Merging hashtables in PowerShell: how?
Solution 1
I see two problems:
- The open brace should be on the same line as
Foreach-object - 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
}
Solution 2
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 $h3
All above examples return the same hash table:
Name Value
---- -----
e 1
d 2
b 6
c 3
a 9
If there are any duplicate keys in the supplied hash tables, the value of the last hash table is taken.
(Added 2017-07-09)
Merge-Hashtables version 2
In general, I prefer more global functions which can be customized with parameters to specific needs as in the original question: "overwriting key-value pairs in the first if the same key exists in the second". Why letting the last one overrule and not the first? Why removing anything at all? Maybe someone else want to merge or join the values or get the largest value or just the average...
The version below does no longer support supplying hash tables as arguments (you can only pipe hash tables to the function) but has a parameter that lets you decide how to treat the value array in duplicate entries by operating the value array assigned to the hash key presented in the current object ($_).
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
Solution 3
Not a new answer, this is functionally the same as @Josh-Petitt with improvements.
In this answer:
Merge-HashTableuses the correct PowerShell syntax if you want to drop this into a module- Wasn't idempotent. I added cloning of the HashTable input, otherwise your input was clobbered, not an intention
- added a proper example of usage
function Merge-HashTable {
param(
[hashtable] $default, # Your original set
[hashtable] $uppend # The set you want to update/append to the original set
)
# Clone for idempotence
$default1 = $default.Clone();
# We need to remove any key-value pairs in $default1 that we will
# be replacing with key-value pairs from $uppend
foreach ($key in $uppend.Keys) {
if ($default1.ContainsKey($key)) {
$default1.Remove($key);
}
}
# Union both sets
return $default1 + $uppend;
}
# Real-life example of dealing with IIS AppPool parameters
$defaults = @{
enable32BitAppOnWin64 = $false;
runtime = "v4.0";
pipeline = 1;
idleTimeout = "1.00:00:00";
} ;
$options1 = @{ pipeline = 0; };
$options2 = @{ enable32BitAppOnWin64 = $true; pipeline = 0; };
$results1 = Merge-HashTable -default $defaults -uppend $options1;
# Name Value
# ---- -----
# enable32BitAppOnWin64 False
# runtime v4.0
# idleTimeout 1.00:00:00
# pipeline 0
$results2 = Merge-HashTable -default $defaults -uppend $options2;
# Name Value
# ---- -----
# idleTimeout 1.00:00:00
# runtime v4.0
# enable32BitAppOnWin64 True
# pipeline 0
Solution 4
In case you want to merge the whole hashtable tree
function Join-HashTableTree {
param (
[Parameter(Mandatory = $true, ValueFromPipeline = $true)]
[hashtable]
$SourceHashtable,
[Parameter(Mandatory = $true, Position = 0)]
[hashtable]
$JoinedHashtable
)
$output = $SourceHashtable.Clone()
foreach ($key in $JoinedHashtable.Keys) {
$oldValue = $output[$key]
$newValue = $JoinedHashtable[$key]
$output[$key] =
if ($oldValue -is [hashtable] -and $newValue -is [hashtable]) { $oldValue | ~+ $newValue }
elseif ($oldValue -is [array] -and $newValue -is [array]) { $oldValue + $newValue }
else { $newValue }
}
$output;
}
Then, it can be used like this:
Set-Alias -Name '~+' -Value Join-HashTableTree -Option AllScope
@{
a = 1;
b = @{
ba = 2;
bb = 3
};
c = @{
val = 'value1';
arr = @(
'Foo'
)
}
} |
~+ @{
b = @{
bb = 33;
bc = 'hello'
};
c = @{
arr = @(
'Bar'
)
};
d = @(
42
)
} |
ConvertTo-Json
It will produce the following output:
{
"a": 1,
"d": 42,
"c": {
"val": "value1",
"arr": [
"Foo",
"Bar"
]
},
"b": {
"bb": 33,
"ba": 2,
"bc": "hello"
}
}
Solution 5
The open brace has to be on the same line as ForEach-Object or you have to use the line continuation character (backtick).
This is the case because the code within { ... } is really the value for the -Process parameter of ForEach-Object cmdlet.
-Process <ScriptBlock[]>
Specifies the script block that is applied to each incoming object.
This will get you past the current issue at hand.
Comments
-
Andrew J. Brehm almost 2 years
I am trying to merge two hashtables, overwriting key-value pairs in the first if the same key exists in the second.
To do this I wrote this function which first removes all key-value pairs in the first hastable if the same key exists in the second hashtable.
When I type this into PowerShell line by line it works. But when I run the entire function, PowerShell asks me to provide (what it considers) missing parameters to foreach-object.
function mergehashtables($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?