[Scip] Question about branching on master variables in SCIP

[Felipe Serrano]

Hi Yongjia

I can imagine that SCIP doesn't deal with the extra complications
automatically.
Actually, in the binpacking example explicit logicor constraints are added
to the pricing problem in order to avoid generating the same packing.
So I am guessing that you would need to add some constraint that avoids the
generation of a fixed route to your pricing problem.

I do not have any explanation for the behavior you describe apart from:
SCIP might be generating the same variable on the branch where it is
supposed to be fix to 0.

Best,
Felipe

On Fri, Aug 7, 2015 at 4:02 PM, Yongjia Song <ysong3 at vcu.edu> wrote:

> Hi Benny,
>
> Thanks a lot! I actually tried a set partitioning formulation with only
> master variables, and claim them to be integer so branching has to be done
> for these variables. I only specify a pricer as the one in the VRP example.
> Will Scip automatically take care of the extra complication in pricing
> because of branching? I actually tried this, and sometimes it gave the
> correct solution(maybe out of luck) but it also happened that no progress
> is made at all after the root. Any explanation on this? Thanks a lot!
>
> Best,
> ------------------------------
> From: Benjamin Müller <benjamin.mueller at zib.de>
> Sent: ‎8/‎7/‎2015 3:50 AM
> To: Yongjia Song <ysong3 at vcu.edu>; SCIP mailing list <scip at zib.de>
> Subject: Re: [Scip]  Question about branching on master variables in SCIP
>
> Hi Yongjia,
>
> you are completely right, branching on the reformulated variables does
> change the nature of your pricing problem. If your model does only contain
> the routing variables and you declare them as integral then SCIP will
> branch on those as SCIP would usually do. The user has to respect the
> branching decisions made by SCIP in his pricer. You could add your own
> branching rule in order to branch on the original variables even though
> they are not in the model (see Ryan/Foster branching in the Binpacking
> example).
>
> Note that, in the VRP example the routine variables are continuous and
> thus branching will only be applied on edge variables. Those branching
> decisions does not influence the pricing problem too much (excluding edges
> from the graph).
>
>
> Best,
>
> Benny
>
>
> On 08/06/2015 08:01 PM, Yongjia Song wrote:
>
> Hi all,
>
> I have a question about how SCIP handles branching when only the master
> variables present in the model. Taking VRP as an example, suppose we only
> have the route variables (without the original edge variables or the
> linking constraints between edge variables and route variables), and we
> impose the integrality constraints on these variables, it is not clear how
> branching on these variables are implemented in SCIP. Indeed, when a
> branching decision is made on some route variable, it complicates the
> pricing by forcing the pricing algorithm to avoid generating that route.
> Without specific problem structure, it is not clear to me how SCIP handles
> this case. Thanks a lot for your help!
>
> Best,
>
> --
> SONG, Yongjia
> Assistant Professor
> Department of Statistical Sciences and Operations Research
> Virginia Commonwealth University
> PO Box 843083
> Richmond, VA 23284-3083
> 804-828-1149
> Ysong3 at vcu.edu
>
>
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>
>
> --
> ______________________________
> Benjamin Müller
> Zuse Institute Berlin
> Takustr. 7, 14195 Berlinbenjamin.mueller at zib.de+49 30 841 85-195
>
>
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