finding block at end of line sparseMat

I am using a sparseMat which contains points on a nominal pitch of 4 with respect to the index (for example: PIXEL1, NULL, NULL, NULL, PIXEL2) . I would like to extract a 3x3 matrix which contains the last three valid pixels of the current row and the rows before and after. The sparseMat (sparseLocs) is created by iterating over rows and then cols. Hence, the first element is -3 from the current iterator position. I think I can use something like sparseLocs.addref(-3) and I think I can find the row and col by something like sparseLocs.hash(row, col) for that pixel. I am not getting it from the documentation. Can someone point me to the right documentation to understand this? Thanks, Ralph