title: Sum of diagonal elements in a matrix
body: <p>I am trying to find out the sum of the diagonal elements in a matrix. Here, n is the size of the square matrix and a is the matrix. Can someone explain this to me what is happening here.</p>
  
  <pre><code>n = 3
  a = [[11,2,4],[4,5,6],[10,8,-12]]
  sum_first_diagonal = sum(a[i][i] for i in range(n))
  sum_second_diagonal = sum(a[n-i-1][n-i-1] for i in range(n))
  print(str(sum_first_diagonal)+" "+str(sum_first_diagonal))
  </code></pre>