Tuesday, March 24, 2015

Updated Java Utilities for CPLEX and CP Optimizer

I just finished adding a feature to a utility library I use in Java projects that employ either CPLEX or CP Optimizer. In addition, I moved the files to a new home. The library is free to use under the Eclipse Public License 1.0. The code is mentioned in previous posts, so I'll just quickly summarize the content here and refer interested parties to the earlier posts:
The latest source code is now available from the Michigan State University Gitlab server. You do not need to log in, and you do not need to be a Git user. You can just click the download button near the upper right to get a ZIP archive of the source code. If you run into any bugs, there is an issue tracker on the MSU site where you can report them (please!). If you just want a binary (.jar) file and the Javadoc documentation, you can download a .zip file from my MSU web space.

In the following description, please assume that cplex and cp are instances of IloCplex and IloCP respectively. The main reason I developed the library is that I like to experiment with different parameter settings for CPLEX and CP Optimizer. Rather than hard coding a parameter (e.g., cplex.setParameter(IloCplex.Param.Emphasis.MIP, 3)) and then having to edit and recompile the code to try a different value, I prefer to pass CPLEX and CP Optimizer parameters to the program through the command line (or, if my program has a graphical interface, through a user dialog). The equivalent code to the previous example might look like cplex.setParameter(pname, pval) where pname is a string with value "Emphasis.MIP" (the minimum portion of the full parameter name necessary to uniquely identify the parameter) and pval is a string with the value "3".

With that as background, here is a summary of the contents of the library:
  • CplexParameterSetter: Use an instance of this class to apply parameter settings (each specified by two strings, parameter name and new value) to an instance of IloCplex. Example code appears in the May 2014 post (look for the version 2.0 sample).
  • CPOptimizerParameterSetter: Use an instance of this class to apply parameter settings to an instance of IloCP. Again, sample code is in the May 2014 post.
  • CPDisplay: The static method CPDisplay.asString(cp) will produce a summary of the model in cp, suitable for printing or displaying. For whatever reason, IloCplex.toString() produces a human-readable version of a CPLEX model, but IloCP.toString() just prints the object's hash code. This class provides a workaround. For the output to really be readable, you need to be assiduous about assigning meaningful names to the variables and constraints in the model. I originally released this (in the March 2014 post) as an override to IloCP.toString(), but I decided a static method was easier to use (does not require extending IloCP).
  • Main program: The project comes with a main program. If you run it, you will get an alphabetized list of all parameters known to the versions of CPLEX and CP Optimizer that you are using. For instance, run against CPLEX Studio 12.6.1, the output I get looks like this:
               AbsGap double  IloCplex.Param.MIP.Pool.AbsGap
            AbsMIPGap double  IloCplex.Param.MIP.Tolerances.AbsMIPGap
            AbsMIPGap double  IloCplex.Param.MIP.PolishAfter.AbsMIPGap
              Advance int  IloCplex.Param.Advance
           WriteLevel int  IloCplex.Param.Output.WriteLevel
          ZeroHalfCut int  IloCplex.Param.MIP.Cuts.ZeroHalfCut
                      AllDiffInferenceLevel int  IloCP.IntParam.AllDiffInferenceLevel
               AllMinDistanceInferenceLevel int  IloCP.IntParam.AllMinDistanceInferenceLevel
                               WarningLevel int  IloCP.IntParam.WarningLevel
                                    Workers int  IloCP.IntParam.Workers
    At least for me, the main use of this is to figure out which parameter names are unambiguous and which are repeated. For instance, the CPLEX parameter IloCplex.Param.MIP.Pool.AbsGap can be abbreviated "AbsGap"; but the minimum amount necessary to specify the absolute MIP gap for convergence is "Tolerances.AbsMIPGap", since "AbsMIPGap" could also refer to IloCplex.Param.MIP.PolishAfter.AbsMIPGap.
If you have questions about the proper use of the code, please feel free to post them in as comments here. If you run into bugs (or missing features), please use the issue tracker. That will help me keep tabs on what needs to be done.

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