Map, Reduce And Filter In Powershell

Map (Select in LINQ)

PS> $double = { $_ * 2 }     # a scriptblock is like a lambda
PS> 1..5 | % $double         # % is an alias for ForEach-Object
2
4
6
8
10

Filter (Filter in LINQ)

PS> $equals3 =  { $_ -eq 3 }
PS> 1..5 | ? $equals3        # ? is an alias for Where-Object
3

Composing the two

PS> 1..5 | ? $equals3 | % $double
6

Reduce (Aggregate in LINQ)

There’s no reduce (aggregate) built into PowerShell. The closest seems to be to use a function with a begin and end block.

function Reduce($initial, $sb)
{
  begin
  {
    $result = $initial
  }

  process
  {
    $result = & $sb $result $_
  }

  end
  {
    $result
  }
}

$sb = { param($x, $y) $x + $y }

1..5 | Reduce 0 $sb

Update 9/9/2014: Although, conveniently, the ForEach-Object takes a Begin and End block.

PS> 1..5 | % { $result = 0 } { $result += $_ } { $result } # % is an alias for ForEach-Object
15