如何有效地填充数组在PowerShell中 [英] How to fill an array efficiently in Powershell

问题描述

我想使用PowerShell的快速​​填充用相同的整数值的动态数组越好。结果
这项措施命令表明，它需要我的系统上的7秒填补它。结果
我现在的code（剪断）是这样的：

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}  

^{（全code可以在 gist.github.com 或<一看到href=\"http://superuser.com/questions/307496/how-can-i-set-excel-to-always-import-all-columns-of-csv-files-as-text/527894#527894\">superuser)}

考虑到 $长度可以改变。但是，对于更好地理解我选择了一个固定的长度。

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

问：如何加快这一Powershell的code

Q: How do I speed up this Powershell code?

推荐答案

您可以重复阵列，就像你可以用字符串做的：

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

$myArray = ,2 * $length

这意味着»开始跟单元素数组 2 和重复 $长度次，产生了新的数组«。

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

请注意，你不能真正使用它来创建多维数组，因为以下内容：

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

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

只是创建1000个引用内阵列，使他们无用的操作。在这种情况下，你可以使用一个混合策略。我用

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) }

在过去，但下面的性能的测量表明，

in the past, but below performance measurements suggest that

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

将是一个更快的方法。

would be a much faster way.

某些性能测量：

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

到测量命令摘自运行的每个变种50次，每次与 $长度相同的值，和平均结果。

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

位置3和4是一个有点意外的，其实。显然，这是在一定范围内，以的foreach 高得多，而不是使用普通的为循环。

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生成如上图：

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

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