๐Ÿ’ป Programming Language/C#

40. LINQ ๊ธฐ๋ณธ ๋ฌธ๋ฒ•

S.Honey 2022. 4. 8. 09:16
using System.Collections;

/*
[LINQ]

LINQ์˜ ์งˆ์˜ํ‘œํ˜„์‹ = ์ฟผ๋ฆฌ ์‹ (Query Expression) ์€ from ์ ˆ๋กœ ์‹œ์ž‘ํ•œ๋‹ค.
    - from ์ ˆ ๋‹ค์Œ์—๋Š” ๋ฐ์ดํ„ฐ ์›๋ณธ์ด ์˜จ๋‹ค. ๋ฐ์ดํ„ฐ ์›๋ณธ์€ ๋ฐฐ์—ด, ์ปฌ๋ ‰์…˜ ๋“ฑ์ด ์˜ฌ ์ˆ˜ ์žˆ๋‹ค.
    - from <๋ฒ”์œ„๋ณ€์ˆ˜> in <๋ฐ์ดํ„ฐ ์›๋ณธ> 
    ```csharp
    int[] aaa = {11,22,33,44,55}

    var res = from a in aaa
              where a % 2 != 0
              orderby a 
              select a;
    ```

    - where ์ ˆ์€ ๋ฒ”์œ„ ๋ณ€์ˆ˜์˜ ์กฐ๊ฑด์„ ์ง€์ •ํ•˜์—ฌ ๊ทธ ์กฐ๊ฑด์— ํ•ด๋‹นํ•˜๋Š” ๋ฐ์ดํ„ฐ๋ฅผ ๊ฑธ๋Ÿฌ๋‚ด๋Š” ์—ญํ• ์„ ํ•œ๋‹ค.
    - orderby์ ˆ์€ where ์ ˆ์—์„œ ๊ฑธ๋Ÿฌ๋‚ธ ๋ฐ์ดํ„ฐ๋ฅผ ์ •๋ ฌํ•˜๋Š” ์—ญํ• ์„ ํ•œ๋‹ค.
        - ๊ธฐ๋ณธ ์ •๋ ฌ๊ฐ’์€ ์˜ค๋ฆ„์ฐจ์ˆœ ascending ์œผ๋กœ ์ง€์ •๋˜์–ด ์žˆ๋‹ค. ๋‚ด๋ฆผ์ฐจ์ˆœ์€ descending
    - select ์ ˆ์€ ์งˆ์˜(Query)์˜ ์ตœ์ข… ๊ฒฐ๊ณผ๋ฅผ ๊ฐ€์ ธ์˜จ๋‹ค(์ถ”์ถœํ•œ๋‹ค).

    - ์ฟผ๋ฆฌ๋ฌธ์— ์˜ํ•ด ์กฐํšŒ๋œ ๋ฐ์ดํ„ฐ์˜ ํŠน์ • ์†์„ฑ๊ฐ’๋งŒ ๊ฐ€์ ธ์˜ฌ ์ˆ˜ ๋„ ์žˆ๋‹ค.

    - ์ฟผ๋ฆฌ๋ฌธ์— ์˜ํ•ด ๋ฐ˜ํ™˜๋œ ๋ฐ์ดํ„ฐ ๋ชฉ๋ก์€ IEnumerableํด๋ž˜์Šค๋ฅผ ์ƒ์†ํ•˜๊ธฐ์— foreach๋ฌธ์„ ์ด์šฉํ•ด ์ˆœํšŒํ•  ์ˆ˜ ์žˆ๋‹ค.
*/

namespace LinqEx
{
    class Member
    {
        public string name { get; set; }
        public int age { get; set; }

    }

    class Program
    {
        static void Main(string[] args)
        {
            Member[] ml =
            {
                new Member(){name = "ํ™๊ธธ๋™" , age =23},
                new Member(){name = "์ฒญ๊ธธ๋™" , age =24},
                new Member(){name = "๋ฐฑ๊ธธ๋™" , age =27},
                new Member(){name = "๋…น๊ธธ๋™" , age =25},
                new Member(){name = "๊ฒ€๊ธธ๋™" , age =26},
            };

            List<Member> list = new List<Member>();

            foreach (Member m in ml)
            {
                if (m.age >= 25)
                {
                    list.Add(m);
                }
            }

            list.Sort(
                (member1, member2) =>
                {
                    return member2.age - member1.age;
                }
                );


            foreach (Member m in list)
            {
                Console.WriteLine($"์ด๋ฆ„ : {m.name} ๋‚˜์ด : {m.age}");
            }

            // ์œ„์˜ ์ž‘์—…์„ LINQ ํ™œ์šฉ์‹œ.
            Console.WriteLine();
            Console.WriteLine("---------LINQํ™œ์šฉ----------");

            var ml2 = from member in ml
                      where member.age >= 25
                      orderby member.age
                      select new { member.name, age = member.age + "์„ธ" };

            foreach (var m in ml2)
            {
                Console.WriteLine($"์ด๋ฆ„ : {m.name} ๋‚˜์ด : {m.age}");
            }
        }
    }
}
Output

์ด๋ฆ„ : ๋ฐฑ๊ธธ๋™ ๋‚˜์ด : 27
์ด๋ฆ„ : ๊ฒ€๊ธธ๋™ ๋‚˜์ด : 26
์ด๋ฆ„ : ๋…น๊ธธ๋™ ๋‚˜์ด : 25

