From: Tim Hunter <cyclists@...>
Date: 2006-03-12T09:18:44+09:00
Subject: Re: Strange behaviour

Javier Valencia wrote:
> Look this:
> 
> tigre@enigma tigre $ irb
> irb(main):001:0> events = {}
> => {}
> irb(main):002:0> events.default = []
> => []
> irb(main):003:0> events["trying"] << "hello"
> => ["hello"]
> irb(main):004:0> events
> => {}
> irb(main):005:0> events.inspect
> => "{}"
> irb(main):006:0> events["trying"]
> => ["hello"]
> irb(main):007:0> quit
> tigre@enigma tigre $
> 
> 
> why is happening this behaviour?
> 
> 

The "events.default = []" statement establishes a default object to be 
returned when you access a key that is not in the hash. The same object 
is always returned. That is, the same array will be returned every time 
you access a key that is not in the hash. Further, adding an element to 
that array by fetching a non-existent key does not add the key to the 
hash. The 'events["trying"] << "hello"' statement sends [] to the hash, 
which returns the default array, then adds "hello" to the array. To add 
a key to the hash you have to use its []= method*.

Look at this sequence for example:

irb(main):001:0> ary = []
=> []
irb(main):002:0> hsh = {}
=> {}
irb(main):003:0> hsh.default = ary
=> []
irb(main):004:0> hsh[:a] << 1
=> [1]
irb(main):005:0> hsh
=> {}
irb(main):006:0> ary
=> [1]
irb(main):007:0> hsh[:b] = 1
=> 1
irb(main):008:0> hsh
=> {:b=>1}




*Okay, for the picky, there's other ways to add a key to a hash. Work 
with me here.