title: How to access the diagonals of a matrix and change the value?
body: <p>I have 8x8 matrix where I have 15 diagonals. upper left corner “diagonal 1” and the lower
  right corner “diagonal 15”. I want to zero out the specific sets of diagonals like {9, 10, 11, 12, 13, 14, 15} or {5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}. Anyone please give me solution?</p>