How can i retrieve only the indices of elements within a matrix subset?

Hi,

I have a large matrix, C, and a matrix subset, Csub. I want to find the indices of the elements of Csub in C. So essentially, where Csub is in C. I get close with "intersect" but the problem is that I only want the indices of Csub in C, and intersect returns the indices of any element in C who's value is also found in Csub. Is there a way to restrict "intersect" to only the indices that directly correspond to the boundaries (not sure if that's the correct word) of Csub?

For example, if I have the following 2x5 matrix:

C = [1,2,3,4,5;1,2,3,4,3]

And the subset Csub corresponds to elements with linear indices 5 and 6, what is a way to retrieve these indices without also retrieving the index that lies outside of the subset (element with a value of 3 as well, with linear index of 10 in this case)?

[Rik]

You could use a convolution and search for values that are exactly 1. Due to rounding that might not work for very large subsets, but in cases like your example it should work.