From: 7stud -- <lists@...>
Date: 2012-12-28T17:15:16+09:00
Subject: Re: Ruby Koans regarding Hashes.

Derrick B. wrote in post #1090445:
>
> hash = Hash.new([])
> => {}
> hash[:foo]
> => []
> hash[:foo] << "hello"
> => ["hello"]
> hash[:foo]
> => ["hello"]
> hash.inspect
> => {}
>
> The last line "hash.inspect" is what I also find interesting, because if
> there are default values, why is it still empty?  I then assign a value,
> overriding the default value:
>

1) You never write

Hash.new([])

...because the same array is the default for all keys, and that is never 
useful.

2) When you write:

hash[:non_existent_key]

... you get a reference to the single default array.  Unless you 
*assign* that value to the key, then that key will still have no value. 
So you need to write:

hash[:non_existent_key] <<= "hello"


> hash[:foo] = "bar"
> => "bar"
> hash.inspect
> => "{:foo=>\"bar\"}"
>
> Now, inspect no longer shows an empty hash.  Hmmm...
>

Yes. In this example you assigned something to the key.

> Ruby hashes are an interesting topic and I look forward to your veteran
> insight on this topic.
>

This is what you want:

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

hash[:one] << 'hello'
hash[:two] << 'goodbye'

p hash

--output:--
{:one=>["hello"], :two=>["goodbye"]}

In this case, the block executes when you access a non-existent key, and 
the block creates a new array and assigns it to the key *and* returns a 
reference to the array that is assigned to the key.  As a result, all 
you need to do is append to the array.

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