From: Ned Konz <ned@...>
Date: 2002-06-07T00:13:33+09:00
Subject: Re: Array.foldr (Array.reduce)

On Thursday 06 June 2002 07:55 am, Kontra, Gergely wrote:
> folr e + array = array1 + (array2 + ... + (arrayn + e)...)
>
>
> And foldl is the other way (if + is associative, then it gives the
> same result)
>
>
> If inject will be available, will foldl and foldr (=reduce) also?
> Thanks in advance.

If I understand you, you could just do this:

# used like:
#   foldr(e, array) { |partial,b| partial+b }

def foldr(e, array)
  result = e
  array.reverse_each { |elem| result = yield(result, elem) }
  result 
end

# Same thing, but left associative
def foldl(e, array)
  result = e
  array.each { |elem| result = yield(result, elem) }
  result 
end

# and you should provide it for Enumerables too:
module Enumerable
  def foldr(e)
    result = e
    reverse_each { |elem| result = yield(result, elem) }
    result 
  end

  # Same thing, but left associative
  def foldl(e)
    result = e
    each { |elem| result = yield(result, elem) }
    result 
  end
end

arr = [1,2,3,4,5]
p foldr(6, arr) { |a,b| a + b } => 21
p arr.foldr(6) { |a,b| a + b } => 21

-- 
Ned Konz
http://bike-nomad.com
GPG key ID: BEEA7EFE