Using java cplex to solve TSP, getValues of variables gives an error

Question

I use cplex to solve the travelling salesman problem (TSP). Given that if x[i][j]=1, then the path goes from the city i to the city j, otherwise, ther is no path between these cities. The corresponding matrix:

IloNumVar[][] x = new IloNumVar[n][];
for(int i = 0; i < n; i++){
    x[i] = cplex.boolVarArray(n);
}

When cplex finishes solving, I want to get the values of x like this:

if(cplex.solve()){
    double[][] var = new double[n][n];
    for(int i = 0; i<x.length; i++){
        for(int j = 0 ; j < x[i].length; j ++){
            var[i][j] = cplex.getValue(x[i][j]);
            if(var[i][j] ==1){
                System.out.print(i + "  "+ j);
            }       
        }   
    }
}

but it gives an error. I will appreciate anyone who can give some advice. the error is in :

 var[i][j] = cplex.getValue(x[i][j]);

the explain is :

Exception in thread "main"  ilog.cplex.IloCplex$UnknownObjectException: CPLEX Error: object is unknown to IloCplex
at ilog.cplex.IloCplex.getValue(IloCplex.java:6495)

the entire code is following:

import java.io.IOException;
import ilog.concert.*;
import ilog.cplex.IloCplex;

public class TSP {
public static void solveMe(int n) throws IloException, IOException{

    //random data
    double[] xPos = new double[n];
    double[] yPos = new double[n];
    for (int i = 0; i < n; i ++){
        xPos[i] = Math.random()*100;
        yPos[i] = Math.random()*100;
    }

    double[][] c = new double[n][n];
    for (int i = 0 ; i < n; i++){
        for (int j = 0 ; j < n; j++)
        c[i][j] = Math.sqrt(Math.pow(xPos[i]-xPos[j], 2)+ Math.pow(yPos[i]-yPos[j],2));
    }

    //model

    IloCplex cplex = new IloCplex();

    //variables
    IloNumVar[][] x = new IloNumVar[n][];

    for(int i = 0; i < n; i++){
        x[i] = cplex.boolVarArray(n);
    }

    IloNumVar[] u = cplex.numVarArray(n, 0, Double.MAX_VALUE);

    //Objective
    IloLinearNumExpr obj = cplex.linearNumExpr();
    for(int i =0 ; i <n ; i++){
        for (int j = 0; j< n ;j++){
            if(j != i){
                obj.addTerm(c[i][j], x[i][j]);
            }
        }
    }
    cplex.addMinimize(obj);

    //constraints
    for(int j = 0; j < n; j++){
        IloLinearNumExpr expr = cplex.linearNumExpr();
        for(int i = 0; i< n ; i++){
            if(i!=j){
                expr.addTerm(1.0, x[i][j]);
            }
        }
        cplex.addEq(expr, 1.0);
    }

    for(int i = 0; i < n; i++){
        IloLinearNumExpr expr = cplex.linearNumExpr();
        for(int j = 0; j< n ; j++){
            if(j!=i){
                expr.addTerm(1.0, x[i][j]);
            }
        }
        cplex.addEq(expr, 1.0);
    }


    for(int i = 1; i < n; i++){
        for(int j = 1; j < n; j++){
            if(i != j){
                IloLinearNumExpr expr = cplex.linearNumExpr();
                expr.addTerm(1.0, u[i]);
                expr.addTerm(-1.0, u[j]);
                expr.addTerm(n-1, x[i][j]);
                cplex.addLe(expr, n-2);
            }
        }
    }

    //solve mode
    if(cplex.solve()){
        System.out.println();
        System.out.println("Solution status = "+ cplex.getStatus());
        System.out.println();
        System.out.println("cost = " + cplex.getObjValue());
        for(int i = 0; i<x.length; i++){
            for(int j = 0 ; j < x[i].length; j ++){
                    System.out.print(cplex.getValue(x[i][j]));                          
            }   
        }   
    }

    //end
    cplex.end();    
}
}

Answer 1

Looking at this page at the IBM knowledge center, the argument to getValue() must be something that was used to construct the model you are solving, as in the example here. Since var is n x n, you should probably initialize x as IloNumVar[][] x = new IloNumVar[n][n];. At some point before calling solve(), each element of x should be added to your model.

In your setup, the diagonal elements of x are never referenced by the model. You will notice that all your setup loops have a condition if(i != j). To fix your issue, either add an else clause to at least one of the loops, or if that is meaningless (as I suspect it is), do your printout consistently with the input:

for(int i = 0; i<x.length; i++) {
    for(int j = 0 ; j < x[i].length; j ++) {
        if(i != j)
            System.out.print(cplex.getValue(x[i][j]));
        else
            System.out.print("-");
    }   
}