Prolog

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<language>Prolog</language>
<code>
mann(adam).
mann(tobias).
mann(frank).
frau(eva).
frau(daniela).
frau(ulrike).
vater(adam,tobias).
vater(tobias,frank).
vater(tobias,ulrike).
mutter(eva,tobias).
mutter(daniela,frank).
mutter(daniela,ulrike).

grossvater(X,Y) :-
       vater(X,Z),
       vater(Z,Y).

grossvater(X,Y) :-
       vater(X,Z),
       mutter(Z,Y).

</code>
<eval>
grossvater(X,frank).
</eval>


<code>
%queens(N,[N1,N2,...,NN]) is true if you can put a queen on an NxN
%                         in the ith rank of the nth file, s.t. no
%                         queen attacks any other queen.

queens(N,Qs) :-
	range(1,N,Ns), queens(Ns,[],Qs).

queens(UnplacedQs, SafeQs, Qs) :-
	selectq(Q, UnplacedQs, UnplacedQs1),
	\+ attack(Q,SafeQs),
	queens(UnplacedQs1,[Q|SafeQs],Qs).
queens([],Qs,Qs).

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).

range(M,N,[M|Ns]) :- M < N, M1 is M+1, range(M1,N,Ns).
range(N,N,[N]).

selectq(X,[X|Xs],Xs).  
selectq(X,[Y|Ys],[Y|Zs]) :- selectq(X,Ys,Zs). 

%Try
%queens(4,P).
%
% The result P=[3,1,4,2] means the queens go like this:
%    *****************
% 4  *   *   * Q *   *
%    *****************
% 3  * Q *   *   *   *
%    *****************
% 2  *   *   *   * Q *
%    *****************
% 1  *   * Q *   *   *
%    *****************
%      1   2   3   4
</code>
<eval>
queens(8,P).
</eval>