[SCIP] Benders - error during generating cuts

[Schrotenboer, Albert]

Dear SCIP Community,

I am using the Benders (default) plugin for the first time. I consider a
classic network design problem with some side constraints and some bits and
pieces of column generation, but that is all done and covered by the Master
Problem (MP). The MP has only binary variables, encoding to `open' an arc
or not, and will take value 0 in the first iteration as no benders cuts are
yet included. (If this is the wise thing to do, that must be found out, but
assume for now this is what I want ;p)

The subproblem (SP) then matches the stream of commodities to the opened
arcs defined by the MP. Obviously, as no arcs will be opened, it will be
infeasible and corresponding infeasibility cuts should be generated, right?

Whatever I do, SCIP keeps telling me to merge my infeasible SP into the
master ;p!  See the error messages  below.  Is the sole solution for me to
make my own benders plugin? Should I consider doing it differently? Why is
SCIP not able to generate these cuts in its default plugin?  What are the
possible causes? I run in debug mode.

At the moment, I've chosen to make a single subproblem. I.e., different
types of commodities need to be routed along the opened arcs. I can
decompose it for each commodity individually if required. But, still, it
should be solvable with a single subproblem, right (before I spend hours on
coding it differently and see it still wont work.. )

[src/scip/benders.c:2755] ERROR: An error was found when generating cuts
for non-optimal subproblems of Benders' decomposition <default>. Consider
merging the infeasible subproblems into the master problem.
[src/scip/scip_benders.c:682] ERROR: Error <0> in function call
[src/scip/cons_benders.c:254] ERROR: Error <0> in function call
[src/scip/cons_benderslp.c:126] ERROR: Error <0> in function call
[src/scip/cons.c:3471] ERROR: Error <0> in function call
[src/scip/solve.c:3361] ERROR: Error <0> in function call
[src/scip/solve.c:4259] ERROR: Error <0> in function call
[src/scip/solve.c:4935] ERROR: Error <0> in function call
[src/scip/scip_solve.c:2693] ERROR: Error <0> in function call
[src/scip/heur_alns.c:2547] ERROR: Error <0> in function call

Hoping I won't waste your time with a stupid coding mistake,  kind regards,

Albert


-- 

Albert Schrotenboer

Postdoctoral Researcher

Faculty of Economics and Business

University of Groningen

P.O. Box 800, 9700 AV Groningen
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