cplex java constraint set up using assignment operators "+="

Question

I am using Cplex in Java and I have a probem with setting up a constraint.

I've got a matrix x[n][n], where x can be 0 or 1. For every i(1..n) the sum of all j(1..n) = 1

Regarding the answer in this post: Cplex c++ multidimensional decision variable , I set up my code like this:

for (int i=0; i<n; i++){
            IloNumExpr sum_over_i;
            for(int j=0; j<n; j++){
                sum_over_i += x[i][j];
            }   
            cplex.addEq(sum_over_i, 1.0);
        }

For the 4th line of the code java shows this error: "The operator += is undefined for the argument type(s) IloNumExpr, IloNumVar"

Is it possible that assignment operators like "+=" work for cplex c++ but not for cplex java?

As an alternative, I tried this:

for(int i=0; i<n; i++){
            IloNumExpr sum_over_i = null;
            for(int j=0; j<n; j++){
                cplex.eq(sum_over_i, cplex.sum(sum_over_i, x[i][j]));
            }   
            cplex.addEq(sum_over_i, 1.0);
        }

In this case Eclipse comes up with this error note when running:

Exception in thread "main" java.lang.NullPointerException
at ilog.cplex.CpxQextractor.visitLinkedExpr(CpxQextractor.java:73)
at ilog.cplex.CpxLinkedExpr.accept(CpxLinkedExpr.java:46)
at ilog.cplex.CpxRange.<init>(CpxRange.java:1337)
at ilog.cplex.IloCplexModeler.makeRange(IloCplexModeler.java:4316)
at ilog.cplex.IloCplexModeler.eq(IloCplexModeler.java:4902)
at TSP.main(TSP.java:57)

Can anybody tell me whats wrong about my code?

Thanks in advance.

Operator overloading is not supported in java, you can't just take c++ code and expect it to work in java. You should probably read a java tutorial instead of stumbling around in the dark, that would also clear up the confusion about the null pointer exception — Voo
Keep in mind that null is not the same as 0 as it can be in some languages. null is something entirely different. — Erik Nguyen
Thanks for your answers. Operations like "+=" work in java, so I expected that it would be usable for java/cpley too when seeing the c++ code. — Hendrik

user327301 user327301 · Accepted Answer · 2013-07-29T06:31:53

I remember also having a very hard time finding an example of how to do linear expressions in the CPLEX Java API, although the documentation in general for CPLEX is very good.

To do the constraints "For every i(1..n) the sum of all j(1..n) = 1" I would do

for(int i=0; i < n; i++) {
  IloLinearNumExpr lhs = cplex.linearNumExpr(); //lhs as in left hand side
  for(int j=0; j < n; j++) {
    lhs.addTerm(1.0, x[i][j]);        
  }   

  IloRange con = cplex.addEq(lhs, 1);
  con.setName("yourConstraintName(" + i + ")");      
}

You don't need to set a name for your constraint but it will always help you debug your model.

cplex java constraint set up using assignment operators "+="

1 Answers