From: "Derrick B." <lists@...>
Date: 2012-12-28T14:27:43+09:00
Subject: Ruby Koans regarding Hashes.

I am trying to understand this, so let me know how I do.  :)  I know
this is probably a breeze for you veterans, but I need to put this in
words.  heh

I have a C++/Perl background, so I do understand "associative arrays",
but understanding Ruby hash objects, and the methods and operations that
can be used with them is pretty new to me.

In the "Ruby Koans" file, "about_hashes.rb", I completed this method.
It was just a little tough to understand, initially, but I think I got
it.  Here's the test method with helpful line numbers:

  def test_default_value_is_the_same_object
1.    hash = Hash.new([])

2.    hash[:one] << "uno"
3.    hash[:two] << "dos"

4.    assert_equal ["uno", "dos"], hash[:one]
5.    assert_equal ["uno", "dos"], hash[:two]
6.    assert_equal ["uno", "dos"], hash[:three]

7.    assert_equal true, hash[:one].object_id == hash[:two].object_id
  end

Here is my breakdown:

1.  a new hash is created, utilizing an empty array as the default value
for any key created without a value, and assigned to "hash".  This
wasn't too bad, I just had to look up: Hash.new(<place a default key
value here>)

2.  using indexing, the string, "uno", is appended to the default key
value for the key, :one, which was initially an empty array and is now
the value ["uno"].  Since the "<<" was used instead of the assignment
operator "=", it is an append, rather than an assign.  For example, if
the statement was "hash[:one] = "uno", then that replaces the default
array and the value is now a String

3. same as 2., but what I found interesting was how a new key was used,
and the append still modified the general default value for the entire
hash!  So, now the key value default is now: ["uno", "dos"]

4 - 6  test cases to prove that a default key value was created, with
lines 2 and 3 appending to it, and because these were not assignment
expressions, the keys tested (:one, :two, :three) are using the modified
default value.

7.  Final proof that is the same default value, or same Array object_id,
being used for the two utilized keys.

So, when you create a Hash object, you can provide a default value.  In
this case, the default value is an Array object.  Because of this being
an Array object, the "<<" method can be used to modify the default
value.  Using indexing on the Hash, this