determining the sum of top-left to bottom-right diagonal values in a matrix with Ruby?

Question

I have a square matrix of indeterminate row & column length (assume rows and columns are equal as befits a square).

I've plotted out an example matrix as follows:

matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
 ]

My goal is to get a sum from top-left to bottom-right of the diagonal values.

Obviously in this example, this is all i'll need:

diagsum = matrix[0][0]+matrix[1][1]+matrix[2][2]
#=>  15

I see the pattern where it's a +1 incremental for each row & column argument in the matrix, so the code i've developed for my matrix of indeterminate length (supplied as the argument to my method diagsum would preferably need to implement some sort of row_count method on my matrix argument.

Answer 1

If

arr = [[1,2,3],
       [4,5,6],
       [7,8,9]]

then:

require 'matrix'
Matrix[*arr].trace
  #=> 15