Codewars Bonus Challenge 4 - Find The Sum of Diagonal 1's

sum_diagonal.rb

#Given a "square" array of subarrays, find the sum of values from the first value of the first array, 
#the second value of the second array, the third value of the third array, and so on...

#Example 1:

#exampleArray = [[1, 0, 0, 0],
            #    [0, 1, 0, 0],
            #    [0, 0, 1, 0],
            #    [0, 0, 0, 1]]

#diagonalSum(exampleArray) # => 4

#Extra Credit: Try to complete this exercise in three lines or less!

My Solution:

def diagonalSum(matrix)
  sum = 0
  matrix.each{ |arr| sum += arr[matrix.index(arr)] }   #As the matrix is symmetric, the index of the diagonal element in the inner array matches that array's index in the outer array.
  sum
end