problem with writing if statement using Boolean variable in CPLEX - c++

Question

here is a part of my cplex code. i defined the PreReq as boolean variable but in the constraint i get the error for PreReq that "expression must have bool type" what am i doing wrong?
IloBoolVarArray2 PreReq(env), CoRec(env), Offered(env);
//Constraint 6: if course d is taken and course c is its pre-req, then course c must be taken before d
IloExpr constraint6a(env);
IloExpr constraint6b(env);
IloExpr constraint6c(env);
IloExpr constraint6d(env);
for (c = 0; c < NumberOfCourses; c++) {
for (d = 0; d < NumberOfCourses; d++) {
if (PreReq[c][d] == 1 ) {
for (s = 0; s < NumberOfSemesters; s++) {
constraint6a += X[d][s];
constraint6b += X[c][s];
constraint6c += s*X[c][s];
constraint6d += s*X[d][s];
}
}
}
mod.add(constraint6a <= constraint6b);
mod.add(constraint6c <= constraint6d + (NumberOfSemesters)*(1 - onstraint6a) );
constraint6a.end();
constraint6b.end();
constraint6c.end();
constraint6d.end();
}

Answer 1

You'd rather use IloIfThen (logical constraints)
The example in CPLEX documentation:
IloIfThen(env, (x >= y && x >= z), IloNot(x <= 300 || y >= 700))

Answer 2

Expanding on Alex's answer: your problem is that you are mixing constraints with constraint generation: In the statement if ( PreReq[c][d] == 1 ) the operator== is overloaded to generate a constraint that requires PreReq[c][d] to take the value 1. So the result in this expression is of type IloConstraint, which is not a boolean type (it is a constraint object instead). That is why you get the error message. What you want instead is to add a constraint to your model that requires additional constraints in case PreReq[c][d] is 1.
So you probably want something like this:
for (d = 0; d < NumberOfCourses; d++) {
for (s = 0; s < NumberOfSemesters; s++) {
mod.add(IloIfThen(PreReq[c][d] == 1, constraint6a + X[d][s] <= constraint6b + X[c][s]));
mod.add(IloIfThen(PreReq[c][d] == 0, constraint6a <= constraint6b));
}
}
Which adds two variants of the constraint, only one of which will ever be active (depending on the value of PreReq[c][d]). Also see the reference documentation of class IloIfThen.