[SCIP] Different results using SCIPchgVarUb and SCIPchgVarUbNode

[Diego Delle Donne]

Dear SCIP communit,

I'm coding a Branch & Price with a branching rule quite similar to the
Ryan-Foster rule. So, when a branching is performed, some variables need to
be removed from the created nodes (i.e., fixed to 0).

As a first approach (approach A), I had an ObjEventhdlr which is called
when a node is created. It checks every existing variable and if it needs
to be removed the variable is fixed to 0. I do this by
calling SCIPchgVarUb(...) within the scip_exec(...) method of my
ObjEventhdlr.

This works fine, but I noted that it shouldn't be necessary to iterate
through EVERY existing variable each time I branch, since the number of
variables to be fixed is usually just a small portion of the existing ones.
Moreover, in my code, when I perform a branching, I already have access to
the (small) list of variables which should be fixed. Hence, I tried to fix
them at the moment of the node creation (within the scip_execlp(...) method
of my ObjBranchrule object) by using SCIPchgVarUbNode(...), and forgetting
about my ObjEventhdlr.

This new approach (approach B) seems to work fine also and it is usually
faster than the first one (of course, that was the goal!), but I have found
some funny differences in some cases. For instance, sometimes the rounding
heuristic in approach A finds better solutions than the one in approach B.
This allows approach A to end the solving process faster than approach B,
although being "slower" for the first part of the process.

Any ideas on what's causing this behavior? I would think that the solving
process should be the same for both approaches, as I am just omitting to
iterate through a bunch of variables which I won't change anyway. Does
SCIPchgVarUb(...) behave differently from SCIPchgVarUbNode(...) in some way?

Thanks in advance!
Best regards to all
Diego





<http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail>
Libre
de virus. www.avg.com
<http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail>
<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listserv.zib.de/pipermail/scip/attachments/20190423/859293e4/attachment.html>