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### Qp.java

/* Copyright 2019, Gurobi Optimization, LLC */

/* This example formulates and solves the following simple QP model:

minimize    x^2 + x*y + y^2 + y*z + z^2 + 2 x
subject to  x + 2 y + 3 z >= 4
x +   y       >= 1
x, y, z non-negative

It solves it once as a continuous model, and once as an integer model.
*/

import gurobi.*;

public class Qp {
public static void main(String[] args) {
try {
GRBEnv    env   = new GRBEnv("qp.log");
GRBModel  model = new GRBModel(env);

// Create variables

GRBVar x = model.addVar(0.0, 1.0, 0.0, GRB.CONTINUOUS, "x");
GRBVar y = model.addVar(0.0, 1.0, 0.0, GRB.CONTINUOUS, "y");
GRBVar z = model.addVar(0.0, 1.0, 0.0, GRB.CONTINUOUS, "z");

// Set objective

model.setObjective(obj);

// Add constraint: x + 2 y + 3 z >= 4

GRBLinExpr expr = new GRBLinExpr();

// Add constraint: x + y >= 1

expr = new GRBLinExpr();

// Optimize model

model.optimize();

System.out.println(x.get(GRB.StringAttr.VarName)
+ " " +x.get(GRB.DoubleAttr.X));
System.out.println(y.get(GRB.StringAttr.VarName)
+ " " +y.get(GRB.DoubleAttr.X));
System.out.println(z.get(GRB.StringAttr.VarName)
+ " " +z.get(GRB.DoubleAttr.X));

System.out.println("Obj: " + model.get(GRB.DoubleAttr.ObjVal) + " " +
obj.getValue());
System.out.println();

// Change variable types to integer

x.set(GRB.CharAttr.VType, GRB.INTEGER);
y.set(GRB.CharAttr.VType, GRB.INTEGER);
z.set(GRB.CharAttr.VType, GRB.INTEGER);

// Optimize again

model.optimize();

System.out.println(x.get(GRB.StringAttr.VarName)
+ " " +x.get(GRB.DoubleAttr.X));
System.out.println(y.get(GRB.StringAttr.VarName)
+ " " +y.get(GRB.DoubleAttr.X));
System.out.println(z.get(GRB.StringAttr.VarName)
+ " " +z.get(GRB.DoubleAttr.X));

System.out.println("Obj: " + model.get(GRB.DoubleAttr.ObjVal) + " " +
obj.getValue());

// Dispose of model and environment

model.dispose();
env.dispose();

} catch (GRBException e) {
System.out.println("Error code: " + e.getErrorCode() + ". " +
e.getMessage());
}
}
}


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