Filtering blocks


#1

Is it possible to filter blocks, online, in an elegant way?

for instance if I have the block

class Integer
def divisors
(1…self).each{ |i| yield i if self % i == 0 }
end
end

and I want to pass this block through an ‘online’ filter.

I want to do something elegant like
10.divisors.odd{ |n| puts n }

instead of

10.divisors{ |n| puts n if n.odd? } #, which is pretty much the same as
10.divisors{ |n| yield n if n.odd? }{|n| puts n}

I suppose it’s not possible at all to treat a block as an object in this
manner?

If so, suppose I want to use a generic filter that takes a block and a
condition as its arguments, and returns the block of each elements for
which a condition is fulfilled?

pseudo

def filter (block, condition)
block do |s|
yield s if s.condition
end
end

How would I do anything like this, without using arrays as inbetweens?
Am I again trying to treat blocks wrongly as objects, is it at all
possible to ‘operate’ through them in this way?
I’m new to ruby so this might be a bit stupid. I was just experimenting
and couldn’t find an elegent, modular manner of filtering blocks
run-time in a ‘chain-like’ manner, without needlessly compressing them
to arrays in between.

I got this when googling, btw: www.rubyfilter.com :slight_smile:


#2

Hm, I think the problem is not to filter a block (that’s also possible,
because in ruby blocks are also objects of the class Proc). You want to
filter an Enumeration. Therefore you could use the class Enumerator.

require “enumerator”
class Integer
def odds()
if block_given? then
1.upto(self) do |number| yield number if number % 2 == 1 end
else
Enumerable::Enumerator.new(self, :odds)
end
end
def evens()
if block_given? then
1.upto(self) do |number| yield number if number % 2 == 0 end
else
Enumerable::Enumerator.new(self, :even)
end
end
end

5.odds.each do |number| puts number end

=> 1, 3, 5

5.odds.any? do |number| number % 3 == 0 end

=> true

And of course you can add more “filters” by extending the Enumerator
class, or better, a subclass of Enumerator which handles only with
Integers.

require “enumerator”
class Integer
def odds()
if block_given? then
1.upto(self) do |number| yield number if number % 2 == 1 end
else
Enumerable::Enumerator::Integer.new(self, :odds)
end
end
def evens()
if block_given? then
1.upto(self) do |number| yield number if number % 2 == 0 end
else
Enumerable::Enumerator::Integer.new(self, :even)
end
end
end
module Enumerable
class Enumerator
class Integer < Enumerator

  def prime
    if block_given? then
      each do |number|
        # primitive prime algorithm
        yield(number) unless (2..Math.sqrt(number)).any? do |i|
          number % i == 0
        end
      end
    else
      Enumerable::Enumerator.new(self, :prime)
    end
  end

  def odds
    if block_given? then
      each do |number| yield number if number % 2 == 1 end
    else
      Enumerable::Enumerator.new(self, :odds)
    end
  end

  def evens
    if block_given? then
      each do |number| yield number if number % 2 == 0 end
    else
      Enumerable::Enumerator.new(self, :evens)
    end
  end
end

end
end

12.odds.prime.each {|x| p x}