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(\"Sw         


        

Answer 1

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 {$_}