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+====== N queens ======
+{{tag>chess problem_solving}}
+===== Description =====
+Naive generate-and-test program solving N queens
+**Source**:  The Art of Prolog
+===== Download =====
+Program source code: {{n_queens.pl}}
+===== Listing =====
+<code prolog>
+/*
+   queens(N,Queens) :-
+	Queens is a placement that solves the N queens problem,
+	represented as a permutation of the list of numbers [1,2,...,N].
+*/
+     queens(N,Qs) :- range(1,N,Ns), permutation(Ns,Qs), safe(Qs).
+/*
+	safe(Qs) :- The placement Qs is safe.
+*/
+     safe([Q|Qs]) :- safe(Qs), not attack(Q,Qs).
+     safe([]).
+     attack(X,Xs) :- attack(X,1,Xs).
+     attack(X,N,[Y|_]) :- X is Y+N ; X is Y-N.
+     attack(X,N,[_|Ys]) :- N1 is N+1, attack(X,N1,Ys).
+	permutation(Xs,[Z|Zs]) :- select(Z,Xs,Ys), permutation(Ys,Zs).
+	permutation([],[]).
+	select(X,[X|Xs],Xs).
+	select(X,[Y|Ys],[Y|Zs]) :- select(X,Ys,Zs).
+     range(M,N,[M|Ns]) :- M < N, M1 is M+1, range(M1,N,Ns).
+     range(N,N,[N]).
+%	Program 14.2: Naive generate-and-test pr
+gram solving N queens
+</code>
+===== Comments =====
+RTERR: 12:5 : Syntax error. Operator expected.

pl/prolog/pllib/n_queens.txt · ostatnio zmienione: 2019/06/27 15:50 (edycja zewnętrzna)

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