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POCompare-Object PowerShell performance and Operation VS Loop
 

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  1. POCompare-Object PowerShell performance and Operation VS Loop
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    <p>I was looking at <a href="https://stackoverflow.com/questions/16657743/how-can-i-compare-two-arrays-removing-similar-items-without-iterating-through">this question</a> where the OP wanted to know how to compare items in two arrays without looping through each array.</p> <p>The command given was:</p> <pre><code>$array3 = @(Compare-Object $array1 $array2 | select -Expand InputObject </code></pre> <p>My question is two-fold:</p> <p>One, does this actually avoid iterating through the arrays in any form? Or does it simply obfuscate the operation from the user by doing it behind the scenes.</p> <p>Two, as far as performance goes is this the best method for comparing objects? It appears to me it is actually significantly slower.</p> <p>I made a real crude test:</p> <pre><code>$Array1 = @("1","2","Orchid","Envy","Sam","Map Of the World","Short String","s","V","DM","qwerty","1234567891011") $Array2 = @("Bob", "Helmet", "Jane") $Date1 = Get-Date $Array2 | ForEach-Object ` { if ($Array1 -contains $_){} } $Date2 = Get-Date $Time1 = [TimeSpan]$Date2.Subtract($Date1) Write-Host $Time1 $Date1 = Get-Date $Array3 = @(Compare-Object $Array1 $Array2) $Date2 = Get-Date $Time2 = [TimeSpan]$Date2.Subtract($Date1) Write-Host $Time2 </code></pre> <p>And my times came out:</p> <p>ForEach-Object: 00:00:00.0030001</p> <p>Compare-Object: 00:00:00.0030002</p> <hr> <h2>Edit</h2> <p>I updated the script to make it more fair, and it essentially evened out the times.</p> <p>So what is the behind the scenes difference between Compare-Object and a traditional loop? Am I correct in assuming none?</p> <hr> <h2>Edit 2</h2> <p>I found this code using the decompiler:</p> <pre><code>internal int Compare(ObjectCommandPropertyValue first, ObjectCommandPropertyValue second) { if (first.IsExistingProperty &amp;&amp; second.IsExistingProperty) return this.Compare(first.PropertyValue, second.PropertyValue); if (first.IsExistingProperty) return -1; return second.IsExistingProperty ? 1 : 0; } public int Compare(object first, object second) { if (ObjectCommandComparer.IsValueNull(first) &amp;&amp; ObjectCommandComparer.IsValueNull(second)) return 0; PSObject psObject1 = first as PSObject; if (psObject1 != null) first = psObject1.BaseObject; PSObject psObject2 = second as PSObject; if (psObject2 != null) second = psObject2.BaseObject; try { return LanguagePrimitives.Compare(first, second, !this.caseSensitive, (IFormatProvider) this.cultureInfo) * (this.ascendingOrder ? 1 : -1); } catch (InvalidCastException ex) { } catch (ArgumentException ex) { } return string.Compare(((object) PSObject.AsPSObject(first)).ToString(), ((object) PSObject.AsPSObject(second)).ToString(), !this.caseSensitive, this.cultureInfo) * (this.ascendingOrder ? 1 : -1); } </code></pre> <p>I have traced it around as best as I can, and I believe these are the two worker threads. It appears Compare-Object actually only does a 1 &lt;==> 1 check down the list. Am I missing something here?</p>
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    1. COYour cheating with `ForEach-Object`. To be fair you'll also need construct a result comparison object like `Compare-Object` does.
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      1. USAndy Arismendi
    2. COI thought about that last night, and removed *| select -Expand InputObject* and now the Compare-Object is faster...
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    3. COI asked a similar [question](http://stackoverflow.com/questions/15822046/wildcard-matching-in-get-childitem-in-powershell) a while back: how to check code of specific cmdlets. Apparently, no one knows it. You could try http://www.jetbrains.com/decompiler/ (I never used it), or you could simply put a breakpoint with `Set-PSBreakpoint .\your-script.ps1 -Line x` and then monitor the executed code and try to reconstruct the code. Please post back when you know more.
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