# How to: Write a Custom PLINQ Aggregate Function

This example shows how to use the Aggregate method to apply a custom aggregation function to a source sequence.

Warning

This example is intended to demonstrate usage, and might not run faster than the equivalent sequential LINQ to Objects query. For more information about speedup, see Understanding Speedup in PLINQ.

## Example

The following example calculates the standard deviation of a sequence of integers.

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.

// 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);

}
}
}

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)