Question

I’m writing code that uses a large matrix where the elements are a user defined class. To build this matrix, I use the following vector of vectors.

using namespace std;
vector< vector< userclass > > matrix = vector<vector<userclass> >(sizeX, vector<userclass>(sizeY));

This class, which might also be a struct, will contain a few builtins such as floats and pointers. So here’s the thing:
Let’s say the matrix will have a size of 2000 in one direction, but only a size of 20 in the other, but I have total freedom to choose which one. For the best performance, which one should I make the biggest, sizeX or sizeY?

In other words: which is faster, a small vector of large vectors, or a large vector of small vectors? Is there a difference at all?

The performance optimization should be towards single random accesses.

Answer 1

You should aim for the least number of vectors possible, which means that sizeY should be larger than sizeX for best cache performance, not to mention taking up less space.

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Of course, it depends on how you intend to use them. If you can, try to stay accessing a vector for as long as possible – vec[i][j] is much better than vec[j][i]. If you have to do vec[j][i] then having sizeX be larger may have better performance, or using 1 contiguous array.

Fastest iterating where sizeX > sizeY:

for(int i...)
for(int j...) {
  vec[i][j];
}