Question

Given a matrix A with dimension m x n and the entries in the matrix lies [0,1]
For example

A = [0.5 0   0  0.5 0
     0   0.5 0  0   0.5
     1   0   0  0   0]

I would like to calculate sum(sum(a_ij log(a_ij))), where a_ij is the i th row and j th col entry in the matrix A. Since there exist an 0 entry in the matrix, i always get NAN as a result.

How do i consider only non-zero entries to calculate sum(sum(a_ij log(a_ij))) [entropy of the matrix].

Answer 1

To consider only specific elements of a matrix you can use logical indexing. For example if you only want to select non-zero entries of A you can use A(A~=0). So for your problem the solution can be written:

sum(A(A~=0).*log(A(A~=0)));

EDIT: wow that is some kind of coincidence, I’ve just seen your comment after posting this. Well, glad you’ve worked it out yourself.