How to prepend an element to an array in Powershell?

Question

The Powershell code:

$list += "aa"

appends the element "aa" to the list $list. Is there a way to prepend an element? This is my solution, but there must be a way to do this in a single line.

$tmp = ,"aa";
$tmp += $list
$list = $tmp

Solution 1

In your example above, you should just be able to do:

$list = ,"aa" + $list

That will simply prepend "aa" to the list and make it the 0th element. Verify by getting $list[0].

Solution 2

Using += and + on arrays in PowerShell is making a copy of the array every time you use it. That is fine unless the list/array is really large. In that case, consider using a generic list:

C:\> $list = new-object 'System.Collections.Generic.List[string]'
C:\> $list.Add('a')
C:\> $list.Add('b')
C:\> $list.Insert(0,'aa')
C:\> $list
aa
a
b

Note that in this scenario you need to use the Add/Insert methods. If you fall back to using +=, it will copy the generic list back to an object[].

Solution 3

This combines two arrays into one.

$list = @("aa") + $list

It's impossible to do a pure prepending into a PowerShell array, because PowerShell arrays are fixed length. Combining two arrays into one is a good approach.

Solution 4

If you happen to want to do this to an 'Object' array rather than a 'String', I found the following useful:

$List = $List | foreach {'aa' + $_}

Obviously, this would be relatively slow for a gigantic array.

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