Array.Find and IndexOf for multiple elements that are exactly the same object

Question

I have trouble of getting index of the current element for multiple elements that are exactly the same object:

$b = "A","D","B","D","C","E","D","F"
$b | ? { $_ -contains "D" }

Alternative version:

$b = "A","D","B","D","C","E","D","F"
[Array]::FindAll($b, [Predicate[String]]{ $args[0] -contains "D" })

This will return: D D D

But this code:

$b | % { $b.IndexOf("D") }

Alternative version:

[Array]::FindAll($b, [Predicate[String]]{ $args[0] -contains "D" }) | % { $b.IndexOf($_) }

Returns:

1 1 1

so it's pointing at the index of the first element. How to get indexes of the other elements?

Answer 1

You can do this:

$b = "A","D","B","D","C","E","D","F" 

(0..($b.Count-1)) | where {$b[$_] -eq 'D'}

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Answer 2

mjolinor's answer is conceptually elegant, but slow with large arrays, presumably due to having to build a parallel array of indices first (which is also memory-inefficient).

It is conceptually similar to the following LINQ-based solution (PSv3+), which is more memory-efficient and about twice as fast, but still slow:

$arr = 'A','D','B','D','C','E','D','F'
[Linq.Enumerable]::Where(
 [Linq.Enumerable]::Range(0, $arr.Length), 
   [Func[int, bool]] { param($i) $arr[$i] -eq 'D' }
)

While any PowerShell looping solution is ultimately slow compared to a compiled language, the following alternative, while more verbose, is still much faster with large arrays:

PS C:\> & { param($arr, $val)
         $i = 0
         foreach ($el in $arr) { if ($el -eq $val) { $i } ++$i }
        } ('A','D','B','D','C','E','D','F') 'D'
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Note:

• Perhaps surprisingly, this solution is even faster than Matt's solution, which calls [array]::IndexOf() in a loop instead of enumerating all elements.

• Use of a script block (invoked with call operator & and arguments), while not strictly necessary, is used to prevent polluting the enclosing scope with helper variable $i.

• The foreach statement is faster than the Foreach-Object cmdlet (whose built-in aliases are % and, confusingly, also foreach).

• Simply (implicitly) outputting $i for each match makes PowerShell collect multiple results in an array.

  • If only one index is found, you'll get a scalar [int] instance instead; wrap the whole command in @(...) to ensure that you always get an array.

• While $i by itself outputs the value of $i, ++$i by design does NOT (though you could use (++$i) to achieve that, if needed).

• Unlike Array.IndexOf(), PowerShell's -eq operator is case-insensitive by default; for case-sensitivity, use -ceq instead.

---

It's easy to turn the above into a (simple) function (note that the parameters are purposely untyped, for flexibility):

function get-IndicesOf($Array, $Value) {
  $i = 0
  foreach ($el in $Array) { 
    if ($el -eq $Value) { $i } 
    ++$i
  }
}

# Sample call
PS C:\> get-IndicesOf ('A','D','B','D','C','E','D','F') 'D'
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Answer 3

You would still need to loop with the static methods from [array] but if you are still curious something like this would work.

$b = "A","D","B","D","C","E","D","F"
$results = @()
$singleIndex = -1
Do{
    $singleIndex = [array]::IndexOf($b,"D",$singleIndex + 1)
    If($singleIndex -ge 0){$results += $singleIndex}
}While($singleIndex -ge 0)
$results

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Loop until a match is not found. Assume the match at first by assigning the $singleIndex to -1 ( Which is what a non match would return). When a match is found add the index to a results array.