M is a matrix of prime numbers from 1 to 23 (in row order)
I don't understand how the second code line replaces the diagonal entries of M with 1. I am also confused because the function
sub2ind converts row/column subscript to linear index both of which has nothing to do with the value of the entry.
M = zeros(3); M(:) = primes(23); M = M' M(sub2ind(size(M), 1:3, 1:3)) = 1