---------LINQํ™œ์šฉ----------
์ด๋ฆ„ : ๋…น๊ธธ๋™ ๋‚˜์ด : 25์„ธ
์ด๋ฆ„ : ๊ฒ€๊ธธ๋™ ๋‚˜์ด : 26์„ธ
์ด๋ฆ„ : ๋ฐฑ๊ธธ๋™ ๋‚˜์ด : 27์„ธ
  • 39์žฅ LINQ ์‚ฌ์šฉํ•ด๋ณด๊ธฐ ์ฝ”๋“œ์™€ ๋น„๊ต. => select ๋ถ€๋ถ„
    • ์ฟผ๋ฆฌ๋ฌธ์— ์˜ํ•ด ์กฐํšŒ๋œ ๋ฐ์ดํ„ฐ์˜ ํŠน์ • ์†์„ฑ๊ฐ’๋งŒ ๊ฐ€์ ธ์˜ฌ ์ˆ˜ ๋„ ์žˆ๋‹ค.
    • ์ฟผ๋ฆฌ๋ฌธ์— ์˜ํ•ด ๋ฐ˜ํ™˜๋œ ๋ฐ์ดํ„ฐ ๋ชฉ๋ก์€ IEnumerableํด๋ž˜์Šค๋ฅผ ์ƒ์†ํ•˜๊ธฐ์— foreach๋ฌธ์„ ์ด์šฉํ•ด ์ˆœํšŒํ•  ์ˆ˜ ์žˆ๋‹ค.

#### [LINQ] ๊ธฐ๋ณธ ๋ฌธ๋ฒ•
  • LINQ์˜ ์งˆ์˜ํ‘œํ˜„์‹ = ์ฟผ๋ฆฌ ์‹ (Query Expression) ์€ from ์ ˆ๋กœ ์‹œ์ž‘ํ•œ๋‹ค.

    • from ์ ˆ ๋‹ค์Œ์—๋Š” ๋ฐ์ดํ„ฐ ์›๋ณธ์ด ์˜จ๋‹ค. ๋ฐ์ดํ„ฐ ์›๋ณธ์€ ๋ฐฐ์—ด, ์ปฌ๋ ‰์…˜ ๋“ฑ์ด ์˜ฌ ์ˆ˜ ์žˆ๋‹ค.

    • ์ด๋•Œ from์ ˆ์—์„œ์˜ ๋งค๊ฐœ๋ณ€์ˆ˜๋Š” IEnumerable<T> ์˜ ํŒŒ์ƒํ˜•์‹์ด์–ด์•ผ ํ•œ๋‹ค.

    • from <๋ฒ”์œ„๋ณ€์ˆ˜> in <๋ฐ์ดํ„ฐ ์›๋ณธ>

      int[] aaa = {11,22,33,44,55}
      
      var res = from a in aaa
              where a % 2 != 0
              orderby a 
              select a;
    • where ์ ˆ์€ ๋ฒ”์œ„ ๋ณ€์ˆ˜์˜ ์กฐ๊ฑด์„ ์ง€์ •ํ•˜์—ฌ ๊ทธ ์กฐ๊ฑด์— ํ•ด๋‹นํ•˜๋Š” ๋ฐ์ดํ„ฐ๋ฅผ ๊ฑธ๋Ÿฌ๋‚ด๋Š” ์—ญํ• ์„ ํ•œ๋‹ค.
    • orderby์ ˆ์€ where ์ ˆ์—์„œ ๊ฑธ๋Ÿฌ๋‚ธ ๋ฐ์ดํ„ฐ๋ฅผ ์ •๋ ฌํ•˜๋Š” ์—ญํ• ์„ ํ•œ๋‹ค.
      • ๊ธฐ๋ณธ ์ •๋ ฌ๊ฐ’์€ ์˜ค๋ฆ„์ฐจ์ˆœ ascending ์œผ๋กœ ์ง€์ •๋˜์–ด ์žˆ๋‹ค. ๋‚ด๋ฆผ์ฐจ์ˆœ์€ descending
    • select ์ ˆ์€ ์งˆ์˜(Query)์˜ ์ตœ์ข… ๊ฒฐ๊ณผ๋ฅผ ๊ฐ€์ ธ์˜จ๋‹ค(์ถ”์ถœํ•œ๋‹ค).
    • LINQ์—์„œ ๋ฐ˜ํ™˜๋˜๋Š” ๊ฒฐ๊ณผ IEnumerable<T> ์ธ๋ฐ T๋Š” select ์ ˆ์—์„œ์˜ ๋ฐ˜ํ™˜๊ฒฐ๊ณผ ํ˜•์‹์ด๋‹ค.
    • ๋งŒ์ผ select์ ˆ์„ ์‚ฌ์šฉํ•˜์ง€ ์•Š์œผ๋ฉด where์ ˆ์— ์˜ํ•ด ํ•„ํ„ฐ๋ง๋œ ์ปฌ๋ ‰์…˜ ์›๋ณธ์ด ๋‚˜์˜ค๊ฒŒ ๋œ๋‹ค. ์ด๋•Œ์˜ ํƒ€์ž…์€ IEnumerable<T> ์ด๋ฉฐ, ๋‹จ์ˆœํžˆ ์ˆœํšŒ๋ฅผ ํ•˜๋ฉด์„œ ๊ฐ’์„ ์ฐธ์กฐํ• ๊ฒƒ์ด๋ผ๋ฉด ๋ณ€ํ™˜์ด ํ•„์š”์—†์ง€๋งŒ List ํ˜น์€ Dictionary๋กœ ๋ณ€ํ™˜ํ•ด์•ผํ•œ๋‹ค๋ฉด ToList() ํ˜น์€ ToDictionary()๋ฅผ ์‚ฌ์šฉํ•ด ๋ณ€ํ™˜ํ•ด์ค€๋‹ค.