How to fill an array efficiently in Powershell

You can repeat arrays, just as you can do with strings:

$myArray = ,2 * $length

This means »Take the array with the single element 2 and repeat it $length times, yielding a new array.«.

Note that you cannot really use this to create multidimensional arrays because the following:

$some2darray = ,(,2 * 1000) * 1000

will just create 1000 references to the inner array, making them useless for manipulation. In that case you can use a hybrid strategy. I have used

$some2darray = 1..1000 | ForEach-Object { ,(,2 * 1000) }

in the past, but below performance measurements suggest that

$some2darray = foreach ($i in 1..1000) { ,(,2 * 1000) }

would be a much faster way.

---

Some performance measurements:

Command                                                  Average Time (ms)
-------                                                  -----------------
$a = ,2 * $length                                                 0,135902 # my own
[int[]]$a = [System.Linq.Enumerable]::Repeat(2, $length)           7,15362 # JPBlanc
$a = foreach ($i in 1..$length) { 2 }                             14,54417
[int[]]$a = -split "2 " * $length                                24,867394
$a = for ($i = 0; $i -lt $length; $i++) { 2 }                    45,771122 # Ansgar
$a = 1..$length | %{ 2 }                                         431,70304 # JPBlanc
$a = @(); for ($i = 0; $i -lt $length; $i++) { $a += 2 }       10425,79214 # original code

Taken by running each variant 50 times through Measure-Command, each with the same value for $length, and averaging the results.

Position 3 and 4 are a bit of a surprise, actually. Apparently it's much better to foreach over a range instead of using a normal for loop.

---

Code to generate above chart:

$length = 16384

$tests = '$a = ,2 * $length',
         '[int[]]$a = [System.Linq.Enumerable]::Repeat(2, $length)',
         '$a = for ($i = 0; $i -lt $length; $i++) { 2 }',
         '$a = foreach ($i in 1..$length) { 2 }',
         '$a = 1..$length | %{ 2 }',
         '$a = @(); for ($i = 0; $i -lt $length; $i++) { $a += 2 }',
         '[int[]]$a = -split "2 " * $length'

$tests | ForEach-Object {
    $cmd = $_
    $timings = 1..50 | ForEach-Object {
        Remove-Variable i,a -ErrorAction Ignore
        [GC]::Collect()
        Measure-Command { Invoke-Expression $cmd }
    }
    [pscustomobject]@{
        Command = $cmd
        'Average Time (ms)' = ($timings | Measure-Object -Average TotalMilliseconds).Average
    }
} | Sort-Object Ave* | Format-Table -AutoSize -Wrap

Avoid appending to an array in a loop. It's copying the existing array to a new array with each iteration. Do this instead:

$MyArray = for ($i=1; $i -le $length; $i++) { 2 }

Using PowerShell 3.0 you can use (need .NET Framework 3.5 or upper):

[int[]]$MyArray = ([System.Linq.Enumerable]::Repeat(2, 65000))

Using PowerShell 2.0

$AnArray = 1..65000 | % {2}