====== 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.