Creating dynamic variable array names and then adding object to them

Question

What I'm trying to do is create array variable names dynamically, and then with a loop, add the object to its relevant array based on the hash table value being equal to the counter variable.

$hshSite = @{}  # Values like this  CO,1  NE,2  IA,3

$counter = $hshSite.count

For($i = $counter; $i -gt 0; $i--) {
New-Variable -Name "arr$i" -Value @()
}

If $counter = 3, I would create arrays $arr1, $arr2, $arr3

$csv = Import-CSV....

ForEach ($x in $csv) {
   #if $hshSite.Name = $x.location (ie CO), look up hash value (1),
   and add the object to $arr1.  If $hshSite.Name = NE, add to $arr2

I tried creating the dynamic arrays with New-Variable, but having issues trying to add to those arrays. Is it possible to concatenate 2 variables names into a single variable name? So taking $arr + $i to form $arr1 and $arr2 and $arr3, and then I can essentially just do $arr0 += $_

The end goal is to group things based on CO, NE, IA for further sorting/grouping/processing. And I'm open to other ideas of getting this accomplished. Thanks for your help!

mjolinor · Accepted Answer · 2014-02-21 17:04:04Z

3

Just make your hash table values the arrays, and accumulate the values to them directly:

$Sites = 'CO','NE','IA'
$hshSite = @{}
Foreach ($Site in $Sites){$hshSite[$Site] = @()}

ForEach ($x in $csv)
 {
   $hshSite[$x.location] += <whatever it is your adding>
 }

If there's a lot of entries in the csv, you might consider creating those values as arraylists instead of arrays.

$Sites = 'CO','NE','IA'
$hshSite = @{}
Foreach ($Site in $Sites){ $hshSite[$Site] = New-Object Collections.Arraylist }

ForEach ($x in $csv)
 {
   $hshSite[$x.location].add('<whatever it is your adding>') > $nul
 }

answered Feb 21, 2014 at 17:04

mjolinor

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2 Comments

cluckinchicken Over a year ago

Brilliant! Yes, the CSV does have a lot of records. Can you briefly explain the benefits of using an array list rather than an array? Thanks again for your help!!!

mjolinor Over a year ago

It's a performance issue related to how arrays are handled internally. You can't really add to an array. What the += operation does is create a brand new array consisting of the elements of the original array, plus the new element. This creates a lot of process overhead. Array lists don't do that, so it's a few ms faster per operation. As long as it's only a few entries the performance difference is trivial, and not worth giving up the simpler an more intiutive coding the array method provides. When you start doing a lot of it the difference gets to be substantial.

Robert Westerlund · Accepted Answer · 2014-02-21 17:06:54Z

2

You could quite easily do add items to a dynamically named array variable using the Get-Variable cmdlet. Similar to the following:

$MyArrayVariable123 = @()
$VariableNamePrefix = "MyArrayVariable"
$VariableNameNumber = "123"

$DynamicallyRetrievedVariable = Get-Variable -Name ($VariableNamePrefix + $VariableNameNumber)
$DynamicallyRetrievedVariable.Value += "added item"

After running the above code the $MyArrayVariable123 variable would be an array holding the single string added item.

answered Feb 21, 2014 at 17:06

Robert Westerlund

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