How to find indices of non zero elements in large sparse matrix?
Since you have two dense matrices then the double for loop is the only option you have. You don't need a sparse matrix class at all since you only want to know the list of indices (i,j) for which a[i,j] != b[i,j].
In languages like R and Python the double for loop will perform poorly. I'd probably write this in native code for a double for loop and add the indices to a list object.  But no doubt the wizards of interpreted code (i.e. R, Python etc.) know efficient ways to do it without resorting to native coding.
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).
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?