[Scip] Fractional solution, pruned node

[Cristina Núñez del Toro]

Dear Gerald,

thank you for you response.


About 2) yes, I am sure that pricing is performed at this node, and 3) I
start the B&P with a with a set of initial variables that gives a feasible
primal solution to the integer problem. Both, initial and priced variables
are marked as removable and all constraints are marked as modifiable.

About 1), I think this could be actually the problem. I do not compute the
lower bound at any point. I just followed the binpacking example to create
my own implementation but I missed this issue. In fact, I also noticed that
whenever an integer LP solution gets into the pricing callback, scip do not
update the best upper bound in case of promising one. I read a previous
email about this issue and recommended to use SCIPupdateCutoofbound()
and/or SCIPsetObjlimit(). However, what I am more concerned about why this
integral and feasible solution is not stored as a Primal bound of the
original Integer Master Problem thant setting a new cuttoffbound. If you
can help me explaining me a little bit more about this because I'm find
myself quite lost with that.


Best regards,



2015-03-04 19:35 GMT+01:00 Gerald Gamrath <gamrath at zib.de>:

>  Dear Christina,
>
> sorry for the late reply, but we were quite busy in the last weeks.
>
> There might be different reasons for this behavior.
>
> 1) Does your pricing callback compute a lower bound and sets the
> lowerbound pointer accordingly? If this is higher than the cutoff bound,
> the node will be cut off.
>
> 2) Perhaps the propagation already detected infeasibility? Are you sure
> that you perform pricing at this node?
>
> 3) Are all your variables created by pricing and all constraints marked to
> be modifiable? Otherwise, the enforcement might also detect infeasibility
> of an unmodifiable constraint.
>
> Best,
> Gerald
>
> Am 19.02.2015 um 15:27 schrieb Cristina Núñez del Toro:
>
>     Dear all,
>
>  I am currently implemented a Branch&Price algorithm. For my problem, I
> have 3 types of variables, say "x","y" and "z". I have just finished my on
> branching rule that implies to only branch on the "z" variables.
> Apparently, everything goes ok; I mean, everytime SCIP enters to the
> branchexeclp routine, it looks for the most fractional "z" variable and do
> branch on it. However, I have noticed that a certain point of the
> algorithm, after finishing the pricing loop, SCIP "skips" (sorry for the
> joke) the branching phase (the node is cutted off/pruned), I mean, it does
> not enter to any branching callback method and goes directly to the handler
> constraint to propagate another node. As far I understand, this would be of
> course a normal behaviour if, after finishing the pricing stage :
>
>  a) the objective value of the current LP is greater or equal than the
> incumbent,
> b) the current LP solution is an integer solution,
>  c) the current LP solution is an integer solution and it is optimal.
>
>  However, I found a pruned node with a fractional LP solution (inluding
> some "z" variables with fractional value) but with the objective value strictly
> lower than the incumbent.
>
>  Is there any reason for expecting this?
>
>  Thanks in advances,
>
>  Best regards,
>
>
> --
> ---
> Cristina Nuñez
>
>
> _______________________________________________
> Scip mailing listScip at zib.dehttp://listserv.zib.de/mailman/listinfo/scip
>
>
>


-- 
---
Cristina Nuñez
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