Sum of diagonal elements in a matrix

Question

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 happen

Answer 1

Since you know the positions of the diagonal elements for row i, you can write it quite densely like:

d = sum(row[i] + row[-1-i] for i, row in a)

And, for odd sized matrices, you shouldn't add the center element twice:

if len(a)%2:
    centre = len(a)//2
    d -= a[centre][centre]