我在读 Powershell in Action，看到 Chapter 5 的一段时卡住了。结果我能看懂，不过谁能解释一下这样做的用途和原因？
$nested = 4, (5, 6)
$flattened = $nested | foreach { $_ }
$flattened.length这样 $flattened.length 是 3。可以理解。
$nested = 4, (5, 6)
$flattened = $nested | foreach { ,$_ }
$flattened.length这次 $flattened.length 得到 2。
运行 $flattened[0]; $flattened[1] -join "," 能看出那 2 个对象是什么，但第二个 ForEach 里的逗号究竟做了什么？
大概是在说，“把 4 留作一个对象，把 (5,6) 也留作单独的对象。别拆开它们。”吗？
While reading Powershell in Action, I got stuck on a passage in Chapter 5. I follow the result here, but could someone explain the purpose and the reason behind it?
$nested = 4, (5, 6)
$flattened = $nested | foreach { $_ }
$flattened.lengthThat gives $flattened.length as 3. Fair enough.
$nested = 4, (5, 6)
$flattened = $nested | foreach { ,$_ }
$flattened.lengthThis time $flattened.length gives 2.
Running $flattened[0]; $flattened[1] -join "," explains those 2 objects, but what precisely does the comma inside that second ForEach accomplish?
Does it mean, roughly, “Keep 4 as an object and (5,6) as its seperate object. Leave them intact.”
Powershell 管道会为了方便用户和所谓的“一致性”（说来有点好笑）而“展开”集合。一元运算符会阻止 powershell 做这个展开。那些例子不太理想，我换个方式试试。
看看下面两个片段的输出
(4, 5, 6) | ForEach-Object {
Write-Host "Pipeline element now: $_"
}
Pipeline element now: 4
Pipeline element now: 5
Pipeline element now: 6,(4, 5, 6) | ForEach-Object {
Write-Host "Pipeline element now: $_"
}
Pipeline element now: 4 5 6后一个把整个集合作为单个对象传递，所以不会被展开。它其实是数组，显示空格而不是逗号，是因为默认输出字段分隔符是空格（保存在 $ofs 中）。可以这样观察
"$(4, 5, 6)"
4 5 6把数组放进字符串后，powershell 会用默认 OFS 连接它的元素。可以改，只是很少需要。
$ofs = '/~/'
"$(4, 5, 6)"
4/~/5/~/6我自己经常用这个一元运算符的一种情况，就是拆分值之后，仍然想把所有元素放在一个对象里一起传递。
,(-split 'Tavin enjoys powershell') | ForEach-Object {
[PSCustomObject]@{
Who = $_[0]
Does = $_[1]
Topic = $_[2]
}
}
Who Does Topic
--- ---- -----
Tavin enjoys powershellThe Powershell pipeline “unwraps” collections for convenience and, amusingly, for user “consistency”. That unary operator stops powershell from performing that unwrapping. Those examples aren't ideal, so I'll try another approach.
Look at what these two snippets produce
(4, 5, 6) | ForEach-Object {
Write-Host "Pipeline element now: $_"
}
Pipeline element now: 4
Pipeline element now: 5
Pipeline element now: 6,(4, 5, 6) | ForEach-Object {
Write-Host "Pipeline element now: $_"
}
Pipeline element now: 4 5 6The latter sends the complete collection through as a single object, so it never gets unwrapped. It's an array, and the spaces you see, rather than commas, come from the space used as the default output field separator (held in $ofs). Here's a way to observe that
"$(4, 5, 6)"
4 5 6Embedding that array in a string makes powershell join its elements using the default OFS. Changing it is possible, although seldom necessary.
$ofs = '/~/'
"$(4, 5, 6)"
4/~/5/~/6One fairly frequent use for that unary operator in my own work is splitting values while keeping every element together in a single object.
,(-split 'Tavin enjoys powershell') | ForEach-Object {
[PSCustomObject]@{
Who = $_[0]
Does = $_[1]
Topic = $_[2]
}
}
Who Does Topic
--- ---- -----
Tavin enjoys powershell谢谢！这下清楚多了。尤其是把最后那个例子的一元运算符去掉试试。
书里的例子我就是怎么也理解不了。
Thanks! That makes it FAR clearer now. Particularly trying that final example with the unary operator removed.
I just couldn't get comfortable with the example from the book.
非常感谢你发这个解释，真是帮了忙。多谢。
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