foreach loops: How to update collection variable within loop? - powershell

Question

Is there a way to change to behaviour that the collection variable for a loop cannot be updated from within its loop and use the new values in the next iteration?
For example:
$items = #(1,1,1,2)
$counter = 0
foreach ($item in $items) {
$counter += 1
Write-Host "Iteration:" $counter " | collection variable:" $items
$item
$items = $items | Where-Object {$_ -ne $item}
}
$counter
If you run this code the loop will execute for times.
However, since with the first iteration $items is changed from 1,1,1,2 to only contain 2, the loop should only run once more.
I suspect this is because the collection variable $items is not updated in the foreach part.
Is there a way to fix this?

Answer 1

You cannot use a foreach loop with a collection that is being modified in the loop body.
Attempting to do so will actually result in an error (Collection was modified; enumeration operation may not execute.)
The reason you're not seeing an error is that you're not actually modifying the original collection itself; you're assigning a new collection instance to the same variable, but that has no bearing on the original collection instance being enumerated.
You should use a while loop instead, in whose condition the $items variable reference is re-evaluated in every iteration:
$items = 1, 1, 1, 2
$counter = 0
while ($items) { # Loop as long as the collection has at last 1 item.
$counter += 1
Write-Host "Iteration: $counter | collection variable: $items"
$item = $items[0] # access the 1st element
$item # output it
$items = $items | Where-Object {$_ -ne $item} # filter out all elements with the same val.
}
Now you get just 2 iterations:
Iteration: 1 | collection variable: 1 1 1 2
1
Iteration: 2 | collection variable: 2
2