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Lpmod.java
/* Copyright 2024, Gurobi Optimization, LLC */ /* This example reads an LP model from a file and solves it. If the model can be solved, then it finds the smallest positive variable, sets its upper bound to zero, and resolves the model two ways: first with an advanced start, then without an advanced start (i.e. 'from scratch'). */ import com.gurobi.gurobi.*; public class Lpmod { public static void main(String[] args) { if (args.length < 1) { System.out.println("Usage: java Lpmod filename"); System.exit(1); } try { // Read model and determine whether it is an LP GRBEnv env = new GRBEnv(); GRBModel model = new GRBModel(env, args[0]); if (model.get(GRB.IntAttr.IsMIP) != 0) { System.out.println("The model is not a linear program"); System.exit(1); } model.optimize(); int status = model.get(GRB.IntAttr.Status); if (status == GRB.Status.INF_OR_UNBD || status == GRB.Status.INFEASIBLE || status == GRB.Status.UNBOUNDED ) { System.out.println("The model cannot be solved because it is " + "infeasible or unbounded"); System.exit(1); } if (status != GRB.Status.OPTIMAL) { System.out.println("Optimization was stopped with status " + status); System.exit(0); } // Find the smallest variable value double minVal = GRB.INFINITY; GRBVar minVar = null; for (GRBVar v : model.getVars()) { double sol = v.get(GRB.DoubleAttr.X); if ((sol > 0.0001) && (sol < minVal) && (v.get(GRB.DoubleAttr.LB) == 0.0)) { minVal = sol; minVar = v; } } System.out.println("\n*** Setting " + minVar.get(GRB.StringAttr.VarName) + " from " + minVal + " to zero ***\n"); minVar.set(GRB.DoubleAttr.UB, 0.0); // Solve from this starting point model.optimize(); // Save iteration & time info double warmCount = model.get(GRB.DoubleAttr.IterCount); double warmTime = model.get(GRB.DoubleAttr.Runtime); // Reset the model and resolve System.out.println("\n*** Resetting and solving " + "without an advanced start ***\n"); model.reset(); model.optimize(); double coldCount = model.get(GRB.DoubleAttr.IterCount); double coldTime = model.get(GRB.DoubleAttr.Runtime); System.out.println("\n*** Warm start: " + warmCount + " iterations, " + warmTime + " seconds"); System.out.println("*** Cold start: " + coldCount + " iterations, " + coldTime + " seconds"); // Dispose of model and environment model.dispose(); env.dispose(); } catch (GRBException e) { System.out.println("Error code: " + e.getErrorCode() + ". " + e.getMessage()); } } }