Hashtables and key order

Question

Is there a way to keep the order of keys in a hashtable as they were added? Like a push/pop mechanism.

Example:

$hashtable = @{}

$hashtable.Add("Switzerland", "Bern")
$hashtable.Add("Spain", "Madrid")
$hashtable.Add("Italy", "Rome")
$hashtable.Add("Germany", "Berlin")
$hashtable

I want to retain the order in which I've added the elements to the hashtable.

Answer 1

There is no built-in solution in PowerShell V1 / V2. You will want to use the .NET System.Collections.Specialized.OrderedDictionary:

$order = New-Object System.Collections.Specialized.OrderedDictionary
$order.Add("Switzerland", "Bern")
$order.Add("Spain", "Madrid")
$order.Add("Italy", "Rome")
$order.Add("Germany", "Berlin")


PS> $order

Name                           Value
----                           -----
Switzerland                    Bern
Spain                          Madrid
Italy                          Rome
Germany                        Berlin

In PowerShell V3 you can cast to [ordered]:

PS> [ordered]@{"Switzerland"="Bern"; "Spain"="Madrid"; "Italy"="Rome"; "Germany"="Berlin"}

Name                           Value
----                           -----
Switzerland                    Bern
Spain                          Madrid
Italy                          Rome
Germany                        Berlin

Answer 2

You can use an ordered dictionary instead:

Like this:

$list = New-Object System.Collections.Specialized.OrderedDictionary
$list.Add("Switzerland", "Bern")
$list.Add("Spain", "Madrid")
$list.Add("Italy", "Rome")
$list.Add("Germany", "Berlin")
$list

Answer 3

You can give one sequential key as you add elements:

$hashtable = @{}
$hashtable[$hashtable.count] = @("Switzerland", "Bern")
$hashtable[$hashtable.count] = @("Spain", "Madrid")
$hashtable[$hashtable.count] = @("Italy", "Rome")
$hashtable[$hashtable.count] = @("Germany", "Berlin")
$hashtable

Then, you can get elements sorted by the key:

echo "`nHashtable keeping the order as they were added"
foreach($item in $hashtable.getEnumerator() | Sort Key)
{
    $item
}

Answer 4

The PowerShell 1 way is to add a hashtable member to retain the add order. There is no need to use System.Collections.Specialized.OrderedDictionary:

$Hash = New-Object PSObject                                       
$Hash | Add-Member -MemberType NoteProperty -Name key1 -Value val1
$Hash | Add-Member -MemberType NoteProperty -Name key2 -Value val2
$Hash | Add-Member -MemberType NoteProperty -Name key3 -Value val3

Answer 5

For compatibility with older PowerShell versions you might consider this cmdlet:

Function Order-Keys {
    param(
        [Parameter(Mandatory = $true, ValueFromPipeline = $true)][HashTable]$HashTable,
        [Parameter(Mandatory = $false, Position = 1)][ScriptBlock]$Function,
        [Switch]$Descending
    )
    $Keys = $HashTable.Keys | ForEach {$_} # Copy HashTable + KeyCollection
    For ($i = 0; $i -lt $Keys.Count - 1; $i++) {
        For ($j = $i + 1; $j -lt $Keys.Count; $j++) {
            $a = $Keys[$i]
            $b = $Keys[$j]
            If ($Function -is "ScriptBlock") {
                $a = $HashTable[$a] | ForEach $Function
                $b = $HashTable[$b] | ForEach $Function
            }
            If ($Descending) {
                $Swap = $a -lt $b
            }
            Else
            {
                $Swap = $a -gt $b
            }
            If ($Swap) {
                $Keys[$i], $Keys[$j] = $Keys[$j], $Keys[$i]
            }
        }
    }
    Return $Keys
}

This cmdlet returns a list of keys ordered by the function definition:

Sort by name:

$HashTable | Order-Keys | ForEach {Write-Host $_ $HashTable[$_]}
Germany Berlin
Italy Rome
Spain Madrid
Switzerland Bern

Sort by value:

$HashTable | Order-Keys {$_} | ForEach {Write-Host $_ $HashTable[$_]}
Germany Berlin
Switzerland Bern
Spain Madrid
Italy Rome

You might also consider to nest hash tables:

$HashTable = @{
    Switzerland = @{Order = 1; Capital = "Berne"}
    Germany     = @{Order = 2; Capital = "Berlin"}
    Spain       = @{Order = 3; Capital = "Madrid"}
    Italy       = @{Order = 4; Capital = "Rome"}
}

E.g. sort by (hashed) order property and return the key (country):

$HashTable | Order-Keys {$_.Order} | ForEach {$_}

Or sort (descending) by the predefined capital:

$HashTable | Order-Keys {$_.Capital} -Descending | ForEach {$_}

Answer 6

Here is a simple routine that works for me.

function sortedKeys([hashtable]$ht) {
  $out = @()
  foreach($k in $ht.keys) {
    $out += $k
  }
  [Array]::sort($out)
  return ,$out
}

and the call to use it

forEach($k in (& sortedKeys $ht)) {
  ...
}

Answer 7

function global:sortDictionaryByKey([hashtable]$dictionary)
{
    return $dictionary.GetEnumerator() | sort -Property name;
}