Unexpected Result: Lambda Function & Regex

[dicey]

I created a means to mimic a fuzzy lookup and store the results in a data table column using a lambda function with a regular expression. However, I’ve run into an issue with the results when there is no match found.

Staying close to the overall functionality in this demonstration code, how should I script GetMyStuff such that CriticalItems.Count -eq 0 when there are no matches?

If there are posts that I should review, please point me to those as my search of forum topics was inconclusive.

Background

The variable matchStuff stores a lambda script block to match key terms against members of an array (props). This is passed into the named function GetMyStuff which creates an ArrayList object.

The problem appears in GetMyStuff when no matches are made. I’ve attempted different approaches that would ignore the Add() method when zero matches are produced. However, the count on CriticalItems is never less than 1 (the appropriate value when Add($null)). There may be something bound up in &$Expression $item or inherent with the column’s datatype [Object] that in my mind works like T-SQL’s sqlvariant type.

Unexpected: the script block returns matched (or zero matched) items, and not $true|$false. I adjusted my original code from a flag logic to compensate for this, which I came to prefer. #auditing tool

Demo Code

using namespace System.Collections;
using namespace System.Data;

$Bc = [Datatable]::new('Product','Production')
    [void]$Bc.Columns.Add('Color',[string]);
    [void]$Bc.Columns.Add('DaysToManufacture',[string]);
    [void]$Bc.Columns.Add('ListPrice',[decimal]);
    [void]$Bc.Columns.Add('Name',[string]);
    [void]$Bc.Columns.Add('ProductID',[int]);
    [void]$Bc.Columns.Add('ProductNumber',[string]);
    [void]$Bc.Columns.Add('ReorderPoint',[int]);
    [void]$Bc.Columns.Add('SellEndDate',[datetime]);
    [void]$Bc.Columns.Add('SellStartDate',[datetime]);
    [void]$Bc.Columns.Add('Style',[string]);
    [void]$Bc.Columns.Add('CriticalItems',[Object]);

$B = Import-Csv A:\resultsB.csv -Header Color,DaysToManufacture,ListPrice,Name,ProductID,ProductNumber,ReorderPoint,SellEndDate,SellStartDate,Style

function SetDateTime($d1)
{
    switch ($d1)
    {
        {$_ -eq 'NULL'}{'1900-01-01'; break}
        {[string]::IsNullOrEmpty($_)}{'1900-01-01'; break}
        default {$d1}
    }
}

$matchStuff = {param($x) $x -match "(Black|BK|\bGrip Tape\b|adjust)"}

function GetMyStuff
{
    Param(
    [Scriptblock] $Expression,
    [Array] $item
    )

    $results = [ArrayList]::new()
    &$Expression $item | ForEach-Object{
        if( !([string]::IsNullOrEmpty($_)))
        {
            [void]$results.Add($_);
        }
    }
    $results;
}

$Error.Clear();
try
{
    foreach($Bn in $B)
    {
        $props = @($Bn.Color,$Bn.Name,$Bn.ProductNumber);

        $row = $Bc.NewRow();
        $row.Color = $Bn.Color;
        $row.DaysToManufacture = $Bn.DaysToManufacture;
        $row.ListPrice = $Bn.ListPrice;
        $row.Name = $Bn.Name;
        $row.ProductID = $Bn.ProductID;
        $row.ProductNumber = $Bn.ProductNumber;
        $row.ReorderPoint = $Bn.ReorderPoint;
        $row.SellEndDate = SetDateTime($Bn.SellEndDate);
        $row.SellStartDate = SetDateTime($Bn.SellStartDate);
        $row.Style = $Bn.Style;
        $row.CriticalItems = GetMyStuff -Expression $matchStuff -item $props;
        $Bc.Rows.Add($row);
    }
}
catch
{
    $Error.InvocationInfo.ScriptLineNumber;
    $Error.InvocationInfo.OffsetInLine;
    $Error.Exception;
    $Error.InvocationInfo.PositionMessage;
}
cls;
$Bc.Rows.Where({$_.CriticalItems.Count -eq 1}) #produces many rows
$Bc.Rows[$Bc.Rows.Count-1];

Results

Sample of CriticalItems.Count -eq 1. Empty and Single Match. Note - no {} appear.

Sample of CriticalItems storing multiple matches based on the regex key terms.

Many thanks in advance!

[krzydoug]

When there is no match it returns a false, adjusting your anonymous function like so should do the trick.

$matchStuff = {param($x) if($match = $x -match "(Black|BK|\bGrip Tape\b|adjust)"){$match}}

[dicey]

@krzydoug - thanks for the response. Unfortunately, that didn’t fix the issue. The matchStuff expression alters the normal boolean output.

# expressions to return example results
$Bc.Rows.Where({$_.CriticalItems.Count -eq 1}) | Select-Object -First 2
$Bc.Rows[$Bc.Rows.Count-1];

Image showing trial fix

Here’s a look at the two different result types (echo text removed).

&$matchStuff $props;
$props | %{$_ -match "(Black|BK|\bGrip Tape\b|adjust)"}

I’ve looked to the GetMyStuff function to see if the empty or null result (both return a count of 1 in the ArrayList) can be handled. Also unfortunate, the following produces the same issue whereby CriticalElements containing no visible members is returned by ~.Count -eq 1.

function GetMyStuff
{
    Param(
    [Scriptblock] $Expression,    
    [Array] $item
    )

    $results = [ArrayList]::new()
    &$Expression $item | ForEach-Object{[void]$results.Add($_)}
    
    if( ($results.Count-eq 1) -and ($results[0].Length -eq 0) )
    {
        $results.RemoveAt(0);
    }
    $results;
}

Test. Rinse. Repeat.

[dicey]

I added count and length values to provide some more visibility to the results (as well as an auto-incrementing identity column in the DataTable for row identification, called _id).

Why is _id = 2 returned by $Bc.Rows.Where({$_.CriticalItems.Count -eq 1}) | Select-Object -First 2 ?

I noticed that Length in _id = 1 is the number of characters in the single array member, while Length in 504 is 3, which is the same as the member count. That’s understood, but doesn’t quite explain (to me) why 2—being (0,0) is returned by Count -eq 1.

[dicey]

“falsy”

The solution involved handling a null collection—not the modification of a function or a class method.

There may be a more efficient, less costly way to handle null collections, but the following produced the results that I was seeking to this riddle.

1UP! mklement0

$Bc.Rows.Where( {$_.CriticalItems -match "\w+" -and $_.CriticalItems.Count -eq 1}) | Select-Object -First 2;

See Also

https://stackoverflow.com/a/49061138