What is the fastest way to initialize all elements in an array to NaN?

Question

In C# .NET, what is the fastest way to initialize an array of doubles to NaN?

Here is how I am presently initializing an array with many elements.

int length = array.Length;
for(int i = 0; i < length; i++)
{
    array[i] = double.NaN;
}

Is there a faster way?

Answer 1

You can multithread it, but it is still an O(N) problem.

Answer 2

Filling the array with byte values of 0xff produces NaN. Try this code to see what is fastest on your machine. Memset() is not always a slam-dunk btw:

using System;
using System.Runtime.InteropServices;
using System.Diagnostics;

class Program {
  static void Main(string[] args) {
    var arr = new double[10 * 1024 * 1024];
    for (int loop = 1; loop < 20; ++loop) {
      var sw1 = Stopwatch.StartNew();
      for (int ix = 0; ix < arr.Length; ++ix)
        arr[ix] = double.NaN;
      sw1.Stop();
      var sw2 = Stopwatch.StartNew();
      memset(arr, 0xff, 8 * arr.Length);
      sw2.Stop();
      Console.WriteLine("Loop: {0}, memset: {1}", sw1.ElapsedMilliseconds, sw2.ElapsedMilliseconds);
    }
    Console.ReadLine();
  }
  [DllImport("msvcrt.dll")]
  private static extern void memset(double[] array, int value, int cnt);
}