How to forumulate an indicator constraint without addGenConstrIndicator

Hi Community

I'm trying to formulate a conditional Constraint without the addGenConstrIndicator Method. The reason is, that my Prof said, that I should formulate it with Big M logical conditions, so I can transfer the code to other solver that I have to evaluate.

Szenario:

The problem I solve is the Examination Timetabling Problem. The constraint I'm implementing is to minimize the number of rooms that are used.

Can you help to find the error in the addGenConstrIndicator formulation? And how can I formulate the same without addGenConstrIndicator? Is there an example?

Kind regards

Carlo

GRBVar[] roomUsed = new GRBVar[getIlpModel().getRooms().size()];

for (Room r : getIlpModel().getRooms()) {
    roomUsed[indexOf(r)] = getGrbModel().addVar(1, 1, 1.0, GRB.BINARY, r.toString());
}

for (Room r : getIlpModel().getRooms()) {

    GRBLinExpr lhs = new GRBLinExpr();

    for (Timeslot t : getIlpModel().getTimeslots()) {
        for (Presentation p : getIlpModel().getPresentations()) {
            if (getX()[indexOf(p)][indexOf(t)][indexOf(r)] == null) continue;
            lhs.addTerm(1.0, getX()[indexOf(p)][indexOf(t)][indexOf(r)]);
        }
    }
    // if lhs >= 1 then roomUsed
    // if lhs <= 0 then !roomUsed
    // At most N of A, B, C,...  a + b + c+. . . ≤ N
    // At least N of A, B, C,... a + b + c+. . . ≥ N
    getGrbModel().addGenConstrIndicator(roomUsed[indexOf(r)], 1, lhs, GRB.GREATER_EQUAL,1.0, r.toString() );
    getGrbModel().addGenConstrIndicator(roomUsed[indexOf(r)], 0, lhs, GRB.LESS_EQUAL,0.0, r.toString() );

    getObjectives().addTerm(USED_ROOM_COST, roomUsed[indexOf(r)]);

}