matrices

matrices

(1)
def diagsum(matrix)
  (0...matrix.length).map{ |i| matrix[i][i] }.reduce(:+)
end
# uses map enumerator for the array of matrix values - remember to use the three-dot notation as it omits the last value since array starts at 0
# matrix[i][i] indicates the array position of each diagonal value for each row (row, column will be the same value for a square matrix
# reduce(:+) is an enumerator method sums up the block values that were just mapped from the array


(2)
require 'matrix'
def diagsum(matrix)
   Matrix[*matrix].trace
end
#uses matrix library's trace method, which sums up all diag values in a matrix.  needs the splat operator to flatten the matrix array and return just values


(3)
def diagsum(matrix)
   (0...matrix.size).inject(0) { |sum,i| sum + matrix[i][i] }
end
#uses an inject method on the matrix array - note .size works just as .length did in prev example - with a starting arg of (0)
#inject enumerates the sum and the individual values in the array (represented as i), adding each incremental value of matrix[i][i] against itself (the sum