Finding and using sparse matrix indices

I have the sparse matrix array L=

(36,67)       1
(39,70)       1
(42,73)       1
(45,76)       1
(48,79)       1
(51,82)       1
(54,85)       1

and a large matrix M. I need to use the index coordinates of each of the elements in L, offset the coordinates by some amount, find the element in matrix M having those coordinates, and multiply the result. The operation must be vectorized.

A pseudocode might look like:

 out=L*M(Lrow+5,Lrow+Lcolumn-15);

where Lrow is the row in L and parentheses are the matrix coordinates in M, determined for each element in L.

Thanks.

Answer 1

[Iain]

If L is the same size as M:

 idx = find(L);

Move the idx around by adding 1 (moves it down one), or by adding the number of columns (moves it right one) - Just be aware that if you move them down far enough, they'll move right.

 idx = idx + 1 + cols; % right AND down one.
 the_values =  M(idx);