[SCIP] Scaling the OF and differences between primal bound and resulting OF value

[Stefan Vigerske]

Hi,

SCIP treats numbers within 1e-9 of zero as zero. If you divided your 
objective by 1e12, this would mean that objective coefficients that were 
within [-1000,1000] before are now treated as 0 by SCIP.

I don't think that there is anything to automatically print the best 
solution while solving or to write all solutions to a file at the end, 
but if you let AMPL generate a .nl file and then run scipampl on this 
.nl file with additional option -i, i.e.,
   /path/to/scipampl /path/to/myproblem.nl -i
then you will get into the interactive shell of SCIP 
(https://www.scipopt.org/doc/html/SHELL.php).
In the shell, there is a "display solutions" command that you can use to 
print all solutions that were found after a solve to the screen.

Stefan

On 3/11/21 12:24 PM, Oreste Fecarotta wrote:
> Hi,
> I'm using SCIP with AMPL, and I have a large problem involving thousands 
> of variables and contraints.
> I found that I can get some good results
> 1) by setting heuristics to aggressive and
> 2) Dividing the objective function by a large number (let me say 10^12)
> 
> With my surprise, the improvement in the primal bound that is shown 
> during the calculation is lost as I print the OF value once I reach the 
> time limit. I cannot understand why this happens. Furthermore, this does 
> not happen if I divide the OF by a smaller number (i.e. 10^6). 
> Unfortunately, in the latter case, I cannot get the same improvement in 
> the solution.
> 
> Could you please help me?
> Is there any option to print the best solution found during the 
> calculation?
> Or to store the whole solution pool?
> I found the option gams/dumpsolutions for GAMS, but I'm using AMPL...
> 
> Thank you,
> 
> Oreste Fecarotta
> PhD in Hydraulic Engineering
> Assistant Professor in Hydraulic Engineering
> University of Naples "Federico II"
> Via Claudio 21, 80125 Napoli (ITALY)
> - ScopusID:36503779200  
> <https://www.scopus.com/authid/detail.uri?authorId=36503779200>
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> - WoSID:P-7250-2016  
> <https://publons.com/researcher/1354138/oreste-fecarotta/>
> - web:Docenti unina  <www.docenti.unina.it/oreste.fecarotta>
> - Ord.Ing.NA Sez.A 20328
> - tel: +39-081-7683462
> - fax: +39-081-1683456
> 
> Il 20/01/2021 09:31, Franzen, Christian ha scritto:
>>
>> Hi Ambros.
>>
>>
>> Thanks for your reponse and sorry for answering late. I have "switched 
>> off" presolving by including no presolver plugins, hence I only got 
>> the presolving code from the propagators and constraint handlers. That 
>> seems to work properly. I started to include one presolver plugin 
>> after the other and there seems to be a problem with "domcol". It 
>> fixed a lot of variables and afterwards my problem is reported to be 
>> unbounded. I did not test all presolver plugins yet, but I will try to 
>> test all of them by the end of this week. Any idea concerning "domcol"?
>>
>>
>> Regards
>> Christian
>>
>> ------------------------------------------------------------------------
>> *Von:* Scip <scip-bounces at zib.de> im Auftrag von Ambros Gleixner 
>> <gleixner at zib.de>
>> *Gesendet:* Mittwoch, 13. Januar 2021 21:15
>> *An:* scip at zib.de
>> *Betreff:* Re: [SCIP] After stopping pricing and doing restart, model 
>> is becoming unbounded during presolve?
>> Hi Christian,
>>
>> Strange indeed.  Have you tried to turn off presolving?  Does it still
>> happen?  Have you tried to successively activate individual presolvers
>> to see when it starts failing?
>>
>> Best,
>> Ambros
>>
>>
>> Am 04.01.21 um 17:10 schrieb Franzen, Christian:
>> > Hi Scip Team.
>> >
>> >
>> > Thanks to the help of Gregor, I managed to make SCIP stop pricing after
>> > some iterations and doing a restart on the reduced master without
>> > pricing. Unfortunately, I am facing some really strang behaviour.
>> > Sometime it happens, that my model (for that I already have found a
>> > feasible solution), is reported to be unbound. I wrote the 
>> corresponding
>> > LP files before and after presolving to disk and I see that some
>> > variable bounds vanished, which seems to be the reason for the
>> > unboundness. Furthermore, some of the objective coefficients changed
>> > their sign. Any ideas how or why that is happening during presolving?
>> >
>> >
>> > Regards
>> >
>> > Christian
>> >
>> >
>> > P.S.: I attach the SCIP log for one of these runs.
>> >
>> >
>> > _______________________________________________
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>> >
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> 
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