Simple variable selection

To chose decision variables, you evaluate the variables with an evaluator.

In the C++ API, a decision variable evaluator is an instance of IloIntVarEval. This class implements the function:

  IloNum IloIntVarEvalI::eval(IloCPEngine cp, IloIntVar x);

that returns the evaluation of the variable x.

Several predefined evaluators exist in CP Optimizer. For instance, the evaluator returned by the C++ API function call IloDomainSize(env) returns the current size of the domain of the variable that is being evaluated. Another example is the evaluator returned by IloVarIndex(env, vars) that returns the index in the array vars of the evaluated variable.

In order to select a decision variable with an evaluator, an instance of a variable selector needs to be created with the evaluator as an argument. Here are two functions in the C++ API that can create such selectors:

  IloVarSelector IloSelectSmallest(IloIntVarEval e);
  IloVarSelector IloSelectLargest(IloIntVarEval e);

The selector created by IloSelectSmallest(IloDomainSize(env)) will select the variable in the search phase that has the smallest domain. The selector returned by IloSelectRandomVar(env) chooses a variable randomly. This selector is useful for breaking ties.

In the C++ API of CP Optimizer, you use the classes IloIntVarEval and IloVarSelector and the functions IloDomainSize, IloVarIndex, IloSelectSmallest and IloSelectRandomVar.

In the Java™ API of CP Optimizer, you use the interfaces IloIntVarEval and IloVarSelector and the methods IloCP.domainSize, IloCP.varIndex, IloCP.selectSmallest and IloCP.selectRandomVar.

In the C# API of CP Optimizer, you use the interfaces IIntVarEval and IVarSelector and the methods CP.DomainSize, CP.VarIndex, CP.SelectSmallest and CP.SelectRandomVar.