From: "Brett H. Williams" <brett_williams@...>
Date: 2003-07-31T07:27:39+09:00
Subject: Re: collect_by

On Jul 31, dblack@superlink.net wrote:
> Hi --
> 
> On Thu, 31 Jul 2003, Simon Strandgaard wrote:
> 
> > Great idea.
> >
> > I were just experimenting, when I discovered a glitch in
> > my understanding of Ruby..
> >
> > What is the difference between 'Enumerable1' and 'Enumerable'.
> > Why does the 'Enumerable' work and the other not ???
> >
> > Please enligthen me :-)
> >
> >
> > module Enumerable1
> >   def collect_by
> >     h = Hash.new []
> >     each{|i| h[yield(i)].push i}
> >     h
> >   end
> > end
> > [...]
> > # Enumerable1
> > #=> {}
> 
> What you're seeing is a result of the fact that fetching a default
> value for a hash does not actually set the key for the hash --
> because, by definition, the default value is what you get when you use
> a non-existent key, and it doesn't imply assignment.  For example:
> 
> irb(main):001:0> h = Hash.new("hi")
> {}
> irb(main):002:0> h[1]
> "hi"
> irb(main):003:0> h
> {}
> 
> (Another issue is that the way you've written it, h[...] will always
> return the *same* array.  But that's another matter :-)  You can
> circumvent that, in 1.8, with:  h = Hash.new { [] } )

Yes, my very favorite 1.8 line:

h = Hash.new { |hash,key| hash[key] = [] }