Does Powershell have an Aggregate/Reduce function?

Question

I realize there are related questions, but all the answers seem to be work-arounds that avoid the heart of the matter. Does powershell have an operation that can use a scriptblock to aggregate elements of an array into a single value? This is what is known in other languages as aggregate or reduce or fold.

I can write it myself pretty easily, but given that its the base operation of any list processing, I would assume there's something built in I just don't know about.

So what I'm looking for is something like this

1..10 | Aggregate-Array {param($memo, $x); $memo * $x}

@Andie2302 ... no. I'm not sure how + fits my criteria? An aggregate function is - like I said - key to all list processing, it can actually be used very easily to construct all the other functions such as map, filter, groupby, foreach, etc — George Mauer
– George Mauer, Commented Aug 6, 2014 at 15:15
I would love to have the time to do a nice pretty answer, but I don't. See if this article heads you down a useful path: powershellmagazine.com/2013/12/23/… — EBGreen
– EBGreen, Commented Aug 6, 2014 at 16:07
Why not use Measure-Object? stackoverflow.com/a/19170783/58553 — Peter
– Peter, Commented Nov 10, 2016 at 12:42
@Peter because that only provides a few of the different use cases that reduce is used for, whereas reduce itself can be used for just about any array manipulation — George Mauer
– George Mauer, Commented Nov 10, 2016 at 18:28

Keith Hill · Accepted Answer · 2014-08-06 18:25:38Z

63

There is not anything so obviously named as Reduce-Object but you can achieve your goal with Foreach-Object:

1..10 | Foreach {$total=1} {$total *= $_} {$total}

BTW there also isn't a Join-Object to merge two sequences of data based on some matching property.

edited Aug 6, 2014 at 18:25

answered Aug 6, 2014 at 16:02

Keith Hill

204k44 gold badges370 silver badges377 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Matthew MacFarland Over a year ago

This is very clever! It fixed up some code I'm writing very nicely.

Peter · Accepted Answer · 2016-11-14 13:51:35Z

13

If you need Maximum, Minimum, Sum or Average you can use Measure-Object sadly it dosen´t handle any other aggregate method.

Get-ChildItem | Measure-Object -Property Length -Minimum -Maximum -Average

edited Nov 14, 2016 at 13:51

answered Nov 13, 2016 at 21:27

Peter

38.9k39 gold badges151 silver badges213 bronze badges

Comments

manojlds · Accepted Answer · 2014-08-06 19:44:45Z

7

This is something I wanted to start for a while. Seeing this question, just wrote a pslinq (https://github.com/manojlds/pslinq) utility. The first and only cmdlet as of now is Aggregate-List, which can be used like below:

1..10 | Aggregate-List { $acc * $input } -seed 1
#3628800

Sum:

1..10 | Aggregate-List { $acc + $input }
#55

String reverse:

"abcdefg" -split '' | Aggregate-List { $input + $acc }
#gfedcba

PS: This is more an experiment

edited Aug 6, 2014 at 19:44

answered Aug 6, 2014 at 19:36

manojlds

303k66 gold badges482 silver badges426 bronze badges

2 Comments

George Mauer Over a year ago

neato. Does it really have to be c# though? I'm pretty sure you can inject variables into a scope from PS (whatever psake does for example). I like c#, but seems a shame to mix paradigms for something like this.

manojlds Over a year ago

@GeorgeMauer - True. I wanted to do this in C# however, nothing else :)

RiverHeart · Accepted Answer · 2018-06-02 15:59:19Z

Ran into a similar issue recently. Here's a pure Powershell solution. Doesn't handle arrays within arrays and strings like the Javascript version does but maybe a good starting point.

function Reduce-Object {
    [CmdletBinding()]
    [Alias("reduce")]
    [OutputType([Int])]
    param(
        # Meant to be passed in through pipeline.
        [Parameter(Mandatory=$True,
                    ValueFromPipeline=$True,
                    ValueFromPipelineByPropertyName=$True)]
        [Array] $InputObject,

        # Position=0 because we assume pipeline usage by default.
        [Parameter(Mandatory=$True,
                    Position=0)]
        [ScriptBlock] $ScriptBlock,

        [Parameter(Mandatory=$False,
                    Position=1)]
        [Int] $InitialValue
    ) 

    begin {
        if ($InitialValue) { $Accumulator = $InitialValue }
    }

    process {
        foreach($Value in $InputObject) {
            if ($Accumulator) {
                # Execute script block given as param with values.
                $Accumulator = $ScriptBlock.InvokeReturnAsIs($Accumulator,  $Value)
            } else {
                # Contigency for no initial value given.
                $Accumulator = $Value
            }
        }
    }

    end {
        return $Accumulator
    }
}

1..10 | reduce {param($a, $b) $a + $b}
# Or
reduce -inputobject @(1,2,3,4) {param($a, $b) $a + $b} -InitialValue 2

js2010 · Accepted Answer · 2020-07-09 16:20:38Z

1

There's a functional module I came upon https://github.com/chriskuech/functional that has a reduce-object, also merge-object, test-equality, etc. It's surprising that powershell has map (foreach-object) and filter (where-object) but not reduce. https://medium.com/swlh/functional-programming-in-powershell-876edde1aadb Even javascript has reduce for arrays. A reducer actually is very powerful. You can define map and filter in terms of it.

1..10 | reduce-object { $a * $b }

3628800

Measure-object can't sum [timespan]'s:

1..10 | % { [timespan]"$_" } | Reduce-Object { $a + $b } | ft

Days Hours Minutes Seconds Milliseconds
---- ----- ------- ------- ------------
55   0     0       0       0

Merge two objects:

@{a=1},@{b=2} | % { [pscustomobject]$_ } | Merge-Object -Strategy fail

b a
- -
2 1

edited Jul 9, 2020 at 16:20

answered Jul 8, 2020 at 15:13

js2010

28.3k6 gold badges82 silver badges88 bronze badges

1 Comment

RiverHeart Over a year ago

Just eyeballing it, Chris' Powershell implementation of Reduce looks better than my own. The scriptblock validation and use of closures is interesting.

Collectives™ on Stack Overflow

Does Powershell have an Aggregate/Reduce function?

5 Answers 5

1 Comment

Comments

2 Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

1 Comment

Comments

2 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related