Try our new documentation site (beta).


int GRBcomputeIIS ( GRBmodel *model )

Compute an Irreducible Inconsistent Subsystem (IIS). An IIS is a subset of the constraints and variable bounds with the following properties:

  • the subsystem represented by the IIS is infeasible, and
  • if any of the constraints or bounds of the IIS is removed, the subsystem becomes feasible.
Note that an infeasible model may have multiple IISs. The one returned by Gurobi is not necessarily the one with minimum cardinality; there may exist others with fewer constraints or bounds.

If an IIS computation is interrupted before completion, Gurobi will return the smallest IIS found to that point.

This routine populates the IISConstr, IISGenConstr, IISQConstr, IISSOS, IISLB, and IISUB attributes. You can also obtain information about the results of the IIS computation by writing a .ilp format file (see GRBwrite). This file contains only the IIS from the original model.

Note that this routine can be used to compute IISs for both continuous and MIP models.

Return value:

A non-zero return value indicates that a problem occurred while computing the IIS. Refer to the Error Code table for a list of possible return values. Details on the error can be obtained by calling GRBgeterrormsg.


model: The infeasible model. This routine will return an error if the input model is feasible.

Important note:

This routine only reports whether the computation ran into an error. Query the IISConstr, IISGenConstr, IISQConstr, IISSOS, IISLB, or IISUB attributes to determine the result of the computation (see the Attributes section for more information on querying attributes).

Example usage:

  error = GRBcomputeIIS(model);

Try Gurobi for Free

Choose the evaluation license that fits you best, and start working with our Expert Team for technical guidance and support.

Evaluation License
Get a free, full-featured license of the Gurobi Optimizer to experience the performance, support, benchmarking and tuning services we provide as part of our product offering.
Academic License
Gurobi supports the teaching and use of optimization within academic institutions. We offer free, full-featured copies of Gurobi for use in class, and for research.
Cloud Trial

Request free trial hours, so you can see how quickly and easily a model can be solved on the cloud.