8 queens 3
Description
Program solves 8 queens problem
Source: PROLOG programming for artificial intelligence, 3rd Edition, Harlow, 2001, ISBN 0-201-40375-7.
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% Figure 4.7 Program 1 for the eight queens problem.
% solution( BoardPosition) if
% BoardPosition is a list of non-attacking queens
solution( [] ).
solution( [X/Y | Others] ) :- % First queen at X/Y, other queens at Others
solution( Others),
member( Y, [1,2,3,4,5,6,7,8] ),
noattack( X/Y, Others). % First queen does not attack others
noattack( _, [] ). % Nothing to attack
noattack( X/Y, [X1/Y1 | Others] ) :-
Y =\= Y1, % Different Y-coordinates
Y1-Y =\= X1-X, % Different diagonals
Y1-Y =\= X-X1,
noattack( X/Y, Others).
member( Item, [Item | Rest] ).
member( Item, [First | Rest] ) :-
member( Item, Rest).
% A solution template
template( [1/Y1,2/Y2,3/Y3,4/Y4,5/Y5,6/Y6,7/Y7,8/Y8] ).