How to add a String to duplicate element in Array in Powershell

Question

I have following array:

$array = 'A', 'B', 'C', 'A'

I want to identify the duplicate and add to the duplicate a string, for example, $string='Part 2'

so that I have

$array = 'A', 'B', 'C', 'A Part 2'

How do I do this in PowerShell?

$array = 'A';'B';'C';'A' <-- ; is not an array item separator, this is just 1 assignment and 3 string literals - did you mean $array = 'A','B','C','A'? — Mathias R. Jessen
– Mathias R. Jessen, Commented May 11, 2021 at 12:19
And is casing important? Is A and a considered duplicates, or two distinct strings? — Mathias R. Jessen
– Mathias R. Jessen, Commented May 11, 2021 at 12:21
I just want to change the duplicate within an array for example by inserting a string — TheCodingKing
– TheCodingKing, Commented May 11, 2021 at 12:21

Mathias R. Jessen · Accepted Answer · 2021-05-11 12:28:19Z

3

Use a hashtable to keep track of strings you've already seen, then loop through all items in the array - if we've already seen the same, modify the item, otherwise leave it as is:

$array = 'A','B','C','A'

# Create hashtable to keep track of strings we've already encountered
$stringsSeen = @{}

# Now iterate over the array
$modifiedArray = $array |ForEach-Object {
  if(!$stringsSeen.ContainsKey($_)){
    # First encounter, add to hashtable
    $stringsSeen[$_] = 1
  }
  else {
    # We've seen it before! Time to update value and modify `$_`
    $number = ++$stringsSeen[$_]
    $_ += " Part $number"
  }

  # finally output `$_`, regardless of whether we modified it or not
  $_
}

$modifiedArray now holds the new array of (partially) modified strings:

PS ~> $modifiedArray
A
B
C
A Part 2

answered May 11, 2021 at 12:28

Mathias R. Jessen

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Comments

mklement0 · Accepted Answer · 2021-05-11 12:42:46Z

3

Mathias R. Jessen's helpful answer shows an effective, readable solution.

The following solution streamlines the approach to make it more concise (though thereby potentially more obscure) and more efficient:

The .ForEach() array method is used to process the input array, as a faster alternative to the ForEach-Object cmdlet. (A foreach statement would be even faster, but is a bit more verbose).
It relies on the (non-obvious) fact that ++ applied to a non-existent hashtable entry implicitly creates the entry with value 1.

$array = 'A', 'B', 'C', 'A'

$ht = @{}
$newArray = $array.ForEach(
  { if (($count = ++$ht[$_]) -eq 1) { $_ } else { "$_ Part $count" } }
)

^{Note: $newArray is technically not an array, but of type [System.Collections.ObjectModel.Collection[psobject]], but that usually won't make a difference.}

In PowerShell (Core) 7+ an even more concise solution is possible, using ?:, the ternary conditional operator:

$array = 'A', 'B', 'C', 'A'

$ht = @{}
$newArray = $array.ForEach({ ($count = ++$ht[$_]) -eq 1 ? $_ : "$_ Part $count" })

answered May 11, 2021 at 12:42

mklement0

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2 Comments

Mathias R. Jessen Over a year ago

I'm afraid I've packed away all my golf clothes for the season xD

mklement0 Over a year ago

@MathiasR.Jessen :) Yes, I know such solutions aren't for everyone, but I hope at least some readers find them helpful, especially those familiar with awk.

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