From: David Alan Black <dblack@...>
Date: 2001-02-20T09:43:00+09:00
Subject: [ruby-talk:11140] Re: Summary: RCR #U002 - proper new name fo r indexes

On Tue, 20 Feb 2001, Dave Thomas wrote:

> David Alan Black <dblack@candle.superlink.net> writes:
> 
> > In spite of being an advocate of having two different names, I do
> > agree that conceptually an array is essentially a hash on positive
> > integers.  (See for example [ruby-talk:6611] and [ruby-talk:6663].)
> > In fact, looking at arrays and hashes that way makes me wonder about
> > what *exactly* is common to them, and what implications that has.  I
> > keep thinking of a phantom module called "Hashable", and wishing it
> > existed....  
> 
> I'm not sure I agree that they are essentially the same. For example:
> 
>   a = []
>   a[0] = 0
>   a[10] = 10
>   p a     #=> [0, nil, nil, nil, nil, nil, nil, nil, nil, nil, 10]
> 
>   b = {}
>   b[0] = 0
>   b[10] = 10
>   p b     #=> {0=>0, 10=>10}
> 
> An array is a vector, while a hash is an indexed collection.  Arrays
> have key-ordering, while hashes are unordered.

There's no question that they behave differently, and offer different
functionality.  But one could also look at ordered keys as a special
case of non-ordered keys -- namely, a case where the keys
automatically get sorted, in exchange for the constraint of their
having to be integers.  An array could be understood as a
special-purpose hash, with extra intelligence in one area and
restricted flexibility in others.

> However, I _would_ agree that both arrays and hashes might be
> derivable from some underlying collection class.

In my non-existent, phantom scheme, being hashable (i.e., mixing in
Hashable) would mean maintaining a collection of key/value pairs
(tuples?).  Every class that mixed in Hashable (e.g., Hash and Array)
would implement constraints on its possible keys.  (I don't know
whether Hashable is a practical or useful idea -- I've gravitated
toward it mainly as a way of mentally refactoring and picturing the
common hashability of arrays and hashes.)


David

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