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:

  • It is still infeasible, and
  • If a single constraint or bound is removed, the subsystem becomes feasible.
Note that an infeasible model may have multiple IISs. The one returned by Gurobi is not necessarily the smallest one; there may exist others with fewer constraints or bounds.

IIS results are returned in a number of attributes: IISConstr, IISLB, IISUB, IISSOS, IISQConstr, and IISGenConstr. Each indicates whether the corresponding model element is a member of the computed IIS.

The IIS log provides information about the progress of the algorithm, including a guess at the eventual IIS size.

If an IIS computation is interrupted before completion, Gurobi will return the smallest infeasible subsystem 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.

Use the IISMethod parameter to adjust the behavior of the IIS algorithm.

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);