Merging hashtables in PowerShell: how?

Question

I am trying to merge two hashtables, overwriting key-value pairs in the first if the same key exists in the second.

To do this I wrote this function which first removes all key-value pairs in the first hastable if the same key exists in the second hashtable.

When I type this into PowerShell line by line it works. But when I run the entire function, PowerShell asks me to provide (what it considers) missing parameters to foreach-object.

function mergehashtables($htold, $htnew) {     $htold.getenumerator() | foreach-object     {         $key = $_.key         if ($htnew.containskey($key))         {             $htold.remove($key)         }     }     $htnew = $htold + $htnew     return $htnew } 

Output:

PS C:\> mergehashtables $ht $ht2  cmdlet ForEach-Object at command pipeline position 1 Supply values for the following parameters: Process[0]: 

$ht and $ht2 are hashtables containing two key-value pairs each, one of them with the key "name" in both hashtables.

What am I doing wrong?

Answer 1

Merge-Hashtables

Instead of removing keys you might consider to simply overwrite them:

$h1 = @{a = 9; b = 8; c = 7} $h2 = @{b = 6; c = 5; d = 4} $h3 = @{c = 3; d = 2; e = 1}   Function Merge-Hashtables {     $Output = @{}     ForEach ($Hashtable in ($Input + $Args)) {         If ($Hashtable -is [Hashtable]) {             ForEach ($Key in $Hashtable.Keys) {$Output.$Key = $Hashtable.$Key}         }     }     $Output } 

For this cmdlet you can use several syntaxes and you are not limited to two input tables: Using the pipeline: $h1, $h2, $h3 | Merge-Hashtables
Using arguments: Merge-Hashtables $h1 $h2 $h3
Or a combination: $h1 | Merge-Hashtables $h2 $h3
All above examples return the same hash table:

Name                           Value ----                           ----- e                              1 d                              2 b                              6 c                              3 a                              9 

If there are any duplicate keys in the supplied hash tables, the value of the last hash table is taken.

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(Added 2017-07-09)

Merge-Hashtables version 2

In general, I prefer more global functions which can be customized with parameters to specific needs as in the original question: "overwriting key-value pairs in the first if the same key exists in the second". Why letting the last one overrule and not the first? Why removing anything at all? Maybe someone else want to merge or join the values or get the largest value or just the average...
The version below does no longer support supplying hash tables as arguments (you can only pipe hash tables to the function) but has a parameter that lets you decide how to treat the value array in duplicate entries by operating the value array assigned to the hash key presented in the current object ($_).

Function

Function Merge-Hashtables([ScriptBlock]$Operator) {     $Output = @{}     ForEach ($Hashtable in $Input) {         If ($Hashtable -is [Hashtable]) {             ForEach ($Key in $Hashtable.Keys) {$Output.$Key = If ($Output.ContainsKey($Key)) {@($Output.$Key) + $Hashtable.$Key} Else  {$Hashtable.$Key}}         }     }     If ($Operator) {ForEach ($Key in @($Output.Keys)) {$_ = @($Output.$Key); $Output.$Key = Invoke-Command $Operator}}     $Output } 

Syntax

HashTable[] <Hashtables> | Merge-Hashtables [-Operator <ScriptBlock>] 

Default By default, all values from duplicated hash table entries will added to an array:

PS C:\> $h1, $h2, $h3 | Merge-Hashtables  Name                           Value ----                           ----- e                              1 d                              {4, 2} b                              {8, 6} c                              {7, 5, 3} a                              9 

Examples To get the same result as version 1 (using the last values) use the command: $h1, $h2, $h3 | Merge-Hashtables {$_[-1]}. If you would like to use the first values instead, the command is: $h1, $h2, $h3 | Merge-Hashtables {$_[0]} or the largest values: $h1, $h2, $h3 | Merge-Hashtables {($_ | Measure-Object -Maximum).Maximum}.

More examples:

PS C:\> $h1, $h2, $h3 | Merge-Hashtables {($_ | Measure-Object -Average).Average} # Take the average values"  Name                           Value ----                           ----- e                              1 d                              3 b                              7 c                              5 a                              9   PS C:\> $h1, $h2, $h3 | Merge-Hashtables {$_ -Join ""} # Join the values together  Name                           Value ----                           ----- e                              1 d                              42 b                              86 c                              753 a                              9   PS C:\> $h1, $h2, $h3 | Merge-Hashtables {$_ | Sort-Object} # Sort the values list  Name                           Value ----                           ----- e                              1 d                              {2, 4} b                              {6, 8} c                              {3, 5, 7} a                              9 

Answer 2

I see two problems:

1. The open brace should be on the same line as Foreach-object
2. You shouldn't modify a collection while enumerating through a collection

The example below illustrates how to fix both issues:

function mergehashtables($htold, $htnew) {     $keys = $htold.getenumerator() | foreach-object {$_.key}     $keys | foreach-object {         $key = $_         if ($htnew.containskey($key))         {             $htold.remove($key)         }     }     $htnew = $htold + $htnew     return $htnew }