Prolog chess knight algorithm

Question

I'm trying to resolve chess knight problem for custom matrix but I don't know where is problem. A chess knight must visit each place once, and when he manages to visit all fields then finish the program. Currently, he is looking for the correct combination of fields all the time and it takes a very long time ... I have no idea what to do next or what to change

append([], POINT, [POINT]).
append([H|T], POINT, [H|R]) :- 
    append(T, POINT, R).

member(POINT,[POINT|_]).
member(POINT,[_|R]) :- 
    member(POINT, R).

try(X, Y, Xmax, Ymax, A, B) :- 
    (X + 1 < Xmax, Y + 2 < Ymax, A is X + 1, B is Y + 2).
try(X, Y, Xmax, Ymax, A, B) :- 
    (X + 2 < Xmax, Y + 1 < Ymax, A is X + 2, B is Y + 1).
try(X, Y, Xmax, Ymax, A, B) :- 
    (X + 2 < Xmax, Y - 1 >= Ymax - Ymax, A is X + 2, B is Y - 1).
try(X, Y, Xmax, Ymax, A, B) :- 
    (X + 1 < Xmax, Y - 2 >= Ymax - Ymax, A is X + 1, B is Y - 2).
try(X, Y, Xmax, Ymax, A, B) :-  
    (X - 1 >= Xmax - Xmax, Y - 2 >= Ymax - Ymax, A is X - 1, B is Y - 2).
try(X, Y, Xmax, Ymax, A, B) :- 
    (X - 2 >= Xmax - Xmax, Y - 1 >= Ymax- Ymax, A is X - 2, B is Y - 1).
try(X, Y, Xmax, Ymax, A, B) :-  
    (X - 2 >= Xmax - Xmax, Y + 1 < Ymax, A is X - 2, B is Y + 1).
try(X, Y, Xmax, Ymax, A, B) :- 
    (X - 1 >= Xmax - Xmax, Y + 2 < Ymax, A is X - 1, B is Y + 2).


move(X, Y, Xmax, Ymax, L) :-
    not(member([X, Y], L)),
    write('X='), write(X),
    write('Y='), write(Y), nl,
    try(X, Y, Xmax, Ymax, A, B),
    append(L, [X, Y], L2),
    move(A, B, Xmax, Ymax, L2).

Example:move(0, 0, 3, 3, []).

Output

X=0Y=0
X=1Y=2
X=2Y=0
X=0Y=1
X=2Y=2
X=1Y=0
X=0Y=2
X=2Y=1
X=2Y=1
X=0Y=2
X=1Y=0
X=2Y=2
X=0Y=1
X=2Y=0
X=1Y=2
false

That's true, because it will never be in the center of the matrix. But for matrix 5x5 he can't find combination...

You should post the output you want also. For me it just fails quickly. — Tomas By, Nov 30 '17 at 19:30
Actually output it is infinity... https://swish.swi-prolog.org/ run with commend: move(0, 4, 5, 5, []). — Sekru, Nov 30 '17 at 19:33
The problem is that, I do not know if it works because all the time he is looking for the correct combination. — Sekru, Nov 30 '17 at 19:38
You say you do not know if it works. Does it print something else other than you were expecting? — Tomas By, Nov 30 '17 at 19:44
I update my question with example of output when im starting from X=0,Y=0 for matrix 3x3 — Sekru, Nov 30 '17 at 20:04
I'm afraid I still don't understand what you think is wrong. — Tomas By, Nov 30 '17 at 20:09
With this code for matrix 5x5 starting from X=0 and Y=4, he should find correct combination to be once in each field but I do not know how long it should take and this is problem... I can't confirm that it works — Sekru, Nov 30 '17 at 20:14
Please see my answer then. I will try not to change it in the near future. — Tomas By, Nov 30 '17 at 20:15

Tomas By · Accepted Answer · 2017-11-30T20:12:52.927

New attempt. This code:

move(X,Y,Xmax,Ymax,L) :-
  ( try(X,Y,Xmax,Ymax,A,B), not(member([A,B],L)) ->
    write('X='), write(A), write(' '), write('Y='), write(B), nl,
    append(L,[A,B],L2),
    move(A,B,Xmax,Ymax,L2)
  ; true ).

Gives me (for move(0,4,5,5,[])):

X=2 Y=3
X=4 Y=4
X=3 Y=2
X=4 Y=0
X=2 Y=1
X=3 Y=3
X=4 Y=1
X=2 Y=0
X=0 Y=1
X=1 Y=3
X=3 Y=4
X=4 Y=2
X=3 Y=0
X=1 Y=1
X=0 Y=3
X=2 Y=4
X=4 Y=3
X=3 Y=1
X=1 Y=0
X=2 Y=2
X=1 Y=4
X=0 Y=2

Is this not what you were expecting?

Prolog chess knight algorithm

1 Answers1