I have this code:
Get-ChildItem FOLDERNAMEHERE *.png| ForEach-Object { $_.Name } > fileNames.txt
It prints off a list of file Names, and I want to change it to out just print out an index of numbers instead of Names.
1 Answer
To output sequence numbers starting with 1 (replace $i = 0 with $i = -1 to start with 0):
Get-ChildItem FOLDERNAMEHERE *.png |
ForEach-Object -Begin { $i = 0 } -Process { (++$i) }
Note that the variable $i lives on after this command, and, if it preexisted before running the command, effectively changes it, because the script blocks passed to ForEach-Object run directly in the caller's scope.
The increment assignment operation (++) is wrapped in (...) so as to also output the incremented value (assignment statements produce no output by default).
As an aside:
GitHub issue #13772 proposes introducing an automatic index variable for use in pipelines, such as
$PSIndex, which would simplify the solution to:Get-ChildItem FOLDERNAMEHERE *.png | ForEach-Object { $PSIndex + 1 }
Non-streaming alternative, using .., the range operator:
if ($count = (Get-ChildItem FOLDERNAMEHERE *.png).Count) {
1..$count
}
Note: Non-streaming means that the results of the Get-ChildItem call are collected in full, up front in order to determine the upper limit for the sequence numbers.
Similarly, the 1..$count range operation collects the elements between the range endpoints in an array up front before beginning to produce visible output. See the comments for a discussion.
In other words: Go with the streaming, ForEach-Object-based solution if the Get-ChildItem command produces a lot of output objects and you want to start emitting sequence numbers as quickly as possible.
10 Comments
zett42
What surprised me about the range operator, is the fact that it actually creates an array which is then enumerated. I would have thought that it creates a lazy-evaluated sequence instead. But you get a noticable delay when doing
1..100MB for instance. Compare with [System.Linq.Enumerable]::Range(0, 100MB) which starts to enumerate immediately. I wonder why PowerShell choose that design.
mklement0
@zett42, that
foreach( $i in 0..100MB ) { $i } effectively streams is surprising, given that streaming command output is collected in full, up front in foreach statements; e.g.: foreach ($f in Get-ChildItem $HOME -File -Recurse -ErrorAction Ignore) { $f; pause } Therefore, in the case at hand, if a Get-ChildItem call determines the number of sequence numbers to produce, using the pipeline with ForEach-Object is your only option if you want to avoid a delay stemming from up-front collection.
Santiago Squarzon
@zett42 that's an interesting question, I was unaware using a loop would start enumerating immediately,
0..[int]::MaxValue will instantly throw but foreach($i in 0..[int]::MaxValue) { $i } enumeratesmklement0
Correction, @Santiago; Streaming use effectively permits [uint]::MaxValue + 1 elements, namely:
([int]::MinValue)..([int]::MaxValue) | % { $_; pause }zett42
@SantiagoSquarzon Another proof that
foreach enumerates ranges: foreach($i in 0..0) { $foreach.GetType().Name } prints RangeEnumerator. Compare with foreach($i in (0..0)) { $foreach.GetType().Name } which prints ArrayEnumerator. |
$PSIndexvariable. :)