private static void ExportSolution(Solution solution, KnapsackModel knapsackModel)
{
// print objective and variable decisions
Console.WriteLine($"{solution.ObjectiveValues.Single()}");
knapsackModel.y.Variables.OrderBy(y => y.Name).ForEach(y => Console.WriteLine($"{y.ToString().PadRight(36)}: {y.Value}"));
knapsackModel.Model.VariableStatistics.WriteCSV(AppDomain.CurrentDomain.BaseDirectory);
}
/// <summary>
/// Adds a constraint to the model that prevents the solver from simply re-using the old solution.
/// At most 4 variables that were active in the current solution are allowed to be active in the next solution.
/// Serves only to demonstrate the modification + resolve functionality of the framework.
/// </summary>
/// <param name="knapsackModel">The current (solved) model</param>
private static void AllowActivationOfAtMost4ActiveVariables(KnapsackModel knapsackModel)
{
var activeVariables = knapsackModel.y.Variables.Where(v => Math.Round(v.Value) >= 1).ToList();
// only add constraint if it will "do something"
if (!activeVariables.Any() || activeVariables.Count <= 4)
{
return;
}
var info = $"Limiting the usage of the following variables to at most 4:{Environment.NewLine.PadRight(9)}{string.Join($",{Environment.NewLine}".PadRight(10), activeVariables.OrderBy(v => v.Name).Select(v => v.Name))}";
Console.WriteLine(info);
var limitUsageOfItems = Expression.Sum(activeVariables) <= 4;
knapsackModel.Model.AddConstraint(limitUsageOfItems);
}
/// <summary>
/// The main method
/// </summary>
/// <param name="args">
/// no arguments required
/// </param>
static void Main(string[] args)
{
// create example Items
var csv = new CsvReader(File.OpenText("knapsackItems.csv"));
csv.Configuration.Delimiter = ";";
csv.Configuration.CultureInfo = new CultureInfo("en-US");
csv.Configuration.RegisterClassMap <KnapsackItemMap>();
var items = csv.GetRecords <KnapsackItem>().ToList();
// maximum weight of all the items
var maxWeight = 10.8;
// Use long names for easier debugging/model understanding.
var config = new Configuration
{
NameHandling = NameHandlingStyle.UniqueLongNames,
ComputeRemovedVariables = true
};
using (var scope = new ModelScope(config))
{
// create a model, based on given data and the model scope
var knapsackModel = new KnapsackModel(items, maxWeight);
// Get a solver instance, change your solver
using (var solver = new GurobiSolver())
{
// solve the model
var solution = solver.Solve(knapsackModel.Model);
ExportSolution(solution, knapsackModel);
// results from solution are already "synced" with variables in knapsack model.
Program.AllowActivationOfAtMost4ActiveVariables(knapsackModel);
// re-solve 1
var solutionB = solver.Solve(knapsackModel.Model);
ExportSolution(solutionB, knapsackModel);
Program.AllowActivationOfAtMost4ActiveVariables(knapsackModel);
// re-solve 2
var solutionC = solver.Solve(knapsackModel.Model);
ExportSolution(solutionC, knapsackModel);
}
}
}
/// <summary>
/// The main method
/// </summary>
/// <param name="args">
/// no arguments required
/// </param>
static void Main(string[] args)
{
// create example Items
var csv = new CsvReader(File.OpenText("knapsackItems.csv"));
csv.Configuration.Delimiter = ";";
csv.Configuration.CultureInfo = new CultureInfo("en-US");
csv.Configuration.RegisterClassMap <KnapsackItemMap>();
var items = csv.GetRecords <KnapsackItem>().ToList();
// maximum weight of all the items
var maxWeight = 10.8;
// use default settings
var config = new Configuration
{
NameHandling = NameHandlingStyle.UniqueLongNames,
ComputeRemovedVariables = true
};
using (var scope = new ModelScope(config))
{
// create a model, based on given data and the model scope
var knapsackModel = new KnapsackModel(items, maxWeight);
// Get a solver instance, change your solver
var solver = new GurobiSolver();
// solve the model
var solution = solver.Solve(knapsackModel.Model);
// import the results back into the model
knapsackModel.Model.VariableCollections.ForEach(vc => vc.SetVariableValues(solution.VariableValues));
// print objective and variable decisions
Console.WriteLine($"{solution.ObjectiveValues.Single()}");
knapsackModel.y.Variables.ForEach(y => Console.WriteLine($"{y.ToString().PadRight(36)}: {y.Value}"));
knapsackModel.Model.VariableStatistics.WriteCSV(AppDomain.CurrentDomain.BaseDirectory);
}
Console.ReadLine();
}
