HOW TO：撰寫自訂 PLINQ 彙總函式

更新：2010 年 5 月

這個範例示範如何使用 Aggregate 方法，將自訂的彙總函式套用至來源序列。

警告
這個範例是為了示範用法，執行速度可能比不上對應的循序 LINQ to Objects 查詢。如需加速的詳細資訊，請參閱認識 PLINQ 中的加速。

範例

下列範例會計算整數序列的標準差。

Class aggregation
    Private Shared Sub Main(ByVal args As String())

        ' Create a data source for demonstration purposes.
        Dim source As Integer() = New Integer(99999) {}
        Dim rand As New Random()
        For x As Integer = 0 To source.Length - 1
            ' Should result in a mean of approximately 15.0.
            source(x) = rand.[Next](10, 20)
        Next

        ' Standard deviation calculation requires that we first
        ' calculate the mean average. Average is a predefined
        ' aggregation operator, along with Max, Min and Count.
        Dim mean As Double = source.AsParallel().Average()


        ' We use the overload that is unique to ParallelEnumerable. The 
        ' third Func parameter combines the results from each thread.
        ' initialize subtotal. Use decimal point to tell 
        ' the compiler this is a type double. Can also use: 0d.

        ' do this on each thread

        ' aggregate results after all threads are done.

        ' perform standard deviation calc on the aggregated result.
        Dim standardDev As Double = source.AsParallel().Aggregate(0.0R, Function(subtotal, item) subtotal + Math.Pow((item - mean), 2), Function(total, thisThread) total + thisThread, Function(finalSum) Math.Sqrt((finalSum / (source.Length - 1))))
        Console.WriteLine("Mean value is = {0}", mean)
        Console.WriteLine("Standard deviation is {0}", standardDev)

        Console.ReadLine()
    End Sub
End Class

namespace PLINQAggregation
{
    using System;
    using System.Linq;

    class aggregation
    {
        static void Main(string[] args)
        {

            // Create a data source for demonstration purposes.
            int[] source = new int[100000];
            Random rand = new Random();
            for (int x = 0; x < source.Length; x++)
            {
                // Should result in a mean of approximately 15.0.
                source[x] = rand.Next(10, 20);
            }

            // Standard deviation calculation requires that we first
            // calculate the mean average. Average is a predefined
            // aggregation operator, along with Max, Min and Count.
            double mean = source.AsParallel().Average();


            // We use the overload that is unique to ParallelEnumerable. The 
            // third Func parameter combines the results from each thread.
            double standardDev = source.AsParallel().Aggregate(
                // initialize subtotal. Use decimal point to tell 
                // the compiler this is a type double. Can also use: 0d.
                0.0,

                // do this on each thread
                 (subtotal, item) => subtotal + Math.Pow((item - mean), 2),

                 // aggregate results after all threads are done.
                 (total, thisThread) => total + thisThread,

                // perform standard deviation calc on the aggregated result.
                (finalSum) => Math.Sqrt((finalSum / (source.Length - 1)))
            );
            Console.WriteLine("Mean value is = {0}", mean);
            Console.WriteLine("Standard deviation is {0}", standardDev);
            Console.ReadLine();

        }
    }
}

此範例會使用 PLINQ 之唯一彙總標準查詢運算子的多載。 此多載會使用額外的 System.Func<T1, T2, TResult> 做為第三個輸入參數。 此委派會先結合所有執行緒的結果，再執行彙總結果的最終計算。 在此範例中，我們會加總所有執行緒的總計。

請注意，如果 lambda 運算式主體是由單一運算式所組成，則 System.Func<T, TResult> 委派的傳回值即為運算式的值。

請參閱

參考

ParallelEnumerable

概念

平行 LINQ (PLINQ)

變更記錄

日期	記錄	原因
2010 年 5 月	加入有關使用方式和 加速的比較注意事項。	客戶回函。