I agree that that description is misleading in the case of set variables.
The idea behind the constraint is symmetry breaking: if the two values are
symmetric, the constraint breaks that symmetry. See e.g. the social golfers
example on page 241 of MPG.
On 28 November 2017 at 08:52, Tim Potter <t...@frungy.org> wrote:
> Thanks Chris and Sebastian for your replies. I agree that your parsing of
> the mathematical description of the constraint is valid but I don't think
> the result is in the spirit of the constraint, as much as I can divine this
> from the documentation.
>
> "Value precedence constraints over set variables enforce that a value
> pieces another value in a set of array variables" is the overview of the
> precede() constraint in Modelling and Programming with Gecode. Surely if
> there is any case where the first value is not placed before, i.e is
> preceded by, the second then there doesn't seem to be any point to the
> constraint.
>
> I think I'll dive into the source code and test case to try and get some
> more insight.
>
> (I forgot to mention in my original message that the constraint is
> documented in MPG, p.93).
>
>
> Thanks,
>
> Tim.
>
> > On 27 Nov 2017, at 11:45 am, Chris Mears <ch...@cmears.id.au> wrote:
> >
> > Hello Tim,
> >
> > I think you might be misreading this version of the constraint. The
> constraint means "if s and t are ever treated differently -- that is, if
> there's a set where one of them is present but not the other -- then you'll
> get a set with only s before you get any set with only t." So your example
> satisfies the constraint because s and t are always either both present or
> both absent.
> >
> > Cheers,
> > Chris
> >
> >
> > On 25 November 2017 at 08:38, Tim Potter <t...@frungy.org> wrote:
> > Hi everyone. I hope this isn't some silly misunderstanding but I'm
> having trouble figuring out how the precede() constraint works when applied
> to SetVarArray.
> >
> > Imagine I have a set of tasks with dependencies, for example 1->2, 2->3
> and 3->4 to keep things simple. I've added the following constraints:
> >
> > // Array of sets of integers from {1..4}
> >
> > SetVarArray deps(*this, 4, IntSet::empty, IntSet(1, 4));
> >
> > // Enforce pairwise disjointedness
> >
> > sequence(*this, deps);
> >
> > // Final task must appear
> >
> > rel(*this, setunion(deps) >= IntSet(IntArgs(1, 4)));
> >
> > // Task ordering constraints
> >
> > precede(*this, deps, 1, 2);
> > precede(*this, deps, 2, 3);
> > precede(*this, deps, 3, 4);
> >
> > Now this should produce at least a solution like {{1}, {2}, {3}, {4}}.
> However I get this as my first solution in the output:
> >
> > {{1..4}, {}, {}, {}}
> >
> > which indicates that the precede() constraint is not behaving as I think
> it should.
> >
> > According to to the documentation the precede(home, x, s, t) version of
> the value precedence constraint as applied to set variables says that s is
> not an element of x_j and t is an element of x_j, then i < j such that s is
> an element of x_i and t is not an element of x_i.
> >
> > I don't understand how the solution above satisfies the above unless
> maybe the i < j is really acting as i <= j. This seems to be the case
> based on the other solutions as they're sorted but that could be a side
> effect of the sequence() constraint.
> >
> > Does anyone know what I'm doing wrong here?
> >
> >
> > Thanks,
> >
> > Tim.
> > _______________________________________________
> > Gecode users mailing list
> > users@gecode.org
> > https://www.gecode.org/mailman/listinfo/gecode-users
> >
>
>
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