes fastest code and why?

Question

Given following data, what is the best way to organize an array of elements so that the fastest random access will be possible?

Each element has some int number,

Answer 1

It depends on the common access patterns. If you plan to iterate over the data, accessing every element as you go, the struct approach is better. If you plan to iterate independently over each component, then parallel arrays are better.

This is not a subtle distinction, either. With main memory typically being around two orders of magnitude slower than L1 cache, using the data structure that is appropriate for the usage pattern can possibly triple performance.

I must say, though, that your approach to implementing parallel arrays leaves much to be desired. You should simply declare three arrays instead of getting "clever" with two-dimensional arrays and casting:

int nums[900000000];
char names[900000000][4];
float vals[900000000];