If you store this matrix as sparse, then yes the (1,1) 0 element will not be physically stored and cannot be recovered with find. So you will need to store these elements differently. Maybe keep track of the derivative element indexing in another variable. I.e., you could have another sparse matrix that simply keeps track of where the derivatives are. e.g.,
(1,1) 1 (50,2) 1 (99,3) 1
Then this could be used to recover the index values for the derivatives, even the 0 ones.
