Unique Combos from powershell array - No duplicate combos

Question

I\'m trying to figure out the best way to get unique combinations from a powershell array. For instance, my array might be

@(B,C,D,E)

I would b

Answer 1

This is an adaptation from a solution for a C# class I took that asked this same question. For any set find all subsets, including the empty set.

function Get-Subsets ($a){
    #uncomment following to ensure only unique inputs are parsed
    #e.g. 'B','C','D','E','E' would become 'B','C','D','E'
    #$a = $a | Select-Object -Unique
    #create an array to store output
    $l = @()
    #for any set of length n the maximum number of subsets is 2^n
    for ($i = 0; $i -lt [Math]::Pow(2,$a.Length); $i++)
    { 
        #temporary array to hold output
        [string[]]$out = New-Object string[] $a.length
        #iterate through each element
        for ($j = 0; $j -lt $a.Length; $j++)
        { 
            #start at the end of the array take elements, work your way towards the front
            if (($i -band (1 -shl ($a.Length - $j - 1))) -ne 0)
            {
                #store the subset in a temp array
                $out[$j] = $a[$j]
            }
        }
        #stick subset into an array
        $l += -join $out
    }
    #group the subsets by length, iterate through them and sort
    $l | Group-Object -Property Length | %{$_.Group | sort}
}

Use like so:

PS C:>Get-Subsets @('b','c','d','e')

b
c
d
e
bc
bd
be
cd
ce
de
bcd
bce
bde
cde
bcde

Note that computational costs go up exponentially with the length of the input array.

Elements     SecondstoComplete
15               46.3488228
14               13.4836299
13                3.6316713
12                1.2542701
11                0.4472637
10                0.1942997
 9                0.0867832