Select-Random v2.1 (modification of post by Joel Bennett view diff)
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Select a user-defined number of random elements from the collection … which can be passed as a parameter or input via the pipeline. An improvement over http://www.powershellcentral.com/scripts/60 which allows you to select more than one item, and doesn’t copy the full collection into RAM. (Also, a bug fix for http://www.powershellcentral.com/scripts/81)
- # ---------------------------------------------------------------------------
- ### <Script>
- ### <Author>
- ### Joel "Jaykul" Bennett
- ### </Author>
- ### <Description>
- ### Selects a random element from the collection either passed as a parameter or input via the pipeline.
- ### If the collection is passed in as an argument, we simply pick a random number between 0 and count-1
- ### for each element you want to return, but when processing pipeline input we want to keep memory use
- ### to a minimum, so we use a "reservoir sampling" algorithm[1].
- ###
- ### [1] http://gregable.com/2007/10/reservoir-sampling.html
- ###
- ### The script stores $count elements (the eventual result) at all times. It continues processing
- ### elements until it reaches the end of the input. For each input element $n (the count of the inputs
- ### so far) there is a $count/$n chance that it becomes part of the result.
- ### * For each previously selected element, there is a $count/($n-1) chance of it being selected
- ### * For the ones selected, there's a ($count/$n * 1/$count = 1/$n) chance of it being replaced, so a
- ### ($n-1)/$n chance of it remaining ... thus, it's cumulative probability of being among the selected
- ### elements after the nth input is processed is $count/($n-1) * ($n-1)/$n = $count/$n, as it should be.
- ###
- ### </Description>
- ### <Usage>
- ### $arr = 1..5; Select-Random $arr
- ### 1..10 | Select-Random -Count 2
- ### </Usage>
- ### <Version>2.1.0.0</Version>
- ### </Script>
- # ---------------------------------------------------------------------------
- param([int]$count=1, [array]$inputObject=$null)
- BEGIN {
- if ($args -eq '-?') {
- @"
- Usage: Select-Random [[-Count] <int>] [-inputObject] <array> (from pipeline) [-?]
- Parameters:
- -Count : The number of elements to select.
- -Collection : The collection from which to select a random element.
- -? : Display this usage information and exit
- Examples:
- PS> $arr = 1..5; Select-Random $arr
- PS> 1..10 | Select-Random -Count 2
- "@
- exit
- }
- elseif ($inputObject)
- {
- ### If you're accepting $args, you need to pass those in...
- # Write-Output $io | &($MyInvocation.InvocationName) $args;
- Write-Output $inputObject | &($MyInvocation.InvocationName) -Count $count
- break;
- }
- else
- {
- $seen = 0
- $selected = new-object object[] $count
- $rand = new-object Random
- }
- }
- PROCESS {
- if($_) {
- $seen++
- if($seen -lt $count) {
- $selected[$seen-1] = $_
- } ## For each input element $n there is a $count/$n chance that it becomes part of the result.
- elseif($rand.NextDouble() -lt ($count/$seen))
- {
- ## For the ones previously selected, there's a 1/$n chance of it being replaced
- $selected[$rand.Next(0,$count)] = $_
- }
- }
- }
- END {
- if (-not $inputObject)
- { ## DO ONCE: (only on the re-invoke, not when using -inputObject)
- Write-Verbose "Selected $count of $seen elements"
- Write-Output $selected
- # foreach($el in $selected) { Write-Output $el }
- }
- }
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