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problem.iiswrite

problem.iiswrite

Purpose
Writes an LP/MPS/CSV file containing a given Irreducible Infeasible Set (IIS). If 0 is passed as the IIS number parameter, the initial infeasible subproblem is written.
Synopsis
problem.iiswrite(num, fn, type, typeflags)
Arguments
num 
The ordinal number of the IIS to be written.
fn 
The name of the file to be created.
type 
Type of file to be created:
0 
creates an lp/mps file containing the IIS as a linear programming problem;
1 
creates a comma separated (csv) file containing the description and supplementary information on the given IIS.
typeflags 
Flags passed to the problem.write function.
Example
This writes the first IIS (if one exists and is already found) as an lp file. 
p.iiswrite(1, "iis.lp", 0, "l")
Further information
1. Please note that there are problems on the boundary of being infeasible or not. For such problems, feasibility or infeasibility often depends on tolerances or even on scaling. This phenomenon makes it possible that after writing an IIS out as an LP file and reading it back, it may report feasibility. As a first check it is advised to consider the following options:
  1. save the IIS using MPS hexadecimal format to eliminate rounding errors associated with conversion between internal and decimal representation.
  2. turn presolve off since the nature of an IIS makes it necessary that during their identification the presolve is turned off.
  3. use the primal simplex method to solve the problem.

2. Note that the original sense of the original objective function plays no role in an IIS.
3. Even though an attempt is made to identify the most infeasible IISs first by the problem.iisfirst, problem.iisnext, and problem.iisall functions, it is also possible that an IIS becomes just infeasible in problems that are otherwise highly infeasible. In such cases, it is advised to try to deal with the more stable IISs first, and consider to use the infeasibility breaker tool if only slight infeasibilities remain.
4. The LP or MPS files created by problem.iiswrite corresponding to an IIS contain no objective function, since infeasibility is independent from the objective.
Related topics
problem.getiisdata, problem.iisall, problem.iisclear, problem.iisfirst, problem.iisisolations, problem.iisnext, problem.iisstatus.

Parent Topic Reference Manual

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