I want to fill up a dynamic array with the same integer value as fast as possible using Powershell.
The Measure-Command shows that it takes 7 seconds on my system to fill it up.
My current code (snipped) looks like:
$myArray = @()
$length = 16385
for ($i=1;$i -le $length; $i++) {$myArray += 2}
(Full code can be seen on gist.github.com or on superuser)
Consider that $length
can change. But for better understanding I chose a fixed length.
Q: How do I speed up this Powershell code?
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