LINQ 學習路程 -- 查詢操作 Aggregate

聚合操作執行數學的運算，如平均數、合計、總數、最大值、最小值

Method	Description
Aggregate	在集合上執行自定義聚集操作
Average	求平均數
Count	求集合的總數
LongCount	求集合的總數
Max	最大值
Min	最小值
Sum	總數

 

 

public static TSource Aggregate<TSource>(this IEnumerable<TSource> source, 
                                         Func<TSource, TSource, TSource> func);

public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, 
                                         TAccumulate seed, 
                                         Func<TAccumulate, TSource, TAccumulate> func);

public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, 
                                         TAccumulate seed, 
                                         Func<TAccumulate, TSource, TAccumulate> func, 
                                         Func<TAccumulate, TResult> resultSelector);

 

Aggregate接受2個參數，一般第一個參數是稱為累積數（默認情況下等於第一個值），而第二個代表了下一個值。第一次計算之后，計算的結果會替換掉第一個參數，繼續參與下一次計算。

 

 

public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, 
                                         TAccumulate seed, 
                                         Func<TAccumulate, TSource, TAccumulate> func);

 

seed作為種子值進行累加 

// Student collection
IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

string commaSeparatedStudentNames = studentList.Aggregate<Student, string>(
                                        "Student Names: ",  // seed value
                                        (str, s) => str += s.StudentName + "," ); 

Console.WriteLine(commaSeparatedStudentNames); 

 

 

 第三個參數對結果進行構造返回

public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, 
                                         TAccumulate seed, 
                                         Func<TAccumulate, TSource, TAccumulate> func, 
                                         Func<TAccumulate, TResult> resultSelector);

 

 

 

IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

string commaSeparatedStudentNames = studentList.Aggregate<Student, string,string>(
                                            String.Empty, // seed value
                                            (str, s) => str += s.StudentName + ",", // returns result using seed value, String.Empty goes to lambda expression as str
                                            str => str.Substring(0,str.Length - 1 )); // result selector that removes last comma

Console.WriteLine(commaSeparatedStudentNames);