如何在 Powershell 中有效地填充数组 [英] How to fill an array efficiently in Powershell

问题描述

我想使用 Powershell 尽快填充具有相同整数值的动态数组.
Measure-Command 显示我的系统需要 7 秒才能将其填满.
我当前的代码(截断)看起来像:

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}  

^{(完整代码可见 gist.github.com 或超级用户)}

考虑 $length 可以改变.但为了更好地理解，我选择了固定长度.

Consider that $length can change. But for better understanding I chose a fixed length.

问:如何加速这个 Powershell 代码?

Q: How do I speed up this Powershell code?

推荐答案

你可以重复数组，就像你可以对字符串做的那样:

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

$myArray = ,2 * $length

这意味着 »获取具有单个元素 2 的数组并重复 $length 次，产生一个新数组.«.

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

请注意，您不能真正使用它来创建多维数组，因为以下原因:

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

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

只会创建对内部数组的 1000 个引用，使它们无法用于操作.在这种情况下，您可以使用混合策略.我用过

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) }

过去，但以下性能测量表明

in the past, but below performance measurements suggest that

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

会是一个更快的方法.

一些性能测量:

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

通过 Measure-Command 运行每个变体 50 次，每个变体都具有相同的 $length 值，然后平均结果.

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

实际上，位置 3 和 4 有点出人意料.显然，在一个范围内 foreach 比使用普通的 for 循环要好得多.

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.

生成上图的代码:

$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

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