24*3 is more than 12 full columns of a diagonal matrix and less than 13.
1 2 4 7
3 5 8
6 9
10
and so on. Now when you start filling in the second such matrix for the second iteration of the loop should it continue from where the previous one left off in mid column, or should it start at the top of the next column?
A diagram of what you want would help.
