From: William James <w_a_x_man@...>
Date: 2006-08-22T01:30:06+09:00
Subject: Re: A use case for an ordered hash

Phillip Hutchings wrote:
> On 8/14/06, Hal Fulton <hal9000@hypermetrics.com> wrote:
> > Robert Klemme wrote:
> > > Hal Fulton wrote:
> > >>
> > >> I never use a hash for fast lookup. I use it because it
> > >> allows me easily to associate an arbitrary key with a
> > >> value.
> > >
> > > Um...  So you're saying that you wouldn't bother if lookups were O(n)?
> > > Wow!
> >
> > I think you mean "wouldn't care"?
> >
> > Anyway: I'm happy that lookups are fast. But 99% of the time,
> > my hashes are small enough that the time savings is probably
> > very small in terms of program execution time.
>
> Unfortunately for you some of us do things that involve searching
> through 30,000 sales records to pull out Joe Bob's statistics.
> Sometimes you just don't want to do that on the production database,
> it just uses far too many resources. Using an ordered hash for one
> person's benefit would kill the speed and jack up memory usage.
>
> I know an ordered associative array (which I think is a better
> description of the functionality) would make some things very easy,
> but replacing the Hash is not the way to go.
>
> Something like this would probably sit better, and is still shorter
> than the PHP equiv.:
>
> # a = [['key 1','1'],['key 2', '2'],['key 3',3]].toAssoc
>
> # a['key 1']
> => 1
>
> # p a
> => [['key 1','1'],['key 2', '2'],['key 3',3]]

# a = ['key 1','1',  'key 2', '2',  'key 3',3].to_assoc
=> [["key 1", "1"], ["key 2", "2"], ["key 3", 3]]
# a.assoc('key 2')
=> ["key 2", "2"]
# a.set 'key 2', 99
=> ["key 2", 99]
# a
=> [["key 1", "1"], ["key 2", 99], ["key 3", 3]]

class Array
  def to_assoc
    f=nil  ;  partition{f=!f}.transpose
  end
  def set k, v
    (pair = assoc(k))  ?  self[ index(pair) ] = [k,v]  :  push( [k,v] )
  end
  def rm k
    (pair = assoc( k ))  &&  slice!( index( pair ) )
  end
end