Creating Arraylist of Arraylist by slicing existing arraylist

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