Get index of current item in a PowerShell loop

powershell

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.

Keith Hill

.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.

Also you probably don't want to look up each index of an array item while iterating those items. That'd be linear search for every item; sounds like making one iteration O(n^2) :-)

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.

Peter Mortensen

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++){...}

Example: > "A,B,C,D,E,F,G" -split "," | % { $i = 0 } { if ( $i -gt 3 ) { $_ }; ++$i } Output: E F G

It worked like a charm when I had to rename files with a counter~ dir | % {$i = 46}{ move-item $_ ("ARM-{0:00000}.pdf" -f $i++)}

The $letters | % {$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.

Thanks for this - I'd never seen that syntax with two sets of curly braces following foreach. It solved the problem of matching up two arrays like this: $selected = $true,$false,$true; @('first','second','third') | % {$i=0}{If($selected[$i]){$_};$i++ } to return 'first' and 'third'

It's a "shortcut" for foreach-object -Begin {first block} -Process {second block}. The first block is run once before processing, the second is run for each item to process. see get-help foreach-item -full for more info

Peter Mortensen

For PowerShell 3.0 and later, there is one built in :)

foreach ($item in $array) {
    $array.IndexOf($item)
}

This is finding the index of the first item matching every item in the array, not iterating with an index value. This is slower, and if the array contains duplicate items you will get the wrong indexes.

Shay Levy

0..($letters.count-1) | foreach { "Value: {0}, Index: {1}" -f $letters[$_],$_}

This is clean enough, but it's too bad there's no direct method that doesn't involve searching for the index in the array.

Jared Forth

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

most people use $_ for the current object. Also, OP's question was about finding the index of the current object, not the object itself.

Get index of current item in a PowerShell loop

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