Compare two hashtable in powershell

The solution has been already provided by @PetSerAl and @LotPings and is one of the following

$alpha.GetEnumerator() | select Key, @{ n='Value'; e={$beta[$_.Value]} }
$alpha.GetEnumerator() | %{[PSCustomObject]@{aKey=$_.Key;aValue=$_.Value;bValue=$beta[$_.Value]}}

Let me explain what exactly happens there.

First of all, as you use hashtables you cannot manipulate them directly using cmdlets like Select-Object. In order to do this you need to use GetEnumerator() method on it. Now you can pipe it to Select-Object.

To use the values from another hashtable you have to use calculated property instead of standard one. The syntax of it is:

@{ n='name'; e={ expression to be executed }

Let's dig into this expression $beta[$_.Value] a bit more. $_ represents the object sent to pipeline so $_.Value is its value (as you know hashtables have key names and values). To better understand check this expression and its result

PS C:\> $alpha.GetEnumerator() | select -Last 1

Name                           Value
----                           -----
A5                             dog

For this entry $_.Value is dog so $beta[$_.Value] is evaluated to $beta["dog"] and its value is:

PS C:\> $beta["dog"]
P5

Additional resources:

1. Microcode: PowerShell Scripting Tricks: The Joy of using Hashtables with Windows PowerShell
2. Add a calculated property with Select-Object in PowerShell
3. Example of the same but with multiple values to be replaced (my answer)