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]] 



USING: kernel math math.matrices prettyprint sequences ;
 
: sum-below-diagonal ( matrix -- sum )
    dup square-matrix? [ "Matrix must be square." throw ] unless
    0 swap [ head sum + ] each-index ;
 
{
    { 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 }
} sum-below-diagonal .