PowerShell Remove item [0] from an array

问题

I'm struggling a bit to remove the first line (item ID) of an array.

$test.GetType()

IsPublic IsSerial Name                                     BaseType                                                                                                      
-------- -------- ----                                     --------                                                                                                      
True     True     Object[]                                 System.Array

To list all the options I tried ,$test | gm and it clearly states:

Remove         Method                void IList.Remove(System.Object value)                                                                                              
RemoveAt       Method                void IList.RemoveAt(int index)

So when I try $test.RemoveAt(0) I get the error:

Exception calling "RemoveAt" with "1" argument(s): "Collection was of a fixed size."At line:1 char:1
+ $test.RemoveAt(1)
+ ~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : NotSupportedException

So I finally found here that my array needs to be of the type System.Object to be able to use $test.RemoveAt(0). Is it best practice to declare all the arrays in the beginning of the script as a list? Or is it better to convert the arrays with $collection = ({$test}.Invoke()) to a list later on when this functionality is needed?

What are the pro's and cons of both types? Thank you for your help.

回答1:

An alternative option is to use Powershell's ability to assign multiple variables (see this other answer).

$arr = 1..5
$first, $rest= $arr

$rest
2
3
4
5

It's been a feature of Powershell for over a decade. I found this functionality from an MSDN blog post:

回答2:

Arrays are fixed-size, like the error says. RemoveAt() is an inherited method that doesn't apply to normal arrays. To remove the first entry in the array, you could overwrite the array by a copy that includes every item except the first, like this:

$arr = 1..5

$arr
1
2
3
4
5

$arr = $arr[1..($arr.Length-1)]

$arr
2
3
4
5

If you need to remove values at different indexes then you should consider using a List. It supports Add(), Remove() and RemoveAt():

#If you only have a specific type of objects, like int, string etc. then you should edit `[System.Object] to [System.String], [int] etc.
$list = [System.Collections.Generic.List[System.Object]](1..5)

$list
1
2
3
4
5

$list.RemoveAt(0)

$list
2
3
4
5

See my earlier SO answer and about_Arrays for more details about how arrays work.

回答3:

This will allow you to remove every occurrence of an arbitrary element from an array without resorting to a more sophisticated .NET object.

$x=<array element to remove>
$test = $test | Where-Object { $_ -ne $test[$x] }

This will do the same, but will only remove one of the elements. If there are duplicates, they will remain.

$x=<array element to remove>
$skip=$true
$test = $test | ForEach-Object { if (($_ -eq $x) -and $skip) { $skip=$false } else { $_ } }

回答4:

You can use Select-Object -Skip <count> to omit the first count item(s):

PS C:\> 1..3 | Select-Object -Skip 1
2
3
PS C:\>

PS C:\> 1 | Select-Object -Skip 1
PS C:\>

回答5:

Just to update - there's an issue with @Frode F. answer

If the number of elements in array is more than 1

$arr = $arr[1..($arr.Length-1)]

If the number of elements is 1, then this doesn't remove the element

if($arr.Length -le 1) {
    $arr = @()
}
else {
    $arr = $arr[1..($arr.length - 1)]
}

回答6:

I think it's going to depend on the circumstances. If you only need to remove that first element once, then you can use array slicing:

$arr = $arr[1..($arr.length-1)]

If you're going to do it repeatedly, then you should start with an arraylist or generic collection. If it's a large array, you might want to just put the expression that's creating it into a scriptblock and do an .invoke() on that rather than letting the pipeline create an array and then convert that to a collection.

回答7:

Excuse the late answer, but I was struggling with this also. For my intents and purposes (writing to a text file), I realized that since the array was a fixed size -- instead of removing it I could just set the value to string.empty.

$results = SQLQuery -connectionString $connectionString  -query $query;
$results[0] = '';
foreach ($r in $results) {
    Add-Content $skus $r[0]; 
}

For me this got rid of the header that I didn't want in my flat file. Hope this helps someone else out there.

回答8:

If we have the case when big array(or ArrayList) must be performed by some parts - I used some lifehack:

#$bigArray with thousands of items
while($bigArray.Count -gt 0)
{
    if($bigArray.Count -gt 100)
    {
    $k = 100
    }else {$k = $bigArray.Count}
    $part = $bigArray | select -First $k
#now we can make some operations with this $part
#in the end of loop we should exclude this part from big array
    if($bigArray.Count -gt 100)
    {
        $bigArray = $bigArray | select -Last ($bigArray.Count - $k)
    }else {$bigArray = @()}#in this step we have been handle last part of array and we should null him for stop loop
}

And now we can handle big array by some parts(by 100 items)

回答9:

removeat() works with arraylists (like $error):

[collections.arraylist]$a = get-process
$a.removeat(0)

来源：https://stackoverflow.com/questions/24754822/powershell-remove-item-0-from-an-array

标签

arrays

powershell

arraylist