解析文件以创建行数组 [英] Parsing a file to create an array of lines

问题描述

这看起来非常简单，但我遗漏了一些东西.我只需要向数组[0]、数组[1]等添加一个数组.我正在获取一个 vcard 文件并尝试读取一个 vcard 的所有行并将它们放入一个数组中，然后将该数组放入一个数组中，这样数组 [0] 将是 vcard 1，数组 [1] 将是下一个，等等.

This seems so incredibly simple but I am missing something. I just need to add an array to array[0], array[1], etc. I am taking a vcard file and trying to read all the lines of one vcard and put them in an array and then place that array in an array so array[0] will be vcard 1, array[1] will be the next, etc.

$c = Get-Content -Path C:\temp\Contacts_Backup.vcf
$counter=0
$contact=@()
$allcontacts=@()

Foreach ($line in $c){
    $contact += $line
    if ($line -eq 'END:VCARD'){
        $allcontacts[$counter++] = $contact
        $contact=@()
        }
}

结果:无法索引到 System.String 类型的对象.

Result: Unable to index into an object of type System.String.

推荐答案

tl;dr:

• 你不能通过分配给一个不存在的索引来增长"一个数组；如果你以 @() 开头 - 一个空数组 - 你必须使用 += 来追加"元素(数组是固定大小的集合，所以真正发生的是每次都必须分配一个新数组，其中包含旧元素，然后是新元素).

• You cannot "grow" an array by assigning to a nonexistent index; if you start with @() - an empty array - you must use += to "append" elements (arrays are fixed-size collections, so what really happens is that a new array must be allocated every time that contains the old elements followed by the new one).

因此在循环中使用 += 效率低下，并且有两种替代方法:

• 使用 .NET 可扩展列表类型更有效地构建类似数组的集合.

• Use a .NET extensible list type to build an array-like collection more efficiently.

最好 - 因为它既方便又快捷 - 让 PowerShell 为您创建数组，只需从 foreach 捕获输出在变量中循环
($array = @(foreach (...) { ... }))

Preferably - because it is both more convenient and faster - let PowerShell create the array for you, simply by capturing the output from a foreach loop in a variable
($array = @(foreach (...) { ... }))

详情如下.

您的代码确实有问题，尽管它产生的症状与您的问题当前陈述的不同；使用一个简化的例子:

Your code indeed has a problem, though the symptom it would produce differs from what your question currently states; using a simplified example:

PS> $allcontacts=@(); $allcontacts[0] = 'one', 'two'
Index was outside the bounds of the array.  # ERROR
...

也就是说，@() 创建一个 空 数组，你不能通过访问一个不存在的索引来隐式地扩展"它.

That is, @() creates an empty array, which you cannot implicitly "extend" by accessing a non-existent index.

使用 +=，就像使用 $contacts 数组一样，确实有效:

Using +=, as you do with your $contacts array, does work:

$allcontacts=@(); $allcontacts += , ('one', 'two')

注意数组构造运算符,的使用，以确保RHS操作数作为一个整体添加为一个单个新元素；没有它，将添加多个元素，每个元素一个.

Note the use of array-construction operator , to ensure that the RHS operand is added as a whole as a single new element; without it, multiple elements would be added, one for each element.

然而，虽然使用 += 来扩展"一个数组是有效的，但实际上你每次都在幕后创建一个 新 数组，因为数组是根据定义固定大小集合.

However, while "extending" an array with += works, in reality you're creating a new array behind the scenes every time, because arrays are by definition fixed-size collections.

对于较大的集合，这可能会成为性能问题，最好使用 list 数据类型代替，例如 [System.Collections.Generic.List[object]]^[1]:

With larger collections, this can become a performance issue, and it is better to use a list data type instead, such as [System.Collections.Generic.List[object]]^[1]:

$allcontacts = New-Object Collections.Generic.List[object]
$allcontacts.Add(('one', 'two'))

注意需要将要添加的数组 - 作为单个列表元素 - 在 (...) 中，以便 .Add() 方法将其识别为一个参数.

Note the need to enclose the array to add - as a single list element - in (...) so that the .Add() method recognizes it as a single argument.

退一步:您可以让 PowerShell 收集整个 $allcontacts 数组中的 $contact 子数组简单地捕获整个 foreach 命令的输出:

Taking a step back: You can let PowerShell collect the $contact sub-arrays in the overall $allcontacts array by simply capturing the output from the entire foreach command:

$c = Get-Content -Path C:\temp\Contacts_Backup.vcf
$contact=@()

$allcontacts = @(foreach ($line in $c){
    $contact += $line
    if ($line -eq 'END:VCARD'){
        # Output the $contact array as a *single* object,
        # using ",", the array-construction operator
        , $contact
        # Reset for the next contact.
        $contact=@()
    }
})

$allcontacts 将最终成为一个常规的 PowerShell 数组，键入 [object[]].仅当您需要确保 $allcontacts 是一个数组时才需要使用数组子表达式运算符 (@(...))，即使 *.vcf 文件仅包含 一个 联系人定义.

$allcontacts will end up as a regular PowerShell array, typed [object[]]. Use of the array-subexpression operator (@(...)) is only necessary if you need to ensure that $allcontacts is an array even if the *.vcf file contains only one contact definition.

^{[1] 一个非通用的替代方案是 [System.Collections.ArrayList]，但它的缺点是它的 .Add() 方法 返回一个值，要求您使用例如 $null = $arrayList.Add(...) 来抑制该值，以免污染 PowerShell 的输出流.}

^{[1] A non-generic alternative is [System.Collections.ArrayList], but its downside is that its .Add() method returns a value, requiring you to suppress that value with, e.g., $null = $arrayList.Add(...) so as not to pollute PowerShell's output stream.}

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