仅在约束Gurobi Java中使用决策变量

Question

Usually in Gurobi's Java interface I create the model by adding variables to the model with this for example:

for (int i = 0; i < n; i++){
    for (int j = 0; j < n; j++) {
        XobjectiveCoef = distance(i, j);
        X[i][j] = model.addVar(0.0, 1.0, XobjectiveCoef, GRB.BINARY, "x" + String.valueOf(i) + "_" + String.valueOf(j));

    }
}

When I am done I have my model which is the sum of all Xs over i and j, and I can use X[i][j] in any constraint I want with something like this:

for (int j = 0; j < n; j++) {
    GRBLinExpr expr = new GRBLinExpr();
    for (int i = 0; i < n; i++){
        expr.addTerm(1.0, X[i][j]);
    }
    model.addConstr(expr, GRB.EQUAL, 1.0, "Name_of_constraint");
}

Now what I am looking for, is a way to use decision variable only in a constraint and not in the model definition. I checked if addVar is available for GRBLinExpr but it is not.

Thanks

Answer 1

You can create a GRBLinExpr and reuse it. For example:

model.addConstr(expr, GRB.EQUAL, 1.0, "ct1");
expr.addTerm(1.0, Y);
expr.addTerm(1.0, Z).
model.addConstr(expr, GRB.GREATER_EQUAL, 2.0, "ct2");

This may or may not be more efficient than creating a new GRBVar object.