C#の概念 LINQ（グループ＋サブクエリ）

　SQLに類似。

成果物

• github

情報源

• https://docs.microsoft.com/ja-jp/dotnet/csharp/linq/perform-a-subquery-on-a-grouping-operation

コード

class Human {
    public string Name { get; }
    public int Age { get; }
    public Human(string name, int age) => (Name, Age) = (name, age);
}
class Main {
    public void Run() {
        List<Human> humans = CreateHumans();
        Show(Query(humans));
    }
    private List<Human> CreateHumans() {
        return new List<Human> {
            new Human("A", 0), new Human("B", 1),
            new Human("A", 2), new Human("B", 3),
            new Human("A", 4), new Human("B", 5),
        };
    }
    private IEnumerable<dynamic> Query(in List<Human> humans) {
        return from h in humans
               group h by h.Name into g
               select new {
                   Name = g.Key,
                   MaxAge = (from h2 in g select h2.Age).Max(),
               };
    }
    private void Show(in IEnumerable<dynamic> groups) {
        foreach (var g in groups) {
            Console.WriteLine($"Name={g.Name}, MaxAge={g.MaxAge}");
        }
    }
}

Name=A, MaxAge=4
Name=B, MaxAge=5

対象環境

• 2019-12-03
• Raspbierry pi 3 Model B+
• Raspbian stretch 9.0 2018-11-13 ※
• bash 4.4.12(1)-release ※
• SQLite 3.29.0 ※
• C# dotnet 3.0.100 ※

$ uname -a
Linux raspberrypi 4.19.42-v7+ #1218 SMP Tue May 14 00:48:17 BST 2019 armv7l GNU/Linux