Pitifulworm, you need to chime in here. If the assignment requires the name of the element, the number of atoms, *and* anything else (like it's abundance in a file), then you need a structure to hold that info.
If your assignment just requires the name of the element and the number of atoms, then a parallel array is fine, OR a structure, either one.
Both can be sorted, loaded, etc., with very little difference in terms of work involved.
So, which way do you prefer?
