[SCIP] SCIP produces different results for the same problem instance

[Gregor Hendel]

Dear Tufan,

we are actually making an effort to keep SCIP deterministic, so you 
should see the same solution after every call on the same problem. A 
time limit might lead to different results, but this does not seem to be 
the case here.

I tend to say you got "Pythoned" here.

My best guess, as I have no other information how you invoke the problem 
solve, is that you use dictionaries
somewhere in the problem creation. Now, depending on the version, 
Python's internal hashing is nondeterministic,
see also here 
<https://stackoverflow.com/questions/14956313/why-is-dictionary-ordering-non-deterministic>. 
The same would be true for Java, I suppose. If you loop over the keys() 
of a dictionary, the order
is different every time you run, effectively generating a different 
permutation of your problem's columns and rows, which may affect the 
performance of the MIP solver and also specific solution values may be 
different between runs, but of course not the objective value of an 
optimal solution.

The link I posted contains a link to a solution to the problem using a 
PYHASHSEED environment variable, which I haven't verified.

Let us know if this is the source of, and solution to, your problem,
Gregor


Am 03.10.2018 um 13:09 schrieb Tufan BAYDEMİR:
>
> I’m using pyscipopt interface within Python to solve a MIP problem.
>
> I run the same code without making any changes and obtain different 
> optimal solutions.
>
> The objective function value is the same for all solutions.
>
> But value of the decision variables are always different ie. The 
> program ganarates alternative optimal solutions in each subsequent run.
>
> How can I insure that i get the same solution?
>
> Regards
>
> Tufan
>
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