From: John Carter <john.carter@...>
Date: 2003-12-03T09:53:05+09:00
Subject: Re: Rite, Block locals and Autovivification

Autovivification in Perl is simplified by the fact that in the case
of "$a{$b}" Perl knows from the '{' that a hash is involved.

In Ruby, it is not so easy, but clearly "a[b]" doesn't make sense if
a.kind_of? Array and !b.kind_of?( FixNum), so "Do What I Mean" says you
should autovivify a Hash if appropriate.

Likewise whether in Perl "$a{$b}++" or ruby "a[b]+=1" the element at a[b]
should be autovivified as 0.

In the case of
  a[b]*=c
the only thing that makes sense is if a[b] autovivified as 1

On Wed, 3 Dec 2003, Bill Kelly wrote:

> Hi,
>
> From: "John Carter" <john.carter@tait.co.nz>
> >
> > def histogram( enum)
> >   # <--- Implicit hash = Hash.new(0) here
> >   enum.each{|e| hash[e]+=1} # Hash autovivifies.
>
> eh?  But... somevar[e] ... Why would Ruby assume I wanted a Hash
> object?  That could be an Array, or a String, or.......
>
> Do you mean the autovivification would happen only if the variable
> was actually _named_ 'hash' ?
>
> Or am I not understanding your example?
>
> >   # Since hash autovivified we expect it to be available
> >   # outside the block scope.
> >   hash.keys.sort{|a,b| hash[a]<=>hash[b]}.each do |k|
> >     puts "#{k}\t#{hash[k]}"
> >   end
> > end
>
>
> Regards,
>
> Bill
>
>
>



John Carter                             Phone : (64)(3) 358 6639
Tait Electronics                        Fax   : (64)(3) 359 4632
PO Box 1645 Christchurch                Email : john.carter@tait.co.nz
New Zealand

A Million Monkeys can inflict worse things than just Shakespeare on
your system.