Given a list of items in PowerShell, how do I find the index of the current item from within a loop?
For example:
$letters = { 'A', 'B', 'C' }
$letters | % {
# Can I easily get the index of $_ here?
}
The goal of all of this is that I want to output a collection using Format-Table and add an initial column with the index of the current item. This way people can interactively choose an item to select.
I am not sure it's possible with an "automatic" variable. You can always declare one for yourself and increment it:
$letters = { 'A', 'B', 'C' }
$letters | % {$counter = 0}{...;$counter++}
Or use a for loop instead...
for ($counter=0; $counter -lt $letters.Length; $counter++){...}
> "A,B,C,D,E,F,G" -split "," | % { $i = 0 } { if ( $i -gt 3 ) { $_ }; ++$i } Output: E F G
dir | % {$i = 46}{ move-item $_ ("ARM-{0:00000}.pdf" -f $i++)}
$letters | % {$counter... option is safe even if $letters is only one object and not an array where as the for option doesn't handle that properly.
$selected = $true,$false,$true; @('first','second','third') | % {$i=0}{If($selected[$i]){$_};$i++ } to return 'first' and 'third'
.NET has some handy utility methods for this sort of thing in System.Array:
PS> $a = 'a','b','c'
PS> [array]::IndexOf($a, 'b')
1
PS> [array]::IndexOf($a, 'c')
2
Good points on the above approach in the comments. Besides "just" finding an index of an item in an array, given the context of the problem, this is probably more suitable:
$letters = { 'A', 'B', 'C' }
$letters | % {$i=0} {"Value:$_ Index:$i"; $i++}
Foreach (%) can have a Begin sciptblock that executes once. We set an index variable there and then we can reference it in the process scripblock where it gets incremented before exiting the scriptblock.
IndexOf works if your sure you always have an array but the $letters | % {$i=0}... method is safe even if $letters ends up being only one object and not an array.For PowerShell 3.0 and later, there is one built in :)
foreach ($item in $array) {
$array.IndexOf($item)
}
0..($letters.count-1) | foreach { "Value: {0}, Index: {1}" -f $letters[$_],$_}
I found Cédric Rup's answer very helpful but if (like me) you are confused by the '%' syntax/alias, here it is expanded out:
$letters = { 'A', 'B', 'C' }
$letters | ForEach-Object -Begin {$counter = 0} -Process {...;$counter++}
Also, you can use this pattern:
$letters | Select @{n="Index";e={$letters.IndexOf($_)}}, Property1, Property2, Property3
(PowerShell 7)
My solution is to create utility functions in profile script to make the call-site shorter.
I have two solution variants, solution 1 is more flexible and solution 2 is more succint. You may use one, or both!
function ConvertTo-ArrayIndexed {
<#
.SYNOPSIS
Indexed variant of `ForEach-Object`.
.PARAMETER Transform
ScriptBlock function: ([Int]$Index, [Object]$Element) -> [Object]$TransformedElement
.EXAMPLE
'a', ('b','c'), 'd' | ConvertTo-ArrayIndexed {"Element #$($args[0])`: $($args[1]) | Type: $($args[1].getType())"}
Element #0: a | Type: string
Element #1: b c | Type: System.Object[]
Element #2: d | Type: string
#>
[CmdletBinding()]
param(
[Parameter(Mandatory, ValueFromPipeline)][AllowNull()]
[Object]$InputObject,
[Parameter(Mandatory, Position = 0)]
[ScriptBlock]$Transform
)
begin {
$index = 0
}
process {
. $Transform ($index++) $InputObject
}
}
"a", ("b","c"), "d" | %i{ param($i, $e) "Element $i is $e" }
Output:
Element 0 is a
Element 1 is b c
Element 2 is d
function ConvertTo-ArrayIndexed2 {
<#
.SYNOPSIS
Indexed variant of `ForEach-Object`.
.PARAMETER Transform
ScriptBlock function with context: ([Int]$Index, [Object]$Element) -> [Object]$TransformedElement
Two local variables are defined: `$i` for index, and `$_` for element.
.EXAMPLE
'a', ('b','c'), 'd' | ConvertTo-ArrayIndexed2 {"Element #$i`: $_ | Type: $($_.getType())"}
Element #0: a | Type: string
Element #1: b c | Type: System.Object[]
Element #2: d | Type: string
.EXAMPLE
ConvertTo-ArrayIndexed2 -InputObject 'a',('b','c'),'d' {"Element #$i`: $_ | Type: $($_.getType())"}
Element #0: a System.Object[] d | Type: System.Object[]
#>
[CmdletBinding()]
param(
[Parameter(Mandatory, ValueFromPipeline)][AllowNull()]
[Object]$InputObject,
[Parameter(Mandatory, Position = 0)][ArgumentCompletions('{}')]
[ScriptBlock]$Transform
)
begin {
$index = 0
}
process {
$variables = @([PSVariable]::new('i', $index++), [PSVariable]::new('_', $InputObject))
$Transform.InvokeWithContext($null, $variables)
}
}
"a", ("b","c"), "d" | %i{ "Element $i is $_" }
Output:
Element 0 is a
Element 1 is b c
Element 2 is d
I added function aliases:
New-Alias -Name %i -Value ConvertTo-ArrayIndexed
New-Alias -Name mapIndexed -Value ConvertTo-ArrayIndexed
Modify the function and alias as you wish.
I came from a background where I understand such a function as mapIndexed, but %i is more succinct and it goes well with the existing % alias for ForEach-Object.
Also note that "ForEach" is a "reserved verb", but if you don't mind I think an alias named ForEach-ObjectIndexed is pretty nice as well.
[scriptblock] can be a type of argument.For those coming here from Google like I did, later versions of Powershell have a $foreach automatic variable. You can find the "current" object with $foreach.Current
$_ for the current object. Also, OP's question was about finding the index of the current object, not the object itself.
$_ doesn't work in a try-catch block, specifically if there's a catch, since then $_ or $PSItem become $error[0]. So $foreach.Current works perfectly to reference the actual item in that scenario.$_ to another variable on entering the loop, if you're in the try-catch scenario, but it's nice not to have to bother.