Question

I’m working through ‘Seven Languages in Seven Weeks’, and I’m just trying to get an example from the book working. It solves a mini sudoku grid (4×4).

The author is using gprolog, but I am using swi-prolog (I couldn’t get gprolog to work on my VM for whatever reason, but swi-prolog worked first try).

I am running Ubuntu 10.04 in VirtualBox 4.0.4 r70112 (hopefully that’s not too relevant!)

Here is the code in my prolog file:

:- use_module(library(clpfd)).

valid([]).
valid([Head|Tail]) :-
    all_different(Head),    % in the book, this is 'fd_all_different'
    valid(Tail).

 % beginning of sudoku rule itself
sudoku(Puzzle, Solution) :- 
Solution = Puzzle,
Puzzle = [S11, S12, S13, S14,
          S21, S22, S23, S24,
          S31, S32, S33, S34,
          S41, S42, S43, S44],
Puzzle ins 1..4,    % in the book, this is 'fd_domain'

Row1 = [S11, S12, S13, S14],
Row2 = [S21, S22, S23, S24],
Row3 = [S31, S32, S33, S34],
Row4 = [S41, S42, S43, S44],

Col1 = [S11, S21, S31, S41],
Col2 = [S12, S22, S32, S42],
Col3 = [S13, S23, S33, S43],
Col4 = [S14, S24, S34, S44],

Square1 = [S11, S12, S21, S22],
Square2 = [S13, S14, S23, S24],
Square3 = [S31, S32, S41, S42],
Square4 = [S33, S34, S43, S44],

valid([Row1, Row2, Row3, Row4,
       Col1, Col2, Col3, Col4,
       Square1, Square2, Square3, Square4]).

The only parts that I (intentionally) changed were:

• adding use_module(library(clpfd)). at the top
• changing fd_all_different(Head), to all_different(Head),
• changing fd_domain(Puzzle, 1, 4), to Puzzle ins 1..4,

Here is the invocation from swipl

?- sudoku([_, _, 2, 3,
           _, _, _, _,
           _, _, _, _,
           3, 4, _, _],
Solution).
Solution = [4, 1, 2, 3, 2, 3, 4, 1, 1|...] ;
false.

The solution is correct right up until it cuts off, at which point prolog seems to determine that there is no solution. But there is:

4 1 2 3
2 3 4 1
1 2 3 4
3 4 1 2

I’ve scoured the code looking for a typo or misplaced column, but have been unable to find the source of this. Any ideas?

Answer 1

It seems to me you are relying on the default display in SWI-Prolog to write the list that represents the solution, and it’s a feature of SWI-Prolog that it doesn’t print all the entries of a long list in this case, replacing the tail after nine items with the “ellipsis” …

You stumbled onto this when you added write(Puzzle) to the goal and thus saw the entire list. SWI-Prolog’s site has a FAQ about this “abbreviation” of lists.