Creating Arraylist of Arraylist by slicing existing arraylist - powershell

Question

I have the following variable defined
$A = New-Object -TypeName "System.Collections.ArrayList"
Now I add n elements to it :
$A.Add(1..n)
Now I want to divide $A into p parts of k elements each(The last one might have lesser elements if p*k>$A.count).
How do I do that?

Answer 1

You can use a function to split an array into several smaller arrays.
Below a slighty adapted version of that function found here:
function Split-Array {
[CmdletBinding(DefaultParametersetName = 'ByChunkSize')]
Param(
[Parameter(Mandatory = $true, Position = 0)]
$Array,
[Parameter(Mandatory = $true, Position = 1, ParameterSetName = 'ByChunkSize')]
[ValidateRange(1,[int]::MaxValue)]
[int]$ChunkSize,
[Parameter(Mandatory = $true, Position = 1, ParameterSetName = 'ByParts')]
[ValidateRange(1,[int]::MaxValue)]
[int]$Parts
)
$items = $Array.Count
switch ($PsCmdlet.ParameterSetName) {
'ByChunkSize' { $Parts = [Math]::Ceiling($items / $ChunkSize) }
'ByParts' { $ChunkSize = [Math]::Ceiling($items / $Parts) }
default { throw "Split-Array: You must use either the Parts or the ChunkSize parameter" }
}
# when the given ChunkSize is larger or equal to the number of items in the array
# use TWO unary commas to return the array as single sub array of the result.
if ($ChunkSize -ge $items) { return ,,$Array }
$result = for ($i = 1; $i -le $Parts; $i++) {
$first = (($i - 1) * $ChunkSize)
$last = [Math]::Min(($i * $ChunkSize) - 1, $items - 1)
,$Array[$first..$last]
}
return ,$result
}
In your case you could use it like:
$p = 4 # the number of parts you want
$subArrays = Split-Array $A.ToArray() -Parts $p
or
$k = 4 # the max number items in each part
$subArrays = Split-Array $A.ToArray() -ChunkSize $k

Answer 2

Here is a function I came up with to chunk your System.Collections.ArrayList into a nested array list of p parts. It uses a System.Collections.Specialized.OrderedDictionary to group the size k chunks by index / chunksize, which is then rounded down to the nearest integer using System.Math.Floor. It then only fetches the groups with keys from 0 to $Parts.
function Split-ArrayList {
[CmdletBinding()]
param (
# Arraylist to slice
[Parameter(Mandatory=$true)]
[System.Collections.ArrayList]
$ArrayList,
# Chunk size per part
[Parameter(Mandatory=$true)]
[ValidateRange(1, [int]::MaxValue)]
[int]
$ChunkSize,
# Number of parts
[Parameter(Mandatory=$true)]
[ValidateRange(1, [int]::MaxValue)]
[int]
$Parts
)
# Group chunks into hashtable
$chunkGroups = [ordered]#{}
for ($i = 0; $i -lt $ArrayList.Count; $i++) {
# Get the hashtable key by dividing the index by the chunk size
# Round down to nearest integer using Math.Floor
[int]$key = [Math]::Floor($i / $ChunkSize)
# Add new arraylist for key if it doesn't exist
# ContainsKey is not supported for ordered dictionary
if ($chunkGroups.Keys -notcontains $key) {
$chunkGroups.Add($key, [System.Collections.ArrayList]::new())
}
# Add number to hashtable
[void]$chunkGroups[$key].Add($ArrayList[$i])
}
# Create nested ArrayList of parts
$result = [System.Collections.ArrayList]::new()
for ($key = 0; $key -lt $Parts; $key++) {
[void]$result.Add($chunkGroups[$key])
}
$result
}
Usage:
$A = [System.Collections.ArrayList]::new(1..10)
Split-ArrayList -ArrayList $A -ChunkSize 4 -Parts 1 |
ForEach-Object { "{ " + ($_ -join ", ") + " }" }
# { 1, 2, 3, 4 }
Split-ArrayList -ArrayList $A -ChunkSize 4 -Parts 2 |
ForEach-Object { "{ " + ($_ -join ", ") + " }" }
# { 1, 2, 3, 4 }
# { 5, 6, 7, 8 }
Split-ArrayList -ArrayList $A -ChunkSize 4 -Parts 3 |
ForEach-Object { "{ " + ($_ -join ", ") + " }" }
# { 1, 2, 3, 4 }
# { 5, 6, 7, 8 }
# { 9, 10 }
Note: I didn't really account for the cases where you might want to exclude Parts, so I made every parameter mandatory. You can amend the function to be more flexible with different inputs.