From: Joachim Schimpf <j.schimpf_at_icparc.ic.ac.uk>

Date: Tue 18 Jan 2005 04:17:55 PM GMT

Message-ID: <41ED36B3.4030708@icparc.ic.ac.uk>

Date: Tue 18 Jan 2005 04:17:55 PM GMT

Message-ID: <41ED36B3.4030708@icparc.ic.ac.uk>

Piero S wrote: > Dear all, > > I need to enforce a constraint so that the domain of some of its variables > are not reduced during (domain) propagation. So far I've been using the > get_bounds/3 predicate to achieve this, as you can see in the following > example. > > %+++ Example > [eclipse 1] lib(ic). > [eclipse 2] X #:: 0..1, get_bounds(X,B,T), > (Y #:: 0..3, X + Y #< 4, indomain(Y), > get_bounds(X,B1,T1), B1#=B, T1#=T). > > Y = 0 > X = X{[0, 1]} > B1 = 0 > B = 0 > T1 = 1 > T = 1 > Yes (0.00s cpu, solution 1, maybe more) ? ; > > [eclipse 3]: X #:: 0..3, get_bounds(X,B,T), (Y #:: 0..1, X #= Y, > get_bounds(X,B1,T1), B1 #= B, T1 #= T). > > No (0.00s cpu) > %+++ You can get this behaviour by triggering failure as soon as the variable gets constrained, e.g. ?- X #:: 0 .. 1, suspend(fail, 2, X->constrained), Y #:: 0 .. 3, X + Y #< 4, indomain(Y). But a more general warning here: your approach might not achieve what you want. I guess what you want is a forall-quantification like forall X in 0..1: Y #:: 0 .. 3, X + Y #< 4 But the fact that propagation does not eliminate a value from X's domain does _not_ (in general) prove that this domain value is a valid solution! Example: ?- [X, Y, Z] :: 0 .. 9, Z #=< Y, Y #\= 1, X #= Y + 1, indomain(Z). X = X{1 .. 9} Y = Y{[0, 2 .. 8]} Z = 0 There is 1 delayed goal. Even though 2 is still in the domain of X, it is not a solution! The delayed goal is an indication of this (within a program, you can use subcall/2 to check for delayed goals). In general, you need to label all variables to be sure that there are no delayed goals left, and the values really are solutions. -- Joachim Schimpf / phone: +44 20 7594 8187 IC-Parc / mailto:J.Schimpf@imperial.ac.uk Imperial College London / http://www.icparc.ic.ac.uk/eclipseReceived on Tue Jan 18 16:20:14 2005

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