PowerShell array unshift

Question

After reading this helpful article on the Windows PowerShell Blog, I realized I could "shift" off the first part of an array, but didn't know how to "unshift" or push elements onto the front of an array in PowerShell.

I am creating an array of hash objects, with the last read item pushed onto the array first. I'm wondering if there is a better way to accomplish this.

## Create a list of files for this collection, pushing item on top of all other items
if ($csvfiles[$collname]) {
    $csvfiles[$collname] = @{ repdate = $date; fileobj = $csv }, $csvfiles[$collname] | %{$_}
}
else {
    $csvfiles[$collname] = @{ repdate = $date; fileobj = $csv }
}

A couple of things to note:

I need to unroll the previous array element with a foreach loop %{$_}, because merely referencing the array would create a nested array structure. I have to have all the elements at the same level.
I need to differentiate between an empty array and one that contains elements. If I attempt to unroll an empty array, it places a $null element at the end of the array.

Thoughts?

PS: The reason the empty hash element produces a NULL value is that $null is treated as a scalar in PowerShell. For details, see https://connect.microsoft.com/PowerShell/feedback/details/281908/foreach-should-not-execute-the-loop-body-for-a-scalar-value-of-null.

ANSWER:

Looks like the best solution is to pre-create the empty array when necessary, rather than code around the $null issue. Here's the rewrite using a .NET ArrayList and a native PoSh array.

if (!$csvfiles.ContainsKey($collname)) {
    $csvfiles[$collname] = [System.Collections.ArrayList]@()
}
$csvfiles[$collname].insert(0, @{ repdate = $repdate; fileobj = $csv })

## NATIVE POSH SOLUTION...
if (!$csvfiles.ContainsKey($collname)) {
    $csvfiles[$collname] = @()
}
$csvfiles[$collname] = @{ repdate = $repdate; fileobj = $csv }, `
                       $csvfiles[$collname] | %{$_}

Sasha · Accepted Answer · 2020-02-25 08:58:30Z

5

You can simply use a plus operator:

$array = @('bar', 'baz')
$array = @('foo') + $array

Note: this re-creates actually creates a new array instead of changing the existing one (but the $head, $tail = $array way of shifting you refer to works extactly in the same way).

answered Feb 25, 2020 at 8:58

Sasha

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Comments

zdan · Accepted Answer · 2012-11-21 23:46:17Z

4

You might want to use ArrayList objects instead, as they support insertions at arbitrary locations. For example:

# C:\> $a = [System.Collections.ArrayList]@(1,2,3,4)
# C:\> $a.insert(0,6)
# C:\> $a
6
1
2
3
4

answered Nov 21, 2012 at 23:46

zdan

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1 Comment

jsuddsjr Over a year ago

I like. Ultimately, I need to proactively create the empty array before I can add items to it. Making it an ArrayList means that my code is a little more readable.

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