Filtering list with another list PowerShell

Question

Coming from a C# background and being a complete PowerShell novice, I am trying to make a simple filtering based on two lists/arrays.

Goal: filter elements of list 'toFilter' based on list 'filter', so to only keep elements of 'toFilter', which are matching at least one of the patterns listed in 'filter'.

Ideally I would like to achieve that using piping or Linq (and not nested loops).

In C#, I would do it like this:

string[] filter  = {"A", "B", "C"};
string[] toFilter = {"AA", "AB", "DD"};

var filtered = toFilter.Where(s1 => filter.Any(s2 => s1.Contains(s2))).ToList();

or even shorther, using method groups:

var filtered = toFilter.Where(s1 => filter.Any(s1.Contains)).ToList();

With the expected outcome of:

AA AB

For Powershell, I found an article which describes powershell equivalents of Linq in C#, but apparently I cannot grasp the syntax.

I came with a following solution:

$filter = @("A", "B", "C")
$toFilter = @("AA", "AB", "DD")

$filtered = $toFilter | where {[Linq.Enumerable]::Any([string[]]$filter, [Func[string,bool]]{ $_ -match $args[0] })}

But it results in an empty list.

What would be a correct way of using those two arrays to come with an expected result?

EDIT: Apparently my solution works, but it is not pure LINQ, so a question is still valid.

i've seen that article. i feel really stupid reading it :) i wonder if someone could help — 4c74356b41
– 4c74356b41, Commented Jan 23, 2018 at 16:13
What version of PowerShell do you have? Your syntax looks flawed but actually generated the correct output for me on v5 — Matt
– Matt, Commented Jan 23, 2018 at 17:11
@Matt Funnily enough it has worked for me today, despite seeing empty output yesterday. I guess it might have something to do with re-assigning variables in the Windows Powershell interactive window. I am running v 5.1. — sirVir
– sirVir, Commented Jan 24, 2018 at 8:47

Mathias R. Jessen · Accepted Answer · 2018-01-23 16:18:16Z

5

The reason your example fails is that $_ and $args[0] refer to the same thing in the inner delegate.

There's no need to employ LINQ here though - you can do the same with a nested Where-Object clause:

$filter = @("A", "B", "C")
$toFilter = @("AA", "AB", "DD")

$filtered = $toFilter | Where-Object {
    $item = $_
    @($filter |Where-Object {$item.Contains($_)}).Count -gt 0
}

answered Jan 23, 2018 at 16:18

Mathias R. Jessen

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1 Comment

sirVir Over a year ago

This is a very nice and readable solution, the only drawback I see is introducing an extra variable. BTW, I have run my solution today and strangely it works, so it seems that my reasoning was correct and '$args[0]' responds to elements passed in from 'filter' and the pipe variable '$_' is representing the values of 'toFilter' which is piped in - it does not change because of being inside of the delegate.

Matt · Accepted Answer · 2018-01-23 18:34:50Z

1

Cards on the table this is my first day working with LINQ. It seems like something I would like to be able to use. I tried to nest LINQ statements as best I could to try and get the right results using only LINQ.

[Linq.Enumerable]::where(
    $toFilter,
    [System.Func[System.Object,bool]]{ 
        param($filterItem)
        [Linq.Enumerable]::Any(
            $filter, 
            [Func[System.Object,bool]]{
                param($singlefilter) 
                # write-host "$filterItem -match $singlefilter"
                $filterItem -match $singlefilter}
        )
    }
)

Where to work over $toFilter and the Any checks each $filter against each $toFilter item.

edited Jan 23, 2018 at 18:34

answered Jan 23, 2018 at 18:06

Matt

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2 Comments

sirVir Over a year ago

This is pure LINQ-esque, nicely done. But seeing all the extra syntax I can see why LINQ in PowerShell is not so popular comparing to "normal" piping and where clauses.

Matt Over a year ago

100 percent agreed. However if you are looking for performance this still has a place

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