CS5L13-3——使用分组统计出现频率

本章代码关键字

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group ... by ... into ...        //第一个空传入集合中的单个元素，第二个空传入分组依据（表达式），第三空为分组得到某个组集合，后续可以使用

IEnumerable<>.GroupBy()            //传入分组方法，对集合进行分组，返回分组出来的集合
IEnumerable<>.Select()            //传入选择方法，返回一个新集合
IEnumerable<>.ToDictionary()    //传入两个方法，一个是获取键的方法，一个是获取值的方法，根据这两个方法将集合转换为字典

统计频率

假设有一个长度为200的数组，其中值是随机的，范围在0到19之间

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Random random = new Random(1334);       //使用种子使数组内部的值是唯一的
var arr = new int[200];
for (int i = 0; i < arr.Length; i++)
    arr[i] = random.Next(0, 20);        //该数组内的数据都是0~19

不使用LINQ

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static void Normal3(int[] arr)
{
    Console.WriteLine("不使用LINQ");
    Dictionary<int, int> dict = new();      //用来装载数组值与对应的频率，键为数组值，值为出现次数
    foreach (int i in arr)
    {
        if (dict.ContainsKey(i))            //若存在，则+1
            dict[i] += 1;
        else
            dict.Add(i, 1);                 //若不存在，则添加键值对
    }
    foreach (int i in dict.Keys)            //全部打印出来
        Console.WriteLine(i + " frequency is " + dict[i]);
}

输出结果:

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12 frequency is 11
4 frequency is 17
5 frequency is 8
9 frequency is 15
6 frequency is 8
13 frequency is 7
0 frequency is 9
3 frequency is 10
1 frequency is 8
18 frequency is 7
14 frequency is 10
2 frequency is 8
15 frequency is 14
16 frequency is 10
8 frequency is 12
10 frequency is 11
17 frequency is 5
11 frequency is 11
7 frequency is 8
19 frequency is 11

分组

group 子句 - C# 参考 - C# | Microsoft Learn

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//查询表达式内
var result = from x in arr                     
             group 要分组的元素 by 按照什么分组 into 分组得到的某个组的集合              
             select new { num = 分组出来的元素.Key, count = 分组出来的元素.Count() };

//连接表达式内
var result = arr
    .GroupBy(要分组的元素 => 按照什么分组)
    .Select(组集合 => new { num = 组集合.Key, count = 组集合.Count() });

由于通过 groupby​ 或者 .GroupBy()​ 分组出的数据类型是类似于字典内的键值对形式的分组IGrouping<>​类型，

其中，IGrouping<>.key​ 可以得到分组的键（具体内容取决于如何分组），IGrouping<>.Count()​ 可以得到该组的元素数量

因此建议使用 Select()​ 返回匿名类来装载，或者使用 ToDictionary()​ 直接转化为Dictionary<>​类型（仅限链接表达式）

LINQ实例

使用LINQ（查询表达式）

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static void QueryExpressionLINQ3(int[] arr)
{
    Console.WriteLine("查询表达式LINQ");
    var result = from x in arr                                      //从arr内取出所有元素并取名x
                 group x by x into g                                //为x的值为x进行分组，装载到g，此时g的类型是类似于键值对形式的
                 select new { num = g.Key, count = g.Count() };     //使用匿名类来装载，num为键也就是数组的元素，count就是该组的值
    foreach (var keyValuePair in result)                            //全部打印出来
        Console.WriteLine(keyValuePair.num + " frequency is " + keyValuePair.count);
}

输出结果：

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12 frequency is 11
4 frequency is 17
5 frequency is 8
9 frequency is 15
6 frequency is 8
13 frequency is 7
0 frequency is 9
3 frequency is 10
1 frequency is 8
18 frequency is 7
14 frequency is 10
2 frequency is 8
15 frequency is 14
16 frequency is 10
8 frequency is 12
10 frequency is 11
17 frequency is 5
11 frequency is 11
7 frequency is 8
19 frequency is 11

使用LINQ（链式表达式）

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static void ChainedExpressionLINQ3(int[] arr)
{
    Console.WriteLine("链式表达式LINQ");
    //对arr使用分组，分组形式为数组元素x通过数组元素x来分组，这时result为分组元素的可迭代集合，
    //再挑选，将分组元素g使用匿名类来装载，num为分组名（数组元素），count为该组的值，这时result为匿名类的可迭代集合
    var result = arr.GroupBy(x => x)
                    .Select(g => new { num = g.Key, count = g.Count() });
    //你也可以这样写，将分组元素直接转化为Dictionary<>类型
    var res = arr.GroupBy(x => x)
                 .ToDictionary(g => g.Key, g => g.Count());
    foreach (var keyValuePair in result)
        Console.WriteLine(keyValuePair.num + " frequency is " + keyValuePair.count);
}

输出结果：

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12 frequency is 11
4 frequency is 17
5 frequency is 8
9 frequency is 15
6 frequency is 8
13 frequency is 7
0 frequency is 9
3 frequency is 10
1 frequency is 8
18 frequency is 7
14 frequency is 10
2 frequency is 8
15 frequency is 14
16 frequency is 10
8 frequency is 12
10 frequency is 11
17 frequency is 5
11 frequency is 11
7 frequency is 8
19 frequency is 11

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