Task

Find and display the sum of elements that are below the main diagonal of a matrix.

The matrix should be a square matrix.



 ───   Matrix to be used:   ───


     [[1,3,7,8,10],
      [2,4,16,14,4],
      [3,1,9,18,11],
      [12,14,17,18,20],
      [7,1,3,9,5]] 



 
def add(s): reduce s as $x (null; . + $x);
 
# input: a square matrix
def sum_below_diagonal:
  add( range(0;length) as $i | .[$i][:$i][] ) ;
 