Using CPLEX in TspIpSolver

First, we need to set up the objective and the degree constraints. First, add the following fields to the class

private IloCplex cplex;
private TspInstance<V,E> tspInstance;
private final ImmutableBiMap<E,IloIntVar> edgeVariables;

and initialize them, as below.

	public TspIpSolver(TspInstance<V,E> tspInstance, EnumSet<Option> options) throws IloException{
		this.options = options;
		this.tspInstance = tspInstance;
		this.cplex = new IloCplex();
		UndirectedGraph<V,E> graph = tspInstance.getGraph(); //for convenience, we will be using this a lot
		this.edgeVariables = Util.makeBinaryVariables(cplex, graph.getEdges());
		//the degree constraints
		//the objective		
	}

The constraints and objective still need to be added to the cplex object. Try adding them yourself! The following methods (as defined in the previous 3 pages) should be useful for making the constraints:

• From Util, the static method integerSum(IloCplex cplex, BiMap<T,IloIntVar> variables, Iterable<T> set)
• From IloCplex, the method addEq(IloNumExpr e, double v)
• From UndirectedGraph<V,E>, the method getIncidentEdges(V vertex)

If you are unfamiliar with Java, consider viewing the solution for the constraint, then trying the objective yourself.
Solution

		//the degree constraints
		for(V vertex: graph.getVertices()){
			cplex.addEq(Util.integerSum(cplex, edgeVariables, 
					graph.getIncidentEdges(vertex)), 2);
		}

For the objective, we need the functions:

• From Util, the static method sum(IloCplex cplex, BiMap<T,IloIntVar> variables, Iterable<T> set, Function<? super T,? extends Number> coefficients)
• From UndirectedGraph<V,E>, the method getEdges()
• From TspInstance, the method getEdgeWeights

Solution

		//the objective
		cplex.addMinimize(Util.integerSum(
				cplex, edgeVariables, graph.getEdges(),tspInstance.getEdgeWeights()));