[C#] Linq

 

ㆍLinq

- 질의어로 필터링

 

<예제>

using System;
using System.Linq;
using System.Collections.Generic;
 
class Class1
{
    static void Main(string[] args)
    {

        List<int> num = new List<int>(Enumerable.Range(0, 100));

        var query = from data in num
                    where data % 2 == 0
                    select data;

        foreach (var i in query)
            Console.WriteLine(i);
    }
}

 

<group 절>

group A(범위변수) by B(분류기준) into C(그룹변수)

추가적 쿼리 작업 없는 경우에는 into와 그룹 변수를 쓰지 않아도 괜찮습니다.

public class Student
{
    public string Name { get; set; }
    public double Score { get; set; }
}

class Ex
{
    static void Main(string[] args)
    {
        List<Student> listStudent = new List<Student>
        {
            new Student {Name = "철수", Score = 87.5},
            new Student {Name = "영희", Score = 77.5},
            new Student {Name = "민수", Score = 67.5},
            new Student {Name = "짱구", Score = 91.2},
            new Student {Name = "맹구", Score = 66.1}
        };

        var queryStudent = from i in listStudent
                           orderby i.Score
                           group i by i.Score < 80.0;

        foreach (var studentGroup in queryStudent)
        {
            Console.WriteLine(studentGroup.Key ? "80점 미만" : "80점 이상");

            foreach (var i in studentGroup)
                Console.WriteLine($"{i.Name}, {i.Score}");
        }
    }
}

 

 

<그룹변수 추가>

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



public class Student
{
    public string Name { get; set; }
    public double Score { get; set; }
}

class Ex
{
    static void Main(string[] args)
    {
        List<Student> listStudent = new List<Student>
        {
            new Student {Name = "철수", Score = 87.5},
            new Student {Name = "영희", Score = 77.5},
            new Student {Name = "민수", Score = 67.5},
            new Student {Name = "짱구", Score = 91.2},
            new Student {Name = "맹구", Score = 66.1}
        };

        var queryStudent = from i in listStudent
                           group i by (int)i.Score / 10 into g
                           orderby g.Key
                           select g;
        

        foreach (var studentGroup in queryStudent)
        {
            int temp = studentGroup.Key * 10;
            Console.WriteLine($"{temp}이상{temp + 10}미만");

            foreach (var i in studentGroup)
                Console.WriteLine($"\t{i.Name}: {i.Score} 점");
        }
    }
}

 

 

<내부 join>

a-field n b-field

from a in A

join b in B on a-field equals b-field

 

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

public class Student
{
    public string Name { get; set; }
    public double Score { get; set; }
}

public class MyHobby
{
    public string Name { get; set; }
    public string Hobby { get; set; }
}

class Ex
{
    static void Main(string[] args)
    {
        List<Student> listStudent = new List<Student>
        {
            new Student {Name = "철수", Score = 87.5},
            new Student {Name = "영희", Score = 77.5},
            new Student {Name = "민수", Score = 67.5},
            new Student {Name = "짱구", Score = 91.2},
            new Student {Name = "맹구", Score = 66.1}
        };

        List<MyHobby> listHobby = new List<MyHobby>
        {
            new MyHobby {Name = "철수", Hobby = "게임"},
            new MyHobby {Name = "영희", Hobby = "당구"},
            new MyHobby {Name = "민수", Hobby = "공부"}
        };

        var queryStudent = from i in listStudent
                           join j in listHobby on i.Name equals j.Name
                           select new
                           {
                               Name = i.Name,
                               Score = i.Score,
                               Hobby = j.Hobby
                           };
        

        foreach (var studentGroup in queryStudent)           
            Console.WriteLine($"이름: {studentGroup.Name}, 점수: {studentGroup.Score}, 취미:{studentGroup.Hobby}");

    }
}

 

<외부 join>

a-field u b-field

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

public class Student
{
    public string Name { get; set; }
    public double Score { get; set; }
}

public class MyHobby
{
    public string Name { get; set; }
    public string Hobby { get; set; }
}

class Ex
{
    static void Main(string[] args)
    {
        List<Student> listStudent = new List<Student>
        {
            new Student {Name = "철수", Score = 87.5},
            new Student {Name = "영희", Score = 77.5},
            new Student {Name = "민수", Score = 67.5},
            new Student {Name = "짱구", Score = 91.2},
            new Student {Name = "맹구", Score = 66.1}
        };

        List<MyHobby> listHobby = new List<MyHobby>
        {
            new MyHobby {Name = "철수", Hobby = "게임"},
            new MyHobby {Name = "영희", Hobby = "당구"},
            new MyHobby {Name = "민수", Hobby = "공부"}
        };

        var queryStudent = from i in listStudent
                           join j in listHobby on i.Name equals j.Name into g
                           from j in g.DefaultIfEmpty(new MyHobby {Hobby = "x"})
                           select new
                           {
                               Name = i.Name,
                               Score = i.Score,
                               Hobby = j.Hobby
                           };
        

        foreach (var studentGroup in queryStudent)           
            Console.WriteLine($"이름: {studentGroup.Name}, 점수: {studentGroup.Score}, 취미:{studentGroup.Hobby}");

    }
}