Display active inactive constraints with AMPL using CPLEX or Gurobi

[Vitali Pawelko]

Dear Sirs and Madams,

I am using AMPL and the two solvers CPLEX and Gurobi.
I know AMPL transmits all model constraints to the solver.
I am solving MILP and am interested in the constraints the solver really
needs and uses for the LP relaxation.

Is it possible to display the amount of all active (binding) constraints
and all inactive constraints? Does CPLEX and Gurobi have a smart Branch
and Cut solver and use the model constraints als lazy constraints?

I discovered all possible forum posts and book of ampl, guobi, cplex and
could not find any answer yet.

Thank you in advance.

[Robert Fourer]

CPLEX and Gurobi have a feature for specifically labelling some constraints as lazy constraints; see the "lazy" option on these pages:

http://ampl.com/products/solvers/solvers-we-sell/cplex/options/
http://www.gurobi.com/documentation/6.0/ampl-gurobi/parameters.html

If you don't use this feature, however, then as far as I know these solvers use all of the constraints in the relaxations. Certainly there is no option to ask which constraints were not used.

Bob Fourer
am...@googlegroups.com

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