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以概率从列表中选取随机元素

probability random list c#

Zied CHAARI · 技术社区 · 8 年前

我有一个包含四项(a、B、C、D)的列表。每个项目都有被选择的可能性。例如,A有74%的机会被选中,B有15%,C有7%,D有4%。

我想创建一个函数,根据概率随机选择一个项目。

需要帮忙吗?

3 回复 | 直到 8 年前

Aleks Andreev Md. Suman Kabir 8 年前

为项目定义一个类,如下所示:

class Items<T>
{
    public double Probability { get; set; }
    public T Item { get; set; }
}

然后初始化它

var initial = new List<Items<string>>
{
    new Items<string> {Probability = 74 / 100.0, Item = "A"},
    new Items<string> {Probability = 15 / 100.0, Item = "B"},
    new Items<string> {Probability = 7 / 100.0, Item = "C"},
    new Items<string> {Probability = 4 / 100.0, Item = "D"},
};

然后需要将其转换为从0到1的概率总和

var converted = new List<Items<string>>(initial.Count);
var sum = 0.0;
foreach (var item in initial.Take(initial.Count - 1))
{
    sum += item.Probability;
    converted.Add(new Items<string> {Probability = sum, Item = item.Item});
}
converted.Add(new Items<string> {Probability = 1.0, Item = initial.Last().Item});

现在你可以从 converted 概率收集:

var rnd = new Random();
while (true)
{
    var probability = rnd.NextDouble();
    var selected = converted.SkipWhile(i => i.Probability < probability).First();
    Console.WriteLine($"Selected item = {selected.Item}");
}

注意:我的实现有 O(n) 复杂性您可以使用二进制搜索对其进行优化(因为 转换 集合已排序)

Kevin 8 年前

我很抱歉这样回答这个问题——我有点把它看作是一种“Euler.Net”谜题,一种玩泛型的方式。

无论如何,我的做法是:

public class WeightedItem<T>
{
    private T value;
    private int weight;
    private int cumulativeSum;
    private static Random rndInst = new Random();

    public WeightedItem(T value, int weight)
    {
        this.value = value;
        this.weight = weight;
    }

    public static T Choose(List<WeightedItem<T>> items)
    {
        int cumulSum = 0;
        int cnt = items.Count();

        for (int slot = 0; slot < cnt; slot++)
        {
            cumulSum += items[slot].weight;
            items[slot].cumulativeSum = cumulSum;
        }

        double divSpot = rndInst.NextDouble() * cumulSum;
        WeightedItem<T> chosen =  items.FirstOrDefault(i => i.cumulativeSum >= divSpot);
        if (chosen == null) throw new Exception("No item chosen - there seems to be a problem with the probability distribution.");
        return chosen.value;
    }
}

用法:

        WeightedItem<string> alice = new WeightedItem<string>("alice", 1);
        WeightedItem<string> bob = new WeightedItem<string>("bob", 1);
        WeightedItem<string> charlie = new WeightedItem<string>("charlie", 1);
        WeightedItem<string> diana = new WeightedItem<string>("diana", 4);
        WeightedItem<string> elaine = new WeightedItem<string>("elaine", 1);

        List<WeightedItem<string>> myList = new List<WeightedItem<string>> { alice, bob, charlie, diana, elaine };
        string chosen = WeightedItem<string>.Choose(myList);

-2

Todor Balabanov 8 年前

using System;

public class Test{
    private static String[] values = {"A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","B","B","B","B","B","B","B","B","B","B","B","B","B","B","B","C","C","C","C","C","C","C","D","D","D","D",};

    private static Random PRNG = new Random();

    public static void Main(){
        Console.WriteLine( values[PRNG.Next(values.Length)] );
    }
}