[SCIP] best solution is not feasible in original problem

[Gregor Hendel]

Hi Tobias,

thanks for sharing the code to reproduce the issue. I actually had a 
hard time to reproduce the issue because I was linking to SCIP 6.0.1 
(which hasn't been released yet). I was able to reproduce your issue 
with SCIP 6.0. WIth 6.0.1, SCIP terminates with a feasible, optimal 
solution, which I verified using CPLEX.

We are currently preparing this bugfix release that will fix the issue 
and be hopefully released before Christmas. If you cannot wait that 
long, I can provide you with a prerelease of the next bugfix version 
that seems to solve your issue.

Best,
Gregor





Am 04.12.18 um 15:42 schrieb Tobias Stengel:
>
> Hello,
>
> I’m solving set cover problems with SCIP 6.0.0. I have one instance 
> where SCIP prints
>
>   [linear] <c_276>: <x_1013>[B] (-0) +<x_1019>[B] (-0) +<x_1021>[B] 
> (-0) +<x_1131>[B] (-0) +<x_1139>[B] (-0) +<x_1142>[B] (-0) 
> +<x_1301>[B] (-0) +<x_1302>[B] (-0) +<x_1317>[B] (-0) +<x_1318>[B] 
> (-0) +<x_1322>[B] (+0) +<x_1323>[B] (+0) +<x_1324>[B] (-0) 
> +<x_1354>[B] (-0) +<x_1446>[B] (-0) +<x_1449>[B] (-0) +<x_1450>[B] 
> (-0) +<x_1451>[B] (-0) +<x_1452>[B] (-0) +<x_1617>[B] (-0) 
> +<x_1621>[B] (+0) +<x_1622>[B] (-0) +<x_1623>[B] (-0) +<x_1624>[B] 
> (-0) +<x_1625>[B] (-0) +<x_1626>[B] (+0) +<x_1627>[B] (-0) 
> +<x_1820>[B] (+0) +<x_1952>[B] (+0) >= 1;
>
> ;
>
> violation: left hand side is violated by 1
>
> best solution is not feasible in original problem
>
> SCIPgetStatus returns SCIP_STATUS_OPTIMAL.
>
> For some reason this error is not reproducible with LP or MPS files – 
> most probably due to slight loss of numeric precision of the objective 
> coefficients.
>
> Therefore I attached a small C++ problem that reproduces the issue.
>
> After compiling setcover.cpp you can run
>
> ./setcover sets.csv
>
> Unfortunatetly the programm requires boost for splitting strings – 
> sorry for that.
>
> For my understanding there is either a bug involved or the presolver 
> is turning a (as far as I can see) numerically stable MIP into 
> something unstable. CBC solves this MIP without issues.
>
> Obvious workaround would be to check the constraint violation and try 
> a second run with the presolver turned of…
>
> Tobias
>
>
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