From: Brian Candler <B.Candler@...>
Date: 2003-07-31T08:35:22+09:00
Subject: Re: collect_by

On Thu, Jul 31, 2003 at 08:14:59AM +0900, Simon Strandgaard wrote:
> On Thu, 31 Jul 2003 08:17:44 +0900, dblac wrote:
> > On Thu, 31 Jul 2003, Simon Strandgaard wrote:
> >> Please enligthen me :-)
> [snip]
> > (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 { [] } )
> 
> Oh.. I luckily forgot about this.  Still it won't work.. 
> What am I doing wrong ?
> 
> 
> module Enumerable
>   def collect_by
>     h = Hash.new { Array.new }     # braces :-)
>     each{|i| h[yield(i)].push i}
>     h
>   end
> end
> 
> a = (1..10).to_a
> p a.collect_by {|i| i % 2} 
> #=> {}

1. h[foo] returns a new empty array
2. you push something onto that array
3. no reference to that array is stored anywhere
4. the array disappears into the ether, to get garbage-collected at
   a later date

The important thing is, step (1) does not assign anything to the hash.
h remains as {} throughout.

Regards,

Brian.