Fastest way to fill an array with a single value
Solution 1
The fastest method I have found uses Array.Copy with the copy size doubling each time through the loop. The speed is basically the same whether you fill the array with a single value or an array of values.
In my test with 20,000,000 array items, this function is twice as fast as a for loop.
using System;
namespace Extensions
{
public static class ArrayExtensions
{
public static void Fill<T>(this T[] destinationArray, params T[] value)
{
if (destinationArray == null)
{
throw new ArgumentNullException("destinationArray");
}
if (value.Length >= destinationArray.Length)
{
throw new ArgumentException("Length of value array must be less than length of destination");
}
// set the initial array value
Array.Copy(value, destinationArray, value.Length);
int arrayToFillHalfLength = destinationArray.Length / 2;
int copyLength;
for(copyLength = value.Length; copyLength < arrayToFillHalfLength; copyLength <<= 1)
{
Array.Copy(destinationArray, 0, destinationArray, copyLength, copyLength);
}
Array.Copy(destinationArray, 0, destinationArray, copyLength, destinationArray.Length - copyLength);
}
}
}
I blogged about this at https://secureapplicationlifestyle.com/2011/11/initialize-array-to-value-in-c-very.html and https://secureapplicationlifestyle.com/2014/04/better-array-fill-function.html
Solution 2
For some related info, see What is the equivalent of memset in C#?.
As mentioned in that question (pretty close to a dupe of this one), a for loop is generally best unless you want to get into unmanaged code.
So this should be pretty fast:
int[] arr = new int[MAX_ELEMENTS];
for (int i = 0; i < arr.Length; ++i)
{
array[i] = MY_VALUE;
}
As with all things performance-related, get something working, then measure what the bottleneck is. Emphasis on "measure." Trying to guess what the bottleneck is is usually a bad idea (:
Solution 3
Array.Copy
is likely to be better optimized than a for loop, so use it.
void FillArray<T>(T[] arr, T fillValue)
{
int i = 0;
if (arr.Length > 16) {
{
do {
array[i++] = fillValue;
} while (i < arr.Length)
while (i + 16 < arr.Length) {
Array.Copy(arr, 0, arr, i, 16);
i = i + 16;
}
}
while (i < arr.Length)
{
array[i++] = fillValue;
}
}
(I'd love to see a performance comparison between this and the naive for loop, for different types and array sizes)
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chadb
Updated on July 09, 2022Comments
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chadb almost 2 years
I would like to fill a 2D array with a single value that I have, however, I would like to do it the quickest way possible has the 2D array's length will be a total of 200k+ and over time there will be over 200 of these arrays. I have looked into Buffer.BlockCopy and Array.Copy, however, they both take in arrays as the source/destination, where the only array I have is the destination, with the source being a single value.
What is the fastest way to fill in an array with the source being a single value and not an array?
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debracey almost 13 yearsThere are a couple of different ways, this guy has listed out a few of the more common ones -- and he was even kind enough to benchmark it: dotnetperls.com/initialize-array Holding 200K items in memory, even if they are primitives, is going to eat up a huge chunk of memory -- what are you doing that you need all 200K items available with constant time access (per item)?
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