From: Alex Shulgin <alex.shulgin@...>
Date: 2007-09-17T18:30:05+09:00
Subject: Re: Modifying a hash key

On Sep 17, 3:42 am, Yukihiro Matsumoto <m...@ruby-lang.org> wrote:
> Hi,
>
> In message "Re: Modifying a hash key"
>     on Sun, 16 Sep 2007 17:10:05 +0900, Alex Shulgin <alex.shul...@gmail.com> writes:
>
> |This suggests me that the keys are actually stored in the hash by
> |reference, not by value as I would expect from my prior experience
> |with other languages.
>
> Hash stores its values, as its name suggests, by hash values from
> keys.  So, if you modifies the key, and subsequently the hash value of
> the key, it screws up.  As a general rule, don't modify the keys, or
> if you really need to modify the key for some unknown reason, call
> rehash method on the hash.

So the real answer to my question (what is the reason for hash to
store it's keys by reference?) might be: trading rule of a thumb for
speed and memory?  If no one is ever going to modify the hash key,
there is no reason to copy it...

OK, I think I got it. :-)

If someone wonder how could I come up with modifying the hash keys:
this is my second program in Ruby and was coding a Ruby version of a
speech generator example from the Pike & Kernigan's "Practical
programming".  Here is the code snippet:

state={}
prefix=[]
while $stdin.eof?
  w=...  # read a word
  state[prefix] << w
  ...
  prefix << w   # <-- Bang!  The hash key is changed... :-)
end


Cheers,
Alex