[c#] LINQ 컬렉션 표준 쿼리 메서드 4 - (조인, 그룹, 집계, 최대, 최소, 합계, 평균, 개수) Join, GroupJoin, GroupBy, ToLookUp, Max, Min, Count, Sum, Average

 

수량자, 생성 관련한 컬렉션 메소드에 대해 알아봅니다.

메서드

메서드 이름	설명	C# 쿼리 식 구문
Join	키를 기준으로 두 시퀀스를 Join한 다음 값 추출	join … in … on … equals …
GroupJoin	키를 기준으로 두 시퀀스를 Join한 다음 결과를 그룹화	join … in … on … equals … into …
GroupBy	공통 특성을 공유하는 요소를 그룹화	group … by group … by … into …
ToLookup	키 선택기 함수에 따라 Lookup<TKey,TElement>(일대다 사전)에 요소를 삽입
Aggregate	컬렉션 값에 대해 사용자 지정 집계 작업을 수행
Average	평균 값
Count	수량
Max	최대값
Min	최소값
Sum	합계

 

Join

키가 같은지 여부에 따라 두 데이터 소스의 일치 여부를 확인하는 조인인 동등 조인을 수행합니다.

Join, GroupJoin

List<Student> students = new List<Student>
{
    new Student{grade = 3, name = "Tom", scores = new int[] {60, 80} },
    new Student{grade = 3, name = "Brian" , scores = new int[] {50, 70} },
    new Student{grade = 4, name = "Chris" , scores = new int[] {80, 80}},
    new Student{grade = 5, name = "James" , scores = new int[] {100, 100} },
};

List<Teacher> teachers = new List<Teacher>
{
    new Teacher{grade = 3, name = "Miss Christina" },
    new Teacher{grade = 4, name = "Mr. Tayler" },
    new Teacher{grade = 6, name = "Mr. Jone" },
    new Teacher{grade = 5, name = "James" },
};

Console.WriteLine("join by grade===");
var join = teachers.Join(students, 
                        t => t.grade, 
                        s => s.grade, 
                        (t, s) => new { grade = t.grade, name = t.name, studentName = s.name });

var query = from teacher in teachers
            join student in students on teacher.grade equals student.grade
            select new { teacher.grade, teacher.name, studentName = student.name  };
            
foreach (var teacher in query)
{
    Console.WriteLine(teacher.grade + "," + teacher.name, 8);
}            
/*
3, Miss Christina
3, Miss Christina
4, Mr. Tayler
*/

Console.WriteLine("join by name===");
var joinName = teachers.Join(students, 
                            t => t.name, 
                            s => s.name, 
                            (t, s) => new { grade = t.grade, name = t.name, studentName = s.name });

foreach (var teacher in joinName)
{
    Console.WriteLine(teacher.grade + ", " + teacher.name + ", " + teacher.studentName);
}
/*
5, James, James
*/


Console.WriteLine("group join===");
var grpJoin = from teacher in teachers
				join student in students on teacher.grade equals student.grade into grp
				select grp;
foreach (var key in grpJoin)
{
    foreach (var teacher in key)
    {
        Console.WriteLine(teacher.grade + "," + teacher.name, 8);
    }
}
/*
3, Tom
3, Brian
4, Chris
6, James
*/

GroupJoin 결과집합 

• Key = 3   Tom, Brain
• Key = 4   Chris
• Key = 5    null
• Key = 6    James

 

그룹

시퀀스의 요소를 그룹화

 

GroupBy, Lookup

List<Student> students = new List<Student>
{
    new Student{grade = 3, name = "Tom", scores = new int[] {60, 80} },
    new Student{grade = 3, name = "Brian" , scores = new int[] {50, 70} },
    new Student{grade = 4, name = "Chris" , scores = new int[] {80, 80}},
    new Student{grade = 5, name = "James" , scores = new int[] {100, 100} },
};


var grpByQuery = from student in students
                 group student by student.grade;


var groupByMethod = students.GroupBy(student => student.grade);

foreach (var grp in groupByMethod) 
{
    Console.WriteLine("key : " + grp.Key);
    foreach(var item in grp)
    {
        Console.WriteLine("   " + item.name);
    }
}
/*
Key : 3
	Tom
	Brain
Key : 4
	Chris
Key : 5
	James  
*/


// ToLookUp
// Key, Element 로 그룹화
var lookup = students.ToLookup(student => student.grade, student=> student.grade + ":" +student.name);
foreach (var grp in lookup)
{
    Console.WriteLine("Lookup key : " + grp.Key);
    foreach (var item in grp)
    {
        Console.WriteLine("   " + item);
    }
}
/*
Lookup key : 3
	3:Tom
	3:Brain
Lookup key : 4
	4:Chris
Lookup key : 5
	5:James
*/

foreach (var grp in lookup)
{
    Console.WriteLine("Lookup key : " + grp.Key);
    foreach (var item in lookup[grp.Key])
    {
        Console.WriteLine("     lookup   " + item);
    }
}
/*
Lookup key : 3
     lookup   3:Tom
     lookup   3:Brain
Lookup key : 4
     lookup   4:Chris
Lookup key : 5
     lookup   5:James
*/

 

집계

Aggregate, Average, Count, Max, MIn, Sum

List<Student> students = new List<Student>
{
    new Student{grade = 3, name = "Tom", scores = new int[] {60, 80} },     
    new Student{grade = 3, name = "Brian" , scores = new int[] {50, 70} },  
    new Student{grade = 4, name = "Chris" , scores = new int[] {80, 80}},   
    new Student{grade = 5, name = "James" , scores = new int[] {100, 100} },
};


Console.WriteLine(students.Average(student => student.scores.Average()));
/*
77.5		// 각 요소의 scores 의 평균 값의 평균 =>  (70 + 60 +80 +100) / 4
*/

Console.WriteLine(students.Count());
/*
4
*/

Console.WriteLine(students.Max(student=> student.scores.Max()));
/*
100			// 각 요소의 scores 의 최대값중 최대값(100)
*/

Console.WriteLine(students.Min(student => student.scores.Min()));
/*
50			// 각 요소의 scores 의 최소값중 최소값(50)
*/

Console.WriteLine(students.Sum(student => student.scores.Sum()));
/*
620			// 각 요소의 scores 의 합계의 총합(620)
*/