An important feature of ILOG CPLEX is that you can modify a previously created model to consider different scenarios. Furthermore, depending on the optimization model and algorithm used, ILOG CPLEX will save as much information from a previous solution as possible when optimizing a modified model.

The most important modification method is IloModel.add, for adding modeling objects to the active model. Conversely, you can use IloModel.remove to remove a modeling object from a model, if you have previously added that object.

When you add a modeling object such as a ranged constraint to a model, all the variables used by that modeling object implicitly become part of the model as well. However, when you remove a modeling object, no variables are implicitly removed from the model. Instead, variables can only be explicitly removed from a model by calling IloMPModeler.delete. (The interface IloMPModeler derives from the class IloModel, among others. It is implemented by the class IloCplex.) This call will cause the specified variables to be deleted from the model, and thus from all modeling objects in the model that are using these variables. In other words, deleting variables from a model may implicitly modify other modeling objects in that model.

The API of specific modeling objects may provide modification methods. For example, you can change variable bounds by using the methods IloNumVar.setLB and IloNumVar.setUB. Similarly, you can change the bounds of ranged constraints by using IloRange.setLB and IloRange.setUB.

Because not all the optimizers that implement the IloModeler interface support the ability to modify a model, modification methods are implemented in IloMPModeler. These methods are for manipulating the linear expressions in ranged constraints and objective functions used with IloCplex. The methods IloMPModeler.setLinearCoef, IloMPModeler.setLinearCoefs, and IloMPModeler.addToExpr apply in this situation.

The type of a variable cannot be changed. However, it can be overwritten for a particular model by adding an IloConversion object, which allows you to specify new types for variables within that model. When ILOG CPLEX finds a conversion object in the active model, it uses the variable types specified in the conversion object instead of the original type specified for the optimization. For example, in a model containing the following lines, ILOG CPLEX will only generate solutions where variable x is an integer (within tolerances), yet the type returned by x.getType will remain IloNumVarType.Float.

IloNumVar x = cplex.numVar(0.0, 1.0);  
cplex.add(cplex.conversion(x, IloNumVarType.Int));

A variable can be used only in at most one conversion object, or the model will no longer be unambiguously defined. This convention does not imply that the type of a variable can be changed only once and never again after that. Instead, you can remove the conversion object and add a new one to implement consecutive variable type changes. To remove the conversion object, use the method IloModel.remove.