From: Trans <transfire@...>
Date: 2007-03-14T07:40:10+09:00
Subject: Re: non-destructive merging of hashes in array

On Mar 13, 8:40 am, "Robert Dober" <robert.do...@gmail.com> wrote:
> On 3/13/07, Trans <transf...@gmail.com> wrote:
>
>
>
> > On Mar 12, 10:01 pm, "Giles Bowkett" <gil...@gmail.com> wrote:
> > > Hi, I have an array of hashes. The keys in the hashes represent the same things.
>
> > > eg:
>
> > > h1 = {:rabbits => 5}
> > > h2 = {:rabbits => 10}
>
> > > bunnies = [h1, h2]
>
> > > I want to end up with this:
>
> > > {:rabbits => 15}
>
> > > It's a trivial task, but what's the quickest way to get there? I'm
> > > certain it can be done on one line.
>
> > > I know there's a Hash#update, but it appears that it would just
> > > replace the 10 with 5, or vice versa, rather than adding them up.
>
> > > This works, but it seems clunky:
>
> > > hashes.each do |h|
> > >   h.each do |k, v|
> > >     if new_hash[k]
> > >       new_hash[k] += v
> > >     else
> > >       new_hash[k] = v
> > >     end
> > >   end
> > > end
>
> > > It can be crammed all onto one line, too, but there must be a nicer way.
>
> > (SORRY IF THIS GETS POSTED TWICE)
>
> > The inject/merge solutions are good, but they are one trick ponies.
> > How about something like:
>
> >    OpenCollection[h1, h2].rabbits.sum
>
> > It shouldn't be too hard to write:
>
> >    require 'ostruct'
>
> >    class OpenCollection
> >      class << self ; alias :[] :new ; end
> >      def initialize(*hashes)
> >        @opens = hashes.collect { |h| OpenStruct.new(h) }
> >      end
>
> >      def method_missing(sym, *args)
> >        @opens.collect{ |o| o.send(sym) }
> >      end
> >    end
>
> > Actually, I'd use Facets OpenObject instead OpenStruct myself, but
> > that's just me. You'll also need:
>
> >   require 'facets/core/enumerable/sum'
>
> > For fun, here's a one line version (more or less):
>
> >    require 'facets/more/functor'
>
> >    oc = Functor.new([h1,h2]){|s,m| m.map{|h| h[s]}}
>
> >    oc.rabbits.sum
>
> > T.
>
> Tom Facet is a great thing and I do not fail to point to it regulary.
> But sometimes I feel we have to flex our muscles in pure Ruby before
> we shall use libraries, even excellent ones like Facets, just to
> understand everything a little better.
>
> This all does not mean that your post is not very valuable, I just
> want to warn from the "Pull In A Library before Do Some Thinking"
> approach.
>
> i fear that this approach hurts the user as much as the library.

I understand what youre saying --and I waited on posting this until
others gave solutions. Though in this particular case I think there's
some pretty good meat here, ie. the OpenCollection class I literally
just made up on the spot. Of course that still leaves Enumerable#sum,
but that's rather straight forward: a.inject(0){|s,n| s+=n;s}.

The Functor was just a little playful plug. If you've ever seen the
functor code you know it's a generalization of what the OpenCollection
class is doing. I actually would like to see Functor included in
Ruby's standard lib. But I haven't been able to convince Matz of it's
usefulness. So I try to publicly use it when ever I get the chance.

HTH,
T.