From: 7stud -- <lists@...>
Date: 2012-12-29T11:09:01+09:00
Subject: Re: Ruby Koans regarding Hashes.

> Derrick B. wrote in post #1090528:

> I had that mostly correct in my head, but two references to the same
> array???  After creating the new Hash object, the hash variable is just
> the Hash reference, but the indexed Hash reference ( like hash[:one] )
> points to the array reference, right?
>
>>> hash = Hash.new { |hash, key| hash[key] = [] }
> => {}
>>> hash
> => {}
>>> hash[:one]
> => []
>>> hash.object_id
> => 269913890
>>> hash[:one].object_id
> => 269881860
>
> Two different id's.  Or did I do that wrong?  Two references to the same
> array in that assignment expression above doesn't sound right.  Your
> other example:

1) The block does not create the hash.   If you look at the block:

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

...you can see that the block takes a hash as an argument. The hash gets
created elsewhere.

2) You cannot look at a block and know what a method returns.  A block
is like an argument to a method.  In essence, you are passing a function
to the method.  Here is an example:

def do_stuff(x, y, &block)  #&block is the syntax to capture the block
                                                #in a variable
  if block_given?
    block_return_val = block.call
    puts "Block returns: %s" % block_return_val
  end

  'hello'   #This is the return val of do_stuff
end


do_stuff_return_val = do_stuff(10, 20) {'goodbye'}
puts "do_stuff returns: %s" % do_stuff_return_val


--output:--
Block returns: goodbye
do_stuff returns: hello


However, it is pretty obvious if you are a programmer that a method
named new is going to return a new object.  So when you write:

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

... hash is going to be a reference to a new Hash.  Therefore, you can
deduce that the new() method of the Hash class returns a reference to a
new Hash.  Note that the block returns whatever a method named []= in
the Hash class returns.

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