title: How to find indices of non zero elements in large sparse matrix?
body: <p>i have two sq matrix (a, b) of size in order of 100000 X 100000. I have to take difference of these two matrix (c = a-b). Resultant matrix 'c' is a sparse matrix. I want to find the indices of all non-zero elements. I have to do this operation many times (>100).</p>
  
  <p>Simplest way is to use two for loops. But that's computationally intensive. Can you tell me any algorithm or package/library preferably in R/python/c to do this as quickly as possible?</p>