How to use values of a vector as indices for a matrix

I have the 20x20 matrix M and the m-by-3 matrix (two column vectors) L.

The values in the 1st and 2nd column of matrix L are to be used as the m and n indices for the matrix M. I attempt to get the corresponding values in M and insert them into column 3 as follows:

 L(:,3)=M(L(:,2),L(:,1));

However, this does not work. What is wrong?

Answer 1

[Andrei Bobrov]

L(:,3) = M(sub2ind(size(M),L(:,2),L(:,1)));