Constructing a Where statement with variables

PowerShell Help
1

I have this statement:

$tracks | ? { ($_.Year -like "1986") } | Select year
Year
----
1986

and was able to add:

$tracks | ? { ($_.Year -like "1986" -or $_.Year -like '2003') } | Select year
Year
----
2003
1986
2003
2003
2003
2003

but I want to be able to select years from checkboxes in a gui and have them append to that statement if checked like:

If ($checkbox1.Checked -eq $true)
	{
		$next += "-or $_.Year -like '2003'"
	}
$tracks | ? { ($_.Year -like "1986" $next) } | Select year

Can this be done like this so I can select multiple years and just add the -or section to the statement?

Thanks,
Scott

2

You can do it like that. It’d probably be easier to use -in, though.

$target_year -in @($choice_one, $choice_two, $choice_three)

Since your GUI will probably make it easy to get a collection of selected values, that can just be the second operand the right side of -in.

3

Thanks.

When I tried it to test like this:

$next = "-or $_.Year -like '2003'"
$tracks | ? { ($_.Year -like "1986" $next) } | Select year

I get:

At line:2 char:37
+ $tracks | ? { ($_.Year -like "1986" $next) } | Select year
+                                     ~~~~~
Unexpected token '$next' in expression or statement.
At line:2 char:36
+ $tracks | ? { ($_.Year -like "1986" $next) } | Select year
+                                    ~
Missing closing ')' in expression.
At line:2 char:13
+ $tracks | ? { ($_.Year -like "1986" $next) } | Select year
+             ~
Missing closing '}' in statement block or type definition.
At line:2 char:42
+ $tracks | ? { ($_.Year -like "1986" $next) } | Select year
+                                          ~
Unexpected token ')' in expression or statement.
At line:2 char:44
+ $tracks | ? { ($_.Year -like "1986" $next) } | Select year
+                                            ~
Unexpected token '}' in expression or statement.
At line:2 char:46
+ $tracks | ? { ($_.Year -like "1986" $next) } | Select year
+                                              ~
An empty pipe element is not allowed.
    + CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : UnexpectedToken

Can you tell me whats wrong with my syntax on this? and would it be a problem if $next was blank…meaning nothing was selected.

Thanks for your help!
Scott

4

Ah, OK. I misunderstood what you were attempting.

$next is illegal in that context. PowerShell won’t evaluate that as code in the way you’re thinking, no. You’d have to put your entire comparison into a variable and then use Invoke-Expression - which is also vulnerable to a number of different injection attacks if any of your data is coming from external sources.

5

OK thanks. All of this is run internally so that should not be a problem. Could you give me a example of what you mean by chance? I have a better time understanding it if I see an example.

Thanks,
Scott

6

Sure, just put your ENTIRE comparison in a string.

$expression = “$tracks | where { $year -like ‘1’ -or $year -like ‘2’ -or $year -like ‘3’ }”
Invoke-Expression $expression

What comes out should be the results you want.

7

To use Don’s earlier suggestion. If you would use a listbox enabled for multi-selection in your GUI instead of checkboxes for the years. It would be very easy to implement the filter without using Invoke-Expression which can become dangerous if used incorrectly.

Example code:

# Dummy source data converted into an array of objects
$tracks = @'
Name,Year
Space Oddity,1969
Heroes,1977
Under Pressure,1982
Lazarus,2016
'@ | ConvertFrom-Csv

Add-Type -AssemblyName System.Windows.Forms

$listbox = New-Object -TypeName System.Windows.Forms.ListBox
$listbox.SelectionMode = [System.Windows.Forms.SelectionMode]::MultiSimple

# Add some items to our listbox
$tracks.Year | ForEach-Object {
    [void]($listbox.Items.Add($_))
}

# Select a couple of items because this example doesn't have a UI
$listbox.SetSelected(0, $true)
$listbox.SetSelected(3, $true)

# Actual code to filter the tracks
$tracks | Where-Object { $_.Year -in $listbox.SelectedItems }

I hope that helps.

8

Thanks for all your help, reading the the docs for Invoke-Expression does seem to be exactly what I need but it does not seem to be reading $tracks.

$express = "$tracks | where { $_.Year -like '1986' }"
Invoke-Expression $express
Invoke-Expression : At line:1 char:8
+        | where { .Year -like '1986' }
+        ~
An empty pipe element is not allowed.
At line:2 char:1
+ Invoke-Expression $express
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ParserError: (:) [Invoke-Expression], ParseException
    + FullyQualifiedErrorId : EmptyPipeElement,Microsoft.PowerShell.Commands.InvokeExpressionCommand

Do I have to pass $tracks somehow?

Viewing the $express variable just shows:

$express
       | where { .Year -like '1986' }

Viewing $tracks does show content so I am not sure what I am doing wrong.

EDIT: Thanks Daniel Krebs, I think I was typing as the same time as you were responding. I will try that out but would still like to know what I did wrong above just so I know :slight_smile:

Thanks,
Scott

9

The dollar-signs in Don’s Invoke-Expression example need to be escaped with a backtick to avoid early variable expansion into the $express string.

# Just some example data
$tracks = @'
Name,Year
Space Oddity,1969
Heroes,1977
Under Pressure,1982
Lazarus,2016
'@ | ConvertFrom-Csv

$express = "`$tracks | where { `$_.Year -like '1969' -or `$_.Year -like '2016' }"
Invoke-Expression $express
10

Got it!!! Thanks.

Now to see if I can get the Listbox idea working.

Thanks!

11

I got it working both ways just to make sure I understood both methods. Thanks to both of you for all the help!!
I went with the Listbox method which you are right is so much easier, I over complicated it a bit I think…lol
I do have another related question though. I want to be able to pick from a list of Genres and Years (2 separate Listboxes) and have it output whatever matches. I was able to get it to populate the list of Genres automatically but wanted to use a range for the years so I just put them in manually. What I have below works just fine but only if you select at least one entry from each of the 2 Listboxes. I don’t know how to tell it to ignore that part of the statement if nothing is selected:

	$FilteredYears = @()
	If ($listboxYears.SelectedItems -eq "1980s")
	{
		$FilteredYears += (1980..1989)
	}
	If ($listboxYears.SelectedItems -eq "1990s")
	{
		$FilteredYears += (1990..1999)
	}
	If ($listboxYears.SelectedItems -eq "2000s")
	{
		$FilteredYears += (2000..2009)
	}
	If ($listboxYears.SelectedItems -eq "2010s")
	{
		$FilteredYears += (2010..2019)
	}
	
	$Filtered = $tracks | Where-Object { ($_.Genre -in $listboxGenres.SelectedItems) -and ($_.Year -in $FilteredYears) }

Thanks,
Scott

12

:slight_smile: you’re still working on this?

You validate both controls and build your query based on that. Use the api’s built in search.

$genre = $listbox1.selecteditem
$songs = $libraryplaylist.search($genre, $this.ITPlaylistSearchFieldall)

if($listbox2.selectedindex -gt 0){

#filter all $songs by year selected

}

13

Unfortunately the search doesn’t include ‘ALL’ fields as the directions say. I tried it by year, no dice :frowning:

14

LOL…yeah I am still working on it and its going great. I am using this to learn more as I am still very new to PS.

So basically I would create a list of $songs from the Genre selections then use that filtered list to list the year(s) that match in that filter list, right? That makes sense.
I was thinking I had to have it done all at once.

Thanks,
Scott