As a newbie, I am surprised that I haven't been able to get iterators for Eigen arrays or matrices. I want to get the number of non-zero entries in a matrix, but it's not clear what is the neatest approach. If there were iterators, I could use std::count_if()
Thanks.

Say we have:

MatrixXd A;

You can get a pair of iterators with A.data() and A.data()+A.size(), but better use Eigen's language:

nnz = (A.array() == 0).count();