From: Martin DeMello <martindemello@...>
Date: 2006-07-14T18:00:29+09:00
Subject: Re: Recursion and Ruby

On 7/14/06, Glenn Cadman <glenn.cadman@gmail.com> wrote:
> Daniel Martin wrote:
> >
> > $fib = Hash.new{|h,k| h[k] = h[k-2] + h[k-1]}
> > $fib[0] = 0
> > $fib[1] = 1
>
> Well whilst my orginal exercise was just to get recursion working in
> ruby, I have to say this solution is very quick to calculate any result
> and is recursive in a way that is quite "foriegn" to me. The problem for
> me is that I find it difficult to understand what is happening in this
> code. I would have though the stack would collapse due to referencing
> something that is not defined yet. eg.  h[k-2] for example in the intial
> state of the $fib hash would not be found unless the k was 3 or 2. eg
> $fib(30) would return "I dont have any thing to reference for h[30] =
> h[28] + h[29], has h[28] and h[29] havent been defined.

The block form of Hash.new calls the block for every
nonexistent-hash-key access. So $fib[30] looks for the key 30, doesn't
find it, and calls the block instead, passing it $fib and 30. The
block tries to return h[29] + h[28], but since they don't exist
either, the calls to [] will again call the block, and so on.

martin