Adding two objective function with weight in CPLEX - c++

Question

in my math model, I have two objective functions which based on its importance I want to allocate weight to them and add them together as one objective function.
here are my two Objectives added together with weight:
IloExpr objExpression(env);
for (cc = 0; cc < NumberOfCourses; cc++)
for (ww = 0; ww < AvailableWeeks; ww++) {
objExpression += Weight * Diff[cc][ww]; // objective a
}
for (cc = 0; cc < NumberOfCourses; cc++) {
objExpression += (1 - Weight) * (M[cc] * Students[cc]); // objective b
}
IloObjective theObjective(env, objExpression, IloObjective::Minimize);
mod.add(theObjective);
objExpression.end();
i have set the parameters and variables as follow:
const int NumberOfCourses = 15;
const int AvailableWeeks = 8;
const float Weight = 0.5;
double Students[NumberOfCourses];
IloNumVarArray2 Diff(env, NumberOfCourses);
for (cc = 0; cc < NumberOfCourses; cc++)
Diff[cc] = IloNumVarArray(env, AvailableWeeks, 0.0, 8);
IloNumVarArray M(env, NumberOfCourses);
when I run the code it freezes and sets the breakpoint at the second objective line.
also, what should I do if I wanted to have two separate objectives and get output for both individually?

Answer 1

Check the reference documentation of the IloNumVarArray(env, IloInt) constructor. It says
This constructor creates an extensible array of n numeric variables in
env. Initially, the n elements are empty handles.
In other words, all elements in the newly created array will be NULL (note that this is different from the IloBoolVarArray constructor which creates an array in which all elements are new variables). Thus M is an array of empty handles in your case.
In order to fix you code you either have to initialize the elements of the array one by one or use a constructor that initializes the variables, like for example
IloNumVarArray M(env, NumberOfCourses, 0, IloInfinity);
Additionally, I suggest that you compile your code in debug mode without NDEBUG being defined. If you compile without NDEBUG then many of the functions in Concert will raise an exception if you use empty handles.
If you want to have separate objectives then your problem becomes a multi-objective problem. You may want to read the respective chapter in the user manual and also refer to the ilodiet.cpp example that ships with CPLEX.