PowerShell function for adding elements to an array - arrays

Question

I'm still quite new to PowerShell and am trying to create a few functions that weaves together for creating and administrating an array. And I'm having some problems with getting one of these functions to work as intended.
I need the second function (AddToArray) to add an element to the specified index. None of the existing elements can be overwritten or removed.
For example, if I have an array with four indexes and all have the value 5 and I call the function AddToArray 2 4. I need the function to write for in the third index and move the existing ones one down step, so the array now looks like this:
5
5
4
5
5
This is my code so far that shows my CreateArray function and the little code piece for AddToArray function. I've been trying for a while now, but I just can't see the solution.
function CreateArray($Item1, $Item2)
{
$arr = New-Object Array[] $Item1;
# Kontrollerar om $Item2 har fått någon input och skriver in det i arrayen
if ($Item2)
{
for($i = 0; $i -lt $arr.length; $i++)
{
$arr[$i] = $Item2;
}
}
# Standard värde på arrayens index om inget värde anges vid funktionens anrop
else
{
$Item2 = "Hej $env:username och välkommen till vårat script!";
for($i = 0; $i -lt $arr.length; $i++)
{
$arr[$i] = $Item2;
}
}
$script:MainArray = $arr;
}
function AddToArray ($index, $add)
{
$MainArray[$index] = $add;
}

Answer 1

Arrays in .NET don't directly support insertion and they are normally fixed size. PowerShell does allow for easy array resizing but if the array gets large and you're appending (causing a resize) a lot, the performance can be bad.
One easy way to do what you want is to create a new array from the pieces e.g.:
if ($index -eq 0) {
$MainArray = $add,$MainArray
}
elseif ($index -eq $MainArray.Count - 1) {
$MainArray += $add
}
else {
$MainArray = $MainArray[0..($index-1)], $add, $MainArray[$index..($MainArray.Length-1)]
}
But that is kind of a spew. I would use a List for this, which supports insertion and is more efficient than an array.
$list = new-object 'System.Collections.Generic.List[object]'
$list.AddRange((1,2,3,4,5))
$list.Insert(2,10)
$list
And if you really need an array, call the $list.ToArray() method when you're done manipulating the list.

Answer 2

Arrays don't have an .insert() method, but collections do. An easy way to produce a collection from an array is to use the .invoke() method of scriptblock:
$array = 5,5,4,5,5
$collection = {$array}.invoke()
$collection
$collection.GetType()
5
5
4
5
5
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Collection`1 System.Object
Now you can use the .insert() method to insert an element at an arbitrary index:
$collection.Insert(2,3)
$collection
5
5
3
4
5
5
If you need it to be an array again, an easy way to convert it back to an array is to use the pipeline:
$collection | set-variable array
$array
$array.GetType()
5
5
3
4
5
5
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array