Question

I’m doing some experiments with Prolog and having difficulties with the following rule:

row(Row, Matrix, [R1,R2,R3,R4]) :-
  cell(1, Row, Matrix, R1),
  cell(2, Row, Matrix, R2),
  cell(3, Row, Matrix, R3),
  cell(4, Row, Matrix, R4).

This rule extracts one row from a matrix, given its row number. For example,

row(2, [1,2,3,4,5,6,7,8], X)
 X = [5,6,7,8]

What nags me is that there is lots of repetition in that code. After finishing with 4×4 matrices, I will have to deal with 9×9 ones. And the code can get very non DRY.

Is there a way to extract that repetition out?

Thanks.

Edit: The complete code giving me trouble is here: https://github.com/kikito/7-languages-in-7-weeks/blob/master/3-prolog/day-3/sudoku-refactor.pl

Answer 1

After writing my first answer, I realized that you can also simplify your program using findall

 row(Row, Matrix, L) :- findall(X,cell(_,Row,Matrix,X),L).