Get indexes of repeated values in array - arrays

Question

I want to find indexes of repeated values in array.
E.g.
Input:
$data = #(1, 2, 3, 2, 1)
Output:
$indexes = #(3, 4)

Answer 1

For a different approach, you can use try-catch blocks with a hash table.
$data = #(1, 2, 3, 2, 1)
$hash = #{}
$indexes = for ($i = 0; $i -lt $data.count; $i++ ) {
try {
$hash.add($data[$i],$i)
}
catch {
$i
continue
}
}
# Output
$indexes
3
4
The idea here is to add each value as a key and the corresponding index as a value to the hash table. Since a [hashtable] object can only have unique keys, an exception will be thrown and caught. In the catch block, we just output the index which is ultimately stored in $indexes. The continue statement allows the loop to increment and keep processing.
Algorithmically speaking this solution is almost identical to the already proposed solution. However, it utilizes the more efficient Add() method of [arraylist] rather than rebuilding (+=) an [array] during each iteration. The performance is negligible in this example but could be worth considering in larger data sets. This also opts for the traditional for loop rather than foreach.
$uniqueValues = [collections.arraylist]#()
$indexes = for ($i = 0; $i -lt $data.count; $i++) {
if ($uniqueValues -contains $data[$i]) {
$i
}
else {
[void]$uniqueValues.Add($data[$i])
}
}
# Output
$indexes
3
4
This solution maintains an [arraylist] of unique values ($uniqueValues). Any value that is not unique, its index ($i) is output and stored in $indexes. Uniqueness is determined by using the -contains operator to compare the current value in the $data iteration to what is already in $uniqueValues.

Answer 2

You could also use a Hashtable for this:
$data = 1, 2, 3, 2, 1
$hash = #{}
$indexes = for ($i = 0; $i -lt $data.Count; $i++) {
# the value you store in the $hash in the else block is not important
if ($hash.ContainsKey($data[$i])) { $i } else {$hash[$data[$i]] = $true}
}
$indexes
Result:
3
4

Answer 3

Foreach loop with if inside should do the job:
$data = #(1, 2, 3, 2, 1)
$uniqueValues = #()
$duplicatesIndexes = #()
$data | ForEach-Object {$counter = 0}{
if ($_ -notin $uniqueValues) {
$uniqueValues += $_
} else {
$duplicatesIndexes += $counter
}
$counter++
}
# Output
PS> $duplicatesIndexes
3
4