From: stephen O'D <stephen.odonnell@...>
Date: 2007-07-20T06:50:00+09:00
Subject: Hash of arrays - whats going on?

I want a hash, where the values of the keys are arrays, eg

h = Hash.new
h.default = []

h['foo'] << 10
h[foo'] << 20
h['bar'] << 23
h['bar'] << 33

I thought this would give me

{ 'foo' => [20 30],
  'bar' => [23, 33] }

but it doesn't - it seems to put the same array in the value of each
key.  Some googling revealed I need to create my hash like:

h = Hash.new { |hash, key|  hash[key] = [] }

So my problem is solved, but why do you have to do it like this?  At
the risk of answering my own question, is it because the block is re-
executed everytime you access a non existent key, creating a brand new
array object, while the first way, it just initialises the value to
the same array each time?

Thanks,

Stephen.