随机加权选择

考虑下面代表经纪人的类:

public class Broker
{
    public string Name = string.Empty;
    public int Weight = 0;

    public Broker(string n, int w)
    {
        this.Name = n;
        this.Weight = w;
    }
}

我想从阵列中随机选择一个Broker,同时考虑它们的权重.

您如何看待下面的代码？

class Program
    {
        private static Random _rnd = new Random();

        public static Broker GetBroker(List brokers, int totalWeight)
        {
            // totalWeight is the sum of all brokers' weight

            int randomNumber = _rnd.Next(0, totalWeight);

            Broker selectedBroker = null;
            foreach (Broker broker in brokers)
            {
                if (randomNumber <= broker.Weight)
                {
                    selectedBroker = broker;
                    break;
                }

                randomNumber = randomNumber - broker.Weight;
            }

            return selectedBroker;
        }


        static void Main(string[] args)
        {
            List brokers = new List();
            brokers.Add(new Broker("A", 10));
            brokers.Add(new Broker("B", 20));
            brokers.Add(new Broker("C", 20));
            brokers.Add(new Broker("D", 10));

            // total the weigth
            int totalWeight = 0;
            foreach (Broker broker in brokers)
            {
                totalWeight += broker.Weight;
            }

            while (true)
            {
                Dictionary result = new Dictionary();

                Broker selectedBroker = null;

                for (int i = 0; i < 1000; i++)
                {
                    selectedBroker = GetBroker(brokers, totalWeight);
                    if (selectedBroker != null)
                    {
                        if (result.ContainsKey(selectedBroker.Name))
                        {
                            result[selectedBroker.Name] = result[selectedBroker.Name] + 1;
                        }
                        else
                        {
                            result.Add(selectedBroker.Name, 1);
                        }
                    }
                }


                Console.WriteLine("A\t\t" + result["A"]);
                Console.WriteLine("B\t\t" + result["B"]);
                Console.WriteLine("C\t\t" + result["C"]);
                Console.WriteLine("D\t\t" + result["D"]);

                result.Clear();
                Console.WriteLine();
                Console.ReadLine();
            }
        }
    }

我不那么自信.当我运行它时,经纪人A总是获得比经纪人D更多的点击量,并且他们具有相同的权重.

有更准确的算法吗？

谢谢!

1> Konrad Rudol..：

---

你的算法几乎是正确的.但是,测试应该是:<而不是<=:

if (randomNumber < broker.Weight)

这是因为0是包含在随机数中,totalWeight而是排他性的.换句话说,权重为0的经纪人仍然很少被选中 - 根本不是你想要的.这说明经纪人A的点击次数比经纪人D多.

除此之外,你的算法很好,实际上是解决这个问题的规范方法.

2> 小智..：

---

class Program
{
    static void Main(string[] args)
    {
        var books = new List {
        new Book{Isbn=1,Name="A",Weight=1},
        new Book{Isbn=2,Name="B",Weight=100},
        new Book{Isbn=3,Name="C",Weight=1000},
        new Book{Isbn=4,Name="D",Weight=10000},
        new Book{Isbn=5,Name="E",Weight=100000}};

        Book randomlySelectedBook = WeightedRandomization.Choose(books);
    }
}

public static class WeightedRandomization
{
    public static T Choose(List list) where T : IWeighted
    {
        if (list.Count == 0)
        {
            return default(T);
        }

        int totalweight = list.Sum(c => c.Weight);
        Random rand = new Random();
        int choice = rand.Next(totalweight);
        int sum = 0;

        foreach (var obj in list)
        {
            for (int i = sum; i < obj.Weight + sum; i++)
            {
                if (i >= choice)
                {
                    return obj;
                }
            }
            sum += obj.Weight;
        }

        return list.First();
    }
}

public interface IWeighted
{
    int Weight { get; set; }
}

public class Book : IWeighted
{
    public int Isbn { get; set; }
    public string Name { get; set; }
    public int Weight { get; set; }
}

---

每次执行新的Random()时,都会使用时钟进行初始化.这意味着在紧密循环中,您可以多次获得相同的值.您应该保留一个Random实例并继续在同一个实例上使用Next.因此,为Random实例创建一个类级别声明.

---

我也想知道为什么需要内部for循环？不只是增加总和的重量,并检查它是否> =选择工作？

3> 小智..：

---

那些可以用于任何数据类型的更通用的东西怎么样？

using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;

public static class IEnumerableExtensions {

    public static T RandomElementByWeight(this IEnumerable sequence, Func weightSelector) {
        float totalWeight = sequence.Sum(weightSelector);
        // The weight we are after...
        float itemWeightIndex =  new Random().NextDouble() * totalWeight;
        float currentWeightIndex = 0;

        foreach(var item in from weightedItem in sequence select new { Value = weightedItem, Weight = weightSelector(weightedItem) }) {
            currentWeightIndex += item.Weight;

            // If we've hit or passed the weight we are after for this item then it's the one we want....
            if(currentWeightIndex >= itemWeightIndex)
                return item.Value;

        }

        return default(T);

    }

}

只需打电话

    Dictionary foo = new Dictionary();
    foo.Add("Item 25% 1", 0.5f);
    foo.Add("Item 25% 2", 0.5f);
    foo.Add("Item 50%", 1f);

    for(int i = 0; i < 10; i++)
        Console.WriteLine(this, "Item Chosen {0}", foo.RandomElementByWeight(e => e.Value));