PowerShell update hashtable values in foreach loop

Question

I'm trying to update each value in my hashtable with a string replacement. I tried solutions mentioned here, but to no avail.

My current code generates the values with the required replacement. But how do I get them to replace the values in my hashtable?

$mactable = [ordered]@{
    "A" = "11:22:33:XX:YY:10"
    "B" = "11:22:33:XX:YY:20"
    "C" = "11:22:33:XX:YY:30"
    "D" = "11:22:33:XX:YY:40"
    "E" = "11:22:33:XX:YY:50"
}

$octet4hex = "10"
$octet5hex = "16"

foreach($value in $mactable.values){
    $value -replace "XX:YY", ($octet4hex+":"+$octet5hex)
}

I tried this (as a mentioned solution):

foreach($value in @($mactable.values)){
    $mactable[$key] = $value -replace "XX:YY", ($octet4hex+":"+$octet5hex)
}

But that gives me the following error:

Index operation failed; the array index evaluated to null.

Expected result is:

"A" = "11:22:33:10:16:10"
"B" = "11:22:33:10:16:20"
"C" = "11:22:33:10:16:30"
"D" = "11:22:33:10:16:40"
"E" = "11:22:33:10:16:50"

Santiago Squarzon · Accepted Answer · 2023-08-28 19:51:01Z

3

Since you're using an OrderedDictionary and it supports Item[Int32] you can do it with a for loop:

for ($i = 0; $i -lt $mactable.Keys.Count; $i++) {
    $value = $mactable[$i] -replace 'XX:YY', ($octet4hex + ':' + $octet5hex)
    $mactable[$i] = $value
}
$mactable['E'] # Outputs: 11:22:33:10:16:50

If however you were using a hashtable instead, you would need to clone it or create a new array from its keys like shown in other answers from the link (foreach ($key in @($hash.Keys)) { ..) in order to update it while at the same time enumerating it or you would get the error:

Collection was modified; enumeration operation may not execute.

$mactable = @{
    'A' = '11:22:33:XX:YY:10'
    'B' = '11:22:33:XX:YY:20'
    'C' = '11:22:33:XX:YY:30'
    'D' = '11:22:33:XX:YY:40'
    'E' = '11:22:33:XX:YY:50'
}

foreach ($key in $mactable.Clone().Keys) {
    $value = $mactable[$key] -replace 'XX:YY', ($octet4hex + ':' + $octet5hex)
    $mactable[$key] = $value
}
$mactable['E'] # Outputs: 11:22:33:10:16:50

answered Aug 28, 2023 at 19:51

Santiago Squarzon

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2 Comments

mclayton Over a year ago

You could also snapshot the hashtable keys into a new array to bypass the "modify while enumerating" issue - e.g. foreach( $key in @($mactable.Keys) ) { ... }. It's basically the same idea, but without cloning the entire hashtable in the process...

WatskeBart Over a year ago

Thank you for this elaborate answer.I totally missed the fact to put the new value in a variable first and then add it to the mactable. That's why I focussed on the value part in my foreach loop. The ordered part was optional. So I opted for your latter solution and worked like a charm.

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