title: Is there overhead of accessing elements in a sparse matrix
body: <p>Let's say I have a sparse matrix A. I want to do heavy calculations on it. The calculations do not modify A, only accessing its elements e.g. take a row of A then multiply with something. I wonder whether I should convert A to a full matrix before doing any calculation, or just do it directly?<br>
  In other words, is accessing elements in a sparse matrix slower than a full matrix?</p>