From: aero6dof@... (Alan Chen)
Date: 2003-09-03T10:54:09+09:00
Subject: Re: show me the ruby way

Hmm, I meant h[k] not h[h] as I originally posted below...

md = Hash.new { |h,k| h[k] = 0; 3 }
# access undefined key 1
p md[1]   # => 3  # returns the block result
p md[1]   # => 0  # now we discover the actual default value

It's clear to me exactly what is happening, but I guess my thought was 
that this behavior is a bug. If the code that runs the autovivication 
block can recognize that the value for the undefined key was set, then 
perhaps the block return value should be ignored. I guess it would 
require an additional st_lookup after the initial lookup fails.

"Warren Brown" <wkb@airmail.net> wrote in message news:<005101c3719d$c091ca50$670b88cf@warrenpc>...
> alan,
> 
> > I was wondering about this behavior:
> >
> > md = Hash.new { |h,k| h[h] = 0; 3 }
> > p md[1]   # => 3
> > p md[1]   # => 0
> >
> > I understand that the first "p md[1]" is returning
> > the result of the block; is that intentional?
> 
>     Yes.  A clearer example:
> 
> md = Hash.new { 3 }
> md[1] = 1
> p md[1]    => 1
> p md[2]    => 3
> p md       => {1=>1}
> 
>     Remember, the main point of the block is to generate a default value for
> nonexistent keys.  The autovivication trick is merely an extension of this
> functionality.
> 
>     I hope this helps.
> 
>     - Warren Brown