An Example

The following small program is intended to illustrate the use of Prolog to solve a simple problem. The problem is a typical ``programming example'' , it is not a typical ``real world'' application. However this program is written in a way that exploits the attributes of Prolog.

The problem is to place eight chess queens on a chess board so that not one can attack any other.

  eight(QS) :- perm([1,2,3,4,5,6,7,8],QS), safe(QS).
 
  perm([],[]).
  perm([H|T],P) :- perm(T,PT), insert(H,PT,P).
 
  insert(X,L,[X|L]).
  insert(X,[Y|L1],[Y|L2]) :- insert(X,L1,L2).
 
  safe([]).
  safe([Q|Rest]) :- safe(Rest), notattacked(Q,Rest,1).
 
  notattacked(Y,[],Xoffset).
  notattacked(Y,[Y1|YList],Xoffset) :-
          Y1 - Y =\= Xoffset, Y - Y1 =\= Xoffset,
          Nxoff is Xoffset + 1,
          notattacked(Y,YList,Nxoff).

Next: Control and Cut Up: A goal directed programming Previous: Structured Data: Lists

Page generated: 2002-11-04 by Bob Dickerson

Disclaimer