[Scip] Pricing in empty problem

[Martin Bergner]

Hello all,

I'm currently trying to solve a cutpacking problem as a MIP. I want to
maximize the number of cuts (imagine variable x_i for each cut i). As
constraints, for every edge in the graph, the sum of all cuts containing
this edge should be as most one. That means that each edge is crossed by
at most one cut. Because there are a lot of cuts, I want to solve that
as a column generation problem. However, the pricer is not called.

As a side note, the problem is basically (variable) empty at the
beginning. Only some empty set packing constraints are added. The output
below contains a dummy variable added to some constraint but that didn't
help either. 

While solving the root node, I want SCIP to call the included pricer.
Looking at it right now, neither pricerRedCost nor pricerFarkas are
called and the problem is reportedly solved to optimality with objective
value 0. However, the pricer's init method is called.

My basic pricer setup code:

SCIP_CALL(SCIPincludeDefaultPlugins(subscip));
SCIP_CALL(SCIPincludePricerMinCut(subscip));
SCIP_CALL(SCIPsetIntParam(subscip, "presolving/maxrestarts", 0));

SCIP_PRICER *pricer = SCIPfindPricer(subscip, "mincut"));
assert(pricer != NULL);
SCIP_CALL(SCIPactivatePricer(subscip, pricer));

As I said above, after inserting the activate pricer line which I forgot
before, the init function is called but no pricing is done. Here is the
output:

=============== >8 ============== >8 ============== >8 ===============
Solving cut packing:
[src/pricer_mincut.c:148] debug: Pricing init!
feasible solution found by trivial heuristic, objective value  0.000000e+00
presolving:
presolving (1 rounds):
 1 deleted vars, 0 deleted constraints, 0 tightened bounds, 0 added holes, 0 changed sides, 0 changed coefficients
 0 implications, 0 cliques
presolved problem has 0 variables (0 bin, 0 int, 0 impl, 0 cont) and 10698 constraints
  10698 constraints of type <setppc>
Presolving Time: 0.02

 time | node  | left  |LP iter|LP it/n| mem |mdpt |frac |vars |cons |cols |rows |cuts |confs|strbr|  dualbound   | primalbound  |  gap   
t 0.1s|     1 |     0 |     0 |     - |7116k|   0 |   - |   0 |  10k|   0 |  10k|   0 |   0 |   0 | 0.000000e+00 | 0.000000e+00 |   0.00%

SCIP Status        : problem is solved [optimal solution found]
Solving Time (sec) : 0.09
Solving Nodes      : 1
Primal Bound       : +0.00000000000000e+00 (4 solutions)
Dual Bound         : +0.00000000000000e+00
Gap                : 0.00 %

=============== >8 ============== >8 ============== >8 ===============

I have a SCIPdebugMessage also in PricerRedCost and PricerFarkas, but
they are not called. I'm using SCIP 1.2.1.4. Any Ideas? 

Regards and thanks already in advance,
Martin


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