[SCIP] gap limit reached without solution

[Benjamin Müller]

Dear Marko,

yes you are right, the behaviour is kind of strange. In this situation 
SCIP does not even know that the problem is feasible. After analysing 
your instance I saw that during presolving SCIP recognize that a linear 
constraint is parallel to the objective. Thus the right hand side will 
be used for the objective cutoff but unfortunately also for checking the 
(absolute and relative) gap limit. I fixed this by checking the gap 
limit only if SCIP has found at least one solution.

Should I send you a patch for this? Or do you use the windows binaries 
of our homepage? I can compile and send them to you if you need them.


Best regards,

Benny


On 09/03/2015 09:56 AM, Horváth Markó wrote:
> Dear all,
>
> I'm using SCIP-3.1.2 with SoPlex 2.2.0 on windows (but this problem 
> also arises with SCIP-3.1.1 or SCIP-3.1.0).
>
> The problem arises when I want to solve certain MIPs with absolute gap 
> limit.
> I have attached a small example file. In this MIP the optimal solution 
> is equal to 0, and of course SCIP can solve easily. But when I set 
> absolute gap to 11 (that is, set /limits/absgap/=11) SCIP terminates 
> according to gap limit, but without solution:
>
> /SCIP Status        : solving was interrupted [gap limit reached]//
> //Solving Time (sec) : 0.01//
> //Solving Nodes      : 0//
> //Primal Bound       : +7.12610000000000e+000 (0 solutions)//
> //Dual Bound         : +0.00000000000000e+000//
> //Gap                : infinite
>
> /So, what I have got is trivially not correct, I guess.
> How can we reach gap limit without a feasible solution? And why do 
> SCIP set finite primal bound without any feasible solution? (Okay, it 
> is indeed an upper bound for the optimum (according to bounds 0 <= 
> x417 <= 7), and it is lower than the absolute gap, but SCIP should not 
> say that the gap limit reached since we have not got any solution.)
>
> Best regards,
> Marko
>
>
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-- 
______________________________
Benjamin Müller
Zuse Institute Berlin
Takustr. 7, 14195 Berlin
benjamin.mueller at zib.de
+49 30 841 85-195

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