[Scip] Postprocessing

[Mathieu Larose]

Hi Gerald,

The aim of my postprocessing is to reduce the upper bound of the non basic
variables which are at their lower bound. So the postprocessing only affects
the descendants of the current node.


Regards,
Mathieu

On Tue, May 24, 2011 at 11:26 AM, Gerald Gamrath <gamrath at zib.de> wrote:

>  Hi Mathieu,
>
> what is the aim of your postprocessing? In SCIP, the propagation is done to
> get tighter bounds of the variables und thus (hopefully) better dual bounds.
> Therefore, the LP is solved again after bounds are reduced. If this would
> not be done, the LP solution would also not necessarily be feasible for the
> current subproblem.
>
> Could you describe in detail why and how you want to do the postprocessing?
>
> Best,
> Gerald
>
> Am 22.05.2011 19:50, schrieb Mathieu Larose:
>
> Hi Timo,
>
>  I implemented a separator which does the bound strenghtening, but I don't
> see how it can act like a postprocessing procedure (i.e. called once after
> the price-and-cut loop).
>
>  The best I can do is to set the "priority" to a negative value, set
> "delayed" to true and do some tests whether it should run or not (taken from
> sepa_redcost.c lines 115-137).
>
>  So the separator acts almost like a postprocessing procedure. But if it
> is sucessful (i.e. the domain of at least one variable is reduced), another
> round of pricing and separation occurs which I do not want. How can I
> prevent this?
>
>
>  Regards,
> Mathieu
>
>
> On Fri, May 20, 2011 at 3:07 AM, Timo Berthold <berthold at zib.de> wrote:
>
>> Hi Mathieu.
>>
>> Bound strenghtening techniques are automatically applied in the
>> propagation callbacks of certain constraint handlers (linear, knapsack,
>> quadratic,...) which interacts with the price-and-cut loop of SCIP,
>> basically in the following way: propagate->solve LP->price->cut->(if new
>> vars or cuts found, start again). Also, reduced cost strengthening (which
>> actually is a form of propagation/bound strenghtening) is applied only
>> during the separation round. The reason is that this one first needs the
>> LP to be solved to optimality.
>> If you just use SCIP as a blackbox solver, I hope this answered your
>> question.
>>
>> If you like to set up your own domain propagation routine (node
>> pre-processing), you will have to implement a propagation plugin (see How
>> to add propagators in the Doxygen-Docs) or the propagation callback of a
>> constraint handler (see How to add constraint handlers). If you need a
>> solved LP for your method (hend it rather is a post-processing), you
>> probably better implement a separator (How to..., see also
>> src/scip/sepa_redcost.c) or the separation callbac of a constraint
>> handler.
>>
>>
>> Best regards,
>> Timo
>>
>>  > Hello everybody,
>> >
>> > My apologies if this is obvious, but how can I do some postprocessing
>> work
>> > (bound strenghtening) after the price-and-cut loop has run?
>> >
>> >
>> > Regards,
>> >
>> > Mathieu
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>>
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