C++: Optimizing speed with vector/array?

Question

I have a nested for-loop structure and right now I am re-declaring the vector at the start of each iteration:

void function (n1,n2,bound,etc){

    for (int          


        

Answer 1

Since you have to reset the vector values to 0 each iteration, in practical terms, this question boils down to "is the cost of allocating and deallocating the memory for the vector cheap or expensive compared to the computations inside the loops".

Assuming the computations are the expensive part of the algorithm, the way you've coded it is both clear, concise, shows the intended scope, and is probably just as fast as alternate approaches.

If however your computations and updates are extremely fast and the allocation/deallocation of the vector is relatively expensive, you could use std::fill to fill zeroes back into the array at the end/beginning of each iteration through the loop.

Of course the only way to know for sure is to measure with a profiler. I suspect you'll find that the approach you took won't show up as a hotspot of any sort and you should leave the obvious code in place.

Answer 2

Well if you are really concerned about performance (and you know the size of n1 and n2 beforehand) but don't want to use a C-style array, std::array may be your friend.

EDIT: Given your edit, it seems an std::array isn't an appropriate substitute since while the vector size does not change each iteration, it still isn't known before compilation.