[C#/LINQ] ToLookupメソッドの使い方 – キーと値のペアに変換する

IEnumerable<IGrouping<TKey, TElement>> ToLookup<TKey, TElement>(
    Func<TElement, TKey> keySelector,
    Func<TElement, TElement> elementSelector = null
);

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6 };
        
        var lookup = numbers.ToLookup(
            number => number % 2, // 偶数と奇数でグループ化
            number => number       // 値はそのまま
        );
        foreach (var group in lookup)
        {
            Console.WriteLine($"キー: {group.Key}");
            foreach (var value in group)
            {
                Console.WriteLine($"  値: {value}");
            }
        }
    }
}

キー: 1
  値: 1
  値: 3
  値: 5
キー: 0
  値: 2
  値: 4
  値: 6

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        List<string> words = new List<string> { "apple", "banana", "cherry", "date", "fig", "grape" };
        
        var lookup = words.ToLookup(
            word => new { Length = word.Length }, // 長さをキーにする
            word => word                          // 値はそのまま
        );
        foreach (var group in lookup)
        {
            Console.WriteLine($"キー: {group.Key.Length}");
            foreach (var value in group)
            {
                Console.WriteLine($"  値: {value}");
            }
        }
    }
}

キー: 5
  値: apple
  値: grape
キー: 6
  値: banana
  値: cherry
キー: 4
  値: date
キー: 3
  値: fig

using System;
using System.Collections.Generic;
using System.Linq;
class Product
{
    public string Name { get; set; }
    public string Category { get; set; }
}
class Program
{
    static void Main()
    {
        List<Product> products = new List<Product>
        {
            new Product { Name = "Apple", Category = "Fruit" },
            new Product { Name = "Banana", Category = "Fruit" },
            new Product { Name = "Carrot", Category = "Vegetable" },
            new Product { Name = "Broccoli", Category = "Vegetable" },
            new Product { Name = "Chicken", Category = "Meat" }
        };
        
        var lookup = products.ToLookup(
            product => product.Category, // カテゴリでグループ化
            product => product.Name       // 値は商品名
        );
        foreach (var group in lookup)
        {
            Console.WriteLine($"カテゴリ: {group.Key}");
            foreach (var value in group)
            {
                Console.WriteLine($"  商品名: {value}");
            }
        }
    }
}

カテゴリ: Fruit
  商品名: Apple
  商品名: Banana
カテゴリ: Vegetable
  商品名: Carrot
  商品名: Broccoli
カテゴリ: Meat
  商品名: Chicken

using System;
using System.Collections.Generic;
using System.Linq;
class Student
{
    public string Name { get; set; }
    public int Score { get; set; }
}
class Program
{
    static void Main()
    {
        List<Student> students = new List<Student>
        {
            new Student { Name = "Alice", Score = 85 },
            new Student { Name = "Bob", Score = 55 },
            new Student { Name = "Charlie", Score = 70 },
            new Student { Name = "David", Score = 40 },
            new Student { Name = "Eve", Score = 90 }
        };
        
        var lookup = students.ToLookup(
            student => student.Score >= 60 ? "合格" : "不合格", // 成績でグループ化
            student => student.Name                             // 値は学生名
        );
        foreach (var group in lookup)
        {
            Console.WriteLine($"ステータス: {group.Key}");
            foreach (var value in group)
            {
                Console.WriteLine($"  学生名: {value}");
            }
        }
    }
}

ステータス: 合格
  学生名: Alice
  学生名: Charlie
  学生名: Eve
ステータス: 不合格
  学生名: Bob
  学生名: David

using System;
using System.Collections.Generic;
using System.Linq;
class Employee
{
    public string Name { get; set; }
    public int DepartmentId { get; set; }
}
class Department
{
    public int Id { get; set; }
    public string DepartmentName { get; set; }
}
class Program
{
    static void Main()
    {
        List<Employee> employees = new List<Employee>
        {
            new Employee { Name = "Alice", DepartmentId = 1 },
            new Employee { Name = "Bob", DepartmentId = 2 },
            new Employee { Name = "Charlie", DepartmentId = 1 },
            new Employee { Name = "David", DepartmentId = 3 }
        };
        List<Department> departments = new List<Department>
        {
            new Department { Id = 1, DepartmentName = "HR" },
            new Department { Id = 2, DepartmentName = "IT" },
            new Department { Id = 3, DepartmentName = "Finance" }
        };
        var lookup = employees.ToLookup(
            emp => emp.DepartmentId, // 部署IDでグループ化
            emp => emp.Name          // 値は社員名
        );
        foreach (var group in lookup)
        {
            var departmentName = departments.FirstOrDefault(d => d.Id == group.Key)?.DepartmentName;
            Console.WriteLine($"部署: {departmentName}");
            foreach (var value in group)
            {
                Console.WriteLine($"  社員名: {value}");
            }
        }
    }
}

部署: HR
  社員名: Alice
  社員名: Charlie
部署: IT
  社員名: Bob
部署: Finance
  社員名: David