[gecode-users] question about lexical ordering constraint and others

[Kish Shen]

Hi Christian,

Thanks for your reply!

On 10/05/2011 19:25, Christian Schulte wrote:
> Hi Kish,
>
> 1) Hmmm, never thought about that. You could emulate this behavior by
> extending one array as you suggested. Do you know any application that
> relies on this behavior?
>

Well, I don't really know applications that well, especially with using 
lexical ordering in a constraint setting, so I can't say if any 
application relies on this behaviour.

However, being able to sort sequences of different lengths in lexical 
order seems very natural and useful -- dictionaries (for languages with 
an alphabet) relies on this.

I think in practice for implementing the constraint behaviour, you don't 
always have to extend the shorter sequence....

> 2) Just ICL_DEF which is as the algorithm by Paul Shaw, see the referenced
> paper.
>

OK, I see. I guess this means the propagation strength doesn't fit well 
into the predefined levels.

> 3) Gecode always chooses the best propagator, cumulative is a good example:
> if all the variables for the use are assigned it'll choose the special case.
> If it is clear that cumulative is not needed (for example, all tasks require
> more than half of the resource), a disjunctive scheduling propagator is
> posted. This is true for all constraints: you ask which constraint you want
> to have and Gecode posts the best propagator(s) it can for the constraint.
>

Great, this is what I was hoping for, so I don't need to do all the 
extra checks to chose the right version of the constraint to call.

Thanks again,

Kish
> Cheers
> Christian
>
> --
> Christian Schulte, KTH, web.it.kth.se/~cschulte/
>
> -----Original Message-----
> From: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
> Of Kish Shen
> Sent: Tuesday, May 10, 2011 6:35 PM
> To: users at gecode.org
> Subject: [gecode-users] question about lexical ordering constraint and
> others
>
> Hi,
>
> I have a few (more) question about various Gecode constraints that I am
> interfacing to ECLiPSe:
>
> 1) the constraint for lexical ordering
>
> rel(Home, IntVarArgs, IntRelType, IntVarArgs)
>
> requires the two arrays to be the same size. It seems the same restriction
> applies in the global constraint catalog for the lexical ordering
> constraints as well. However, the existing ECLiPSe lexical ordering
> constraints does not have this restriction -- i.e. the two sequences of
> numbers being compared can be different lengths. What is the reason for the
> same length restriction?
>
> The lexical ordering for sequences of different length used in ECLiPSe seems
> to be the natural one, where missing numbers in the shorter sequence are
> replaced by 'bottom', i.e. a value smaller than any number.
> So (using . to separate numbers in a sequence)
>
> 1.3>  1.2.5
>
> 1.2.5>  1.2
>
> etc.
>
> Could such a definition of ordering be used in the Gecode version as well?
>
> 2. What IntConLevel is supported in the binpacking constraint? I can't find
> a specification for this in the reference documentation (in
> doc-latest) [beyond the default of ICL_DEF]
>
> 3. For various versions of the same constraint (e.g. cumulative), some
> arguments can be IntArgs or IntVarArgs. In general, does it make a
> difference if I always use the version with IntVarArgs, even if all the
> IntVar has only one element in their domain (i.e. they are integers)?
> I guess what I am asking is if (in general) Gecode does a check of its
> IntVarArgs argument to see if they are all integers and calls a more
> efficient implementation for the constraint if there is one? [I was
> specifically looking at the cumulative constraint, but there are obviously
> others].
>
> The reaosn I ask is that in interfacing such constraints to ECLiPSe, I
> currently don't check if such arguments are all integers or if they contain
> some domain variables (IntVar). I can do this check, but if it is done in
> Gecode already, there is no reason for me to do so.
>
>
> Thanks in advance for any help and information!
>
> Cheers,
>
> Kish
>
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