compare array of string with json object property

Question

This is how my Input looks like:

$AllDrives = '[
    {
        "DriveLetter":  "F",
        "FileSystemLabel":  "Drive_F",
        "AllocationUnitSize":  4096
    },
    {
        "DriveLetter":  "E",
        "FileSystemLabel":  "Drive_E",
        "AllocationUnitSize":  4096
    },
    {
        "DriveLetter":  "H",
        "FileSystemLabel":  "",
        "AllocationUnitSize":  65536
    },
    {
        "DriveLetter":  "I",
        "FileSystemLabel":  "",
        "AllocationUnitSize":  65536
    },
    {
        "DriveLetter":  "G",
        "FileSystemLabel":  "",
        "AllocationUnitSize":  65536
    }
]' | ConvertFrom-Json

I want to compare another array with $AllDrives and return those elements from another array where DriveLetter is not matching with them.

$StandardDrives = @("E","F","G","H","I","J","K")

How do I accomplish this in (PowerShell way) without running two loops ?

Expected output is @("J","K")

Mathias R. Jessen · Accepted Answer · 2021-12-17 16:11:55Z

3

Use Where-Object and take advantage of property enumeration:

$StandardDrives |Where-Object {$_ -notin $AllDrives.DriveLetter}

When PowerShell sees that the $AllDrives array object does not have a DriveLetter property, it'll automatically enumerate the values of DriveLetter on the individual array items instead, and the expression $AllDrives.DriveLetter thus expands to an array equivalent to @('F','E','H','I','G')

answered Dec 17, 2021 at 16:11

Mathias R. Jessen

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5 Comments

mklement0 Over a year ago

Nicely done. Re terminology: Regrettably, the docs do not give the feature an official name. While the term property enumeration is reasonable, the more general term is member enumeration, given that the feature also works with methods. Member enumeration is indeed the term the person who actually implemented the feature coined in the blog post announcing it.

Mathias R. Jessen Over a year ago

@mklement0 The term "property enumeration" pre-dates "member enumeration" by 1 month then - it's the term used in the "New Features in Windows PowerShell 3.0" - part of the on-box documentation released with Windows Server 2012, RC build of which dropped in mid-May 2012 ¯\_(ツ)_/¯

mklement0 Over a year ago

Good to know. But this is isn't a who-came-first issue. If member doesn't make sense to you as the proper abstraction encompassing both properties and methods, then ¯_(ツ)_/

mklement0 Over a year ago

The larger point is: The feature doesn't have an official name in the docs (blog posts don't count), but it should. I've tried to make the docs team introduce one, to no avail. In other cases, they did - whether I liked the name ultimately chosen or not ("intrinsic members"). In the absence of an official name, it would make sense for high-profile answerers here to agree on a name, hence my comment. Testy responses such as yours (a) suggest a lack of awareness that an agreed-upon term is important and (b) prevent reaching an agreement.

Mathias R. Jessen Over a year ago

@mklement0 Fair point. FWIW I agree "member enumeration" is indeed the appropriate way to describe the abstract operation, but I think in this specific case, "property enumeration" is a better label because I'm specifically showcasing implicit enumeration of properties (no second-level methods were invoked during the course of this answer ^_^). I really do appreciate you trying to establish some common ground in describing this core aspect of PowerShell's behavior, but I think the appropriate route is through the docs :)

Lee_Dailey · Accepted Answer · 2021-12-17 17:57:13Z

2

here is yet another way to get the drive letters that are not in use. [grin]

what it does ...

defines the list of possible drive letters
uses the .Where() collection method to filter things
checks to see if the possible letter is in the in-use letter collection
outputs that to the success stream
that can be captured to a $Var by assigning it to one.

the code ...

@('e', 'f', 'g', 'h', 'i', 'j', 'k').Where({
    $_ -notin $AllDrives.DriveLetter
    })

output ...

j
k

answered Dec 17, 2021 at 17:57

Lee_Dailey

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Comments

Paolo · Accepted Answer · 2021-12-17 16:14:10Z

1

Using Compare-Object:

(Compare-Object -ReferenceObject $AllDrives.DriveLetter -DifferenceObject $StandardDrives).InputObject

will return:

J
K

answered Dec 17, 2021 at 16:14

Paolo

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