Add to an array in a function in PowerShell is not working like I thought

Question

When I add to an array in a function, it works in the ISE, but in the command window it does not add.

Clear
$array = @()

FUNCTION AddtoArray() {
    Write "start of function"
    $array
    $global:array += "c "
    Write "result of add to array function"
    $array
}

$array = "a "
Write "New array"
$array
$array += "b "
Write "Add to array"
$array
AddtoArray
Write "Back from Function"
$array

When I run this in PowerGUI or PowerShell ISE I get:

a

a b

a b c

When I run this from the Powershell command windows I get:

a

a b

The function += is not adding the "c" when the script is called from the PS cmd window. When I ran it without $global:array += "c " it would work in PowerGui but PS ISE would return just the addition and lose the "a b".

What version of PowerShell? I see the ISE results in V2 and V4 consoles. — Keith Hill
– Keith Hill, Commented May 21, 2015 at 19:52
V3 and V4, yes, ISE is running as expected, it's when it is ran from the PS cmd window that the issue shows — Michael
– Michael, Commented May 21, 2015 at 21:52

Mathias R. Jessen · Accepted Answer · 2015-05-21 22:15:06Z

4

The Global scope represents the topmost scope of any session, and when you open a prompt and assign $a = "123", then you're implicitly writing to $Global:a.

But when you launch a script, such as:

PS C:\> # Inside our global scope here at the prompt we just opened
PS C:\> .\TestAddToArray.ps1
a 
a b
a b
a b
a b
PS C:\>

Any variable assignment inside the TestAddToArray.ps1 file defaults to a Script scope - one that's shared among all the variables and functions inside that script file, but still a child scope to Global.

So, when you first initialize $array, you believe $array to mean $Global:array, but depending on the execution context, it might actually mean $Script:array (skipped the duplicate output):

PS C:\> # Inside our global scope here at the prompt we just opened
PS C:\> .\TestAddToArray.ps1
a   # $Script:array is now "a" - $Global:array doesn't exist
a b # $Script:array is now "a ","b " - $Global:array doesn't exist
a b # $Script:array is still "a ","b " - $Global:array is now "c "
PS C:\>

As an experiment, try and call the script 4-5 times, and then type $Global:array and see how many "c "s have been added

The dynamic scope model used in PowerShell can be a little confusing, but it's pretty well explained in the help files:

Get-Help about_Scopes -Full | more

As the help file mentions, you can reference ancestor scopes by supplying an [int] as the -Scope parameter argument (where 0 represents the local/current scope, 1 is the direct parent scope etc.), and herein lies a powerful "workaround":

function AddToArray {
    $temp = (Get-Variable -Name array -Scope 1).Value
    $temp += "c "
    Set-Variable -Name array -Scope 1 -Value $temp
}

Now, if AddToArray is defined and called from the Global scope, it'll write back to $Global:array (the immediate parent scope), but if it happens inside a script file, it now writes back to $Script:array since the Script scope is the immediate parent scope in this context.

The script then becomes:

clear
$array = @()

function AddtoArray {
    Write "start of function"
    $array
    $temp = (Get-Variable -Name array -Scope 1).Value
    $temp += "c "
    Set-Variable -Name array -Scope 1 -Value $temp
    Write "result of add to array function"
    $array
}

$array = "a "
Write "New array"        
$array
$array += "b "        
Write "Add to array"
$array
AddtoArray
Write "Back from Function"
$array

And you'll find that the output is consistent no matter if it's pasted into the command line, executed from a nested prompt, a separate script file, the editor in ISE (you get the idea)

edited May 21, 2015 at 22:15

answered May 21, 2015 at 19:56

Mathias R. Jessen

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6 Comments

briantist Over a year ago

Excellent answer! about_Scopes is also available online, which you can also get to with Get-Help about_Scopes -Online.

Michael Over a year ago

Thank you Mathias, that makes sense, and I do have multiple "c" as you explained. What is the correct formatting then to have the array added to instead of replaced?

Keith Hill Over a year ago

@Michael Oh, this was in a script file. You should have mentioned that in your post. I just figured you were F5'ing in ISE and pasting into the console. To fix, change $global:array in your function to $script:array.

Mathias R. Jessen Over a year ago

@Michael updated answer with a solution (ab)using the relativity between child/parent scopes

Michael Over a year ago

Sorry Keith, I wasn't sure the proper name for it so i was just calling it Powershell Command Window, I meant the non ISE run space. Changing to $Script:array worked! Thank you, I can't mark this as the correct answer for you to get the points from the comments section. Can you repost as an answer so I can give you credit?

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