From: dblack@...
Date: 2003-07-31T07:17:44+09:00
Subject: Re: collect_by

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 { [] } )


David

-- 
David Alan Black
home: dblack@superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav