Conditional mean for a matrix (calculate the mean ignoring the negative numbers)

I have a matrix m x n with positive (including zeros) and some negative numbers.

How can I calculate the mean of all the numbers ignoring the negatives?

For example, my matrix is:

-1 2 4

3 -1 5

-1 3 4

This "conditional mean" should calculate (2+4+3+5+3+4)/6 = 3.5

Thanks.

Answer 1

[Allen]

Use something similar to the below example

A = [-1, 2, 4
      3,-1, 5
     -1, 3, 4];
mean(A(A>=0))