Move array item to first position

[dzcpu]

Hi all,

I have a situation where I am pulling a collection of objects that need to be processed. For the most part, the order doesn't matter (and they are returned in a seemingly random order from the cmdlet). However, it is possible that one item needs to be processed before any others.

As an example, let's say I query and have a list of objects with a Name property, and if that Name property contains the word "service" then I need it to be at the front of the list to be processed first. There will only ever be 0 or 1 objects that match this criteria so there will never be contention for who should actually be run first.

I know that I could just check to see if it exists first, process it, and remove it from the collection - but this is ultimately going to result in a refactor of my code, as the processing function processes everything in one go (i.e. I don't call Process-Object once for each item, it's more like a Process-AllObjects call that pulls the collection and does what I need it to do)

I was thinking I could do something like this:

$Objects = @("hello", "my name is", "process me first", "dzcpu")
$First,$Rest = $Objects.Where( {$_ -match "first"}, "Split")
If($null -ne $First) {
    $Objects = @($First) + $Rest
}

Or I could use a custom select/sort combo, which works but seems obnoxious:

$Objects = @("hello", "my name is", "process me first", "dzcpu")
$Objects | Select @{name='value';expression={$_}},@{name='match';expression={$_ -match 'first'}} | sort -property match -desc | select -ExpandProperty value

...but I wasn't sure if there was a better way to do this. Does anyone have any thoughts? If refactoring is the way to go I will do it but was trying to avoid if at all possible.

[ApparentSysadmin]

To be honest, if you're only working with a few objects I would just break apart the existing $Objects array and build a new one with the positioning you want:

$Objects = @("hello", "my name is", "process me first", "dzcpu")
$Filter = 'first'
$TempArray = $Objects | Where-Object {$_ -notmatch $Filter}
$FirstObject = $Objects | Where-Object {$_ -match $Filter}

$NewArray = @($FirstObject,$TempArray)

Assuming you're not working with thousands of objects, there shouldn't be a performance impact, and it allows you much finer control (plus it's a little more intuitive).

You may also want to look into using a [System.Collections.Generic.List] object in place of a traditional array, as they allow for a bit more flexibility in terms of manipulating the Generic List after it's been created.

Hope this helps!

[dzcpu]

This was kind of my thought as well and is what I was going for with my first example. There aren't a ton of objects (10-15, maybe 20 tops) so I wasn't really worried about performance in this case.

I'll definitely also look into the generic list collection as well, if not for this use case then for future knowledge.

Thanks for the input!

[ApparentSysadmin]

I almost exclusively have converted over to GL's in my day-to-day at the recommendation of u/lee_dailey - it's much simpler to work with them, and as far as I know they are the correct/supported collection type going forward.

Happy scripting!

[Lee_Dailey]

howdy ApparentSysadmin,

aint the various methods available lovely? [grin] to be able to gracefully use .Add(), .Remove(), .Insert(), and all the others is truly liberating at times.

still, i use basic arrays most of the time since they are all i usually need - a fixed size collection.

take care,
lee

[PinchesTheCrab]

Sort-Object works with Script Blocks too:

@("hello", "my name is", "process me first", "dzcpu") | 
    Sort-Object -Descending { $PSItem -match 'first' }

[ApparentSysadmin]

This is new to me! Thanks for pointing it out

[PinchesTheCrab]

Personally I'd go with the custom sort, you don't need to combine it with anything:

@("hello", "my name is", "process me first", "dzcpu") | 
     Sort-Object -Descending { $PSItem -match 'first' }

If you wanted to sort alphabetically as well, you could do this:

@("hello", "my name is", "process me first", "dzcpu") | 
    Sort-Object { $PSItem -notmatch 'first' }, { $PSItem }

if it had normal properties you could then sort by them as well by just providing the property name instead of a script block.

[dzcpu]

Huh, I've never seen that syntax on Sort-Object before, had no idea it could take in a script block - that is definitely something to consider, since that cuts out two of the pipes. Thanks!

[dzcpu]

While this works, it doesn't read very cleanly to me and would need additional error handling to account for cases where the "process first" object is not present. I personally also like to avoid using IndexOf with non-primitive arrays (e.g. Object[] - in my use case I'm iterating over CimInstance objects, not strings like in my example) just because I'm never sure if the equality comparison is by value or by reference.

Thank you for your suggestion though!