No IIS for available model

[Luis Galárraga]

Hi all,

I am using Gurobi 7.0 to optimize a linear model with binary variables. By calling  model.optimize() I get the following output:

Optimize a model with 35 rows, 26 columns and 115 nonzeros
Variable types: 0 continuous, 26 integer (26 binary)
Coefficient statistics:
  Matrix range     [5e-01, 3e+01]
  Objective range  [1e+00, 2e+01]
  Bounds range     [1e+00, 1e+00]
  RHS range        [1e+00, 1e+01]
Found heuristic solution: objective 33
Presolve removed 31 rows and 18 columns
Presolve time: 0.00s
Presolved: 4 rows, 8 columns, 14 nonzeros
Found heuristic solution: objective 51.0000000
Variable types: 0 continuous, 8 integer (6 binary)

Root relaxation: objective 5.400000e+01, 2 iterations, 0.00 seconds

    Nodes    |    Current Node    |     Objective Bounds      |     Work
 Expl Unexpl |  Obj  Depth IntInf | Incumbent    BestBd   Gap | It/Node Time

*    0     0               0      54.0000000   54.00000  0.00%     -    0s

Explored 0 nodes (2 simplex iterations) in 0.00 seconds
Thread count was 4 (of 4 available processors)

Solution count 4: 54 51 33 28
Pool objective bound 54

Optimal solution found (tolerance 1.00e-04)
Best objective 5.400000000000e+01, best bound 5.400000000000e+01, gap 0.0000%

I am able to use the values assigned to the variables without any problem and the solution seems reasonable for my purposes. However, if I want to print the model using model.write("somefile.ilp"). I get the following error:

gurobi.GRBException: No IIS available for current model     at gurobi.GRBModel.write(GRBModel.java:156)     at dk.aau.cs.qweb.pec.fragmentsselector.ILPFragmentsSelector.dumpModel(ILPFragmentsSelector.java:245)     at dk.aau.cs.qweb.pec.fragmentsselector.ILPFr

To fix the problem, I called model.computeIIS() right after the call to optimize() which changes the error message to:

gurobi.GRBException: Cannot compute IIS on a feasible model
    at gurobi.GRBModel.computeIIS(GRBModel.java:676)

I want to print the model to be sure it matches my requirements, i.e., a sort of debugging but for some reason I cannot do it. Your help will be kindly appreciated.

Thanks,
Luis

[Renan Garcia]

The .ILP format is intended for irreducible inconsistent subsystem (IIS) models, which are only relevant when a model is infeasible (see http://www.gurobi.com/documentation/7.0/refman/ilp_format.html). Try the .LP format instead (see http://www.gurobi.com/documentation/7.0/refman/lp_format.html#format:LP), or "somefile.lp".

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[Luis Galárraga]

Thanks! It worked!