Generation of variables in Cplex Java API

Question

I´m using a Cplex Java API for the first time, usually I just use GAMS. I am trying to generate x[i] variable and an objective function that is a multiplication of x[k] and a parameter rl[k] which is of the type double. I always get an error that I am not understanding. Following is the code:

public void variable() throws IloException{
    int i=0; 
    this.rl = new double [(int)Data.Count+1];  

    IloCplex cplex = new IloCplex();

    IloNumVar[] x = new IloNumVar[Data.Count+1];

        for(int Pnr = 0; Pnr <= Data.PCount; Pnr++)
        {
            if(Data.X[Pnr]==1)
            {
                i++;

              x[i] = cplex.boolVar(); 
              rl[i]=Data.RL[Pnr];

            }
        }

     // Optimization problem
        IloLinearNumExpr obj = cplex.linearNumExpr();

        for(int k=0; k<=i; k++){
            obj.addTerm(x[k], rl[k]); 
        }

        cplex.addMaximize(obj);

    }

and error:

Exception in thread "main" java.lang.NullPointerException
at ilog.cplex.CpxLinearExpr.unmarkVars(CpxLinearExpr.java:402)
at ilog.cplex.CpxLinearExpr.removeDuplicates(CpxLinearExpr.java:515)
at ilog.cplex.CpxLinearExpr.removeDuplicates(CpxLinearExpr.java:489)
at ilog.cplex.CpxObjective.setExpr(CpxObjective.java:115)
at ilog.cplex.CpxObjective.<init>(CpxObjective.java:369)
at ilog.cplex.IloCplexModeler.objective(IloCplexModeler.java:706)
at ilog.cplex.IloCplexModeler.addObjective(IloCplexModeler.java:768)
at ilog.cplex.IloCplexModeler.addMaximize(IloCplexModeler.java:810)
at ObjCplex.report1(ObjCplex.java:52)
at ObjCplex.<init>(ObjCplex.java:20)
at Main104.main(Main104.java:11)

Answer 1

You are leaving x[0] uninitialized due to a fencepost error. In your first for loop, i starts are 0, but you are incrementing it before you use it as an index so x1 is the first element of x that is not null. You don't need a second look and can just build the obj expression in the first loop. Also, it's idiomatic for for loops use strict < (or !=) as the continuation criteria instead of '<='.

You can just build the obj expression in the first loop, avoid two loops and avoid creating the array x

IloCplex cplex = new IloCplex();
int i = 0;
this.rl = new double [(int)Data.Count+1];  
IloLinearNumExpr obj = cplex.linearNumExpr();
for(int Pnr = 0; Pnr <= Data.PCount; Pnr++)
    {
     if(Data.X[Pnr]==1)
      {
          IloNumVar x = cplex.boolVar(); 
          obj.addTerm(Data.RL[Pnr], x);
          this.rl[i++] = Data.RL[Pnr]; 
        }
    }
cplex.addMaximize(obj);