From: Joachim Schimpf <J.Schimpf_at_icparc.ic.ac.uk>

Date: Mon 20 Dec 1999 04:22:07 PM GMT

Message-ID: <385E57AF.98AA0F65@icparc.ic.ac.uk>

Date: Mon 20 Dec 1999 04:22:07 PM GMT

Message-ID: <385E57AF.98AA0F65@icparc.ic.ac.uk>

Doug Edmunds wrote: > > /*How do I overcome the > delayed goals in this code, to > get the desired result > X=130, Y =70, Z = 80? > > Doug Edmunds > 16 Dec 1999 > */ > > :-lib(fd). > > g :- X #>0, Y#>0, Z #>0, > X + Y + Z #= 280, > X #= 130, > Y #= Z - 10. > > Additional comment: > > I added a line > indomain(Y), > at the end, and it produces the correct answer. What you are observing here is a manifestation of a fundamental property of the constraint programming paradigm: The implementation of the constraints is "incomplete" and you need to add search in order to establish the existence (and the actual values) of solutions. Why is this so? Because _in_general_ there are no known efficient algorithms to solve a given set of constraints. The implementation of the constraints therefore restricts itself to infer consequences (ie. find inconsistencies and bound reductions) that can be found using efficient (polynomial complexity) algorithms. So you can rely on the constraints to _help_ you find a solution, but in most cases they won't do all the work... Now, your example of course happens to be one of the cases where we do know an efficient algorithm to solve the whole problem: the type of constraints is very restricted, they are all linear equations and we could simply do Gauss-elimination. However, the fd-library does not implement that, it only implements domain propagation, which is weaker on linear equations but applicable to a wider range of constraints. Other libraries like lib(clpr) or lib(eplex) can solve that directly: [eclipse 1]: lib(clpr). ... yes. [eclipse 2]: X$>=0, Y$>=0, Z$>=0, X + Y + Z $= 280, X$=130, Y$=Z-10. X = 130.0 Y = 70.0 Z = 80.0 yes. ------------------------------------------------------------------------ Joachim Schimpf / phone: +44 171 594 8187 IC-Parc, Imperial College / mailto:J.Schimpf@ic.ac.uk London SW7 2AZ, UK / http://www.icparc.ic.ac.uk/eclipseReceived on Mon Dec 20 16:28:26 1999

*
This archive was generated by hypermail 2.1.8
: Wed 16 Nov 2005 06:07:05 PM GMT GMT
*