I am trying to run following code from N-Queens Problem..How far can we go? to find solutions to n-queens problem:
generate([],_).
generate([H|T],N) :- H in 1..N , generate(T,N).
lenlist(L,N) :- lenlist(L,0,N).
lenlist([],N,N).
lenlist([_|T],P,N) :- P1 is P+1 , lenlist(T,P1,N).
queens(N,L) :-
generate(L,N),lenlist(L,N),
safe(L),!,
labeling([ffc],L).
notattack(X,Xs) :- notattack(X,Xs,1).
notattack(X,[],N).
notattack(X,[Y|Ys],N) :- X #\= Y,
X #\= Y - N,
X #\= Y + N,
N1 is N + 1,
notattack(X,Ys,N1).
safe([]).
safe([F|T]) :- notattack(F,T), safe(T).
I have swi-prolog installed on Debian-9 (stable) Linux and I am running above using command "swipl -f nqueens.pl". On loading, I get an error:
Syntax error: operator expected (probably on 2nd code line)
Where is the problem and how can this be solved? Thanks for your help.
The question actually mentions that it is writting in CLPFD (A Constraint Logic Programming tool over Finite Domains). You have to import this library:
:- use_module(library(clpfd)).
generate([],_).
generate([H|T],N) :- H in 1..N , generate(T,N).
lenlist(L,N) :- lenlist(L,0,N).
lenlist([],N,N).
lenlist([_|T],P,N) :- P1 is P+1 , lenlist(T,P1,N).
queens(N,L) :-
generate(L,N),lenlist(L,N),
safe(L),!,
labeling([ffc],L).
notattack(X,Xs) :- notattack(X,Xs,1).
notattack(X,[],N).
notattack(X,[Y|Ys],N) :- X #\= Y,
X #\= Y - N,
X #\= Y + N,
N1 is N + 1,
notattack(X,Ys,N1).
safe([]).
safe([F|T]) :- notattack(F,T), safe(T).Then it works, and produces for instance:
?- queens(5,L).
L = [1, 3, 5, 2, 4] ;
L = [1, 4, 2, 5, 3] ;
L = [2, 4, 1, 3, 5] ;
L = [2, 5, 3, 1, 4] ;
L = [3, 1, 4, 2, 5] ;
L = [3, 5, 2, 4, 1] ;
L = [4, 1, 3, 5, 2] ;
L = [4, 2, 5, 3, 1] ;
L = [5, 2, 4, 1, 3] ;
L = [5, 3, 1, 4, 2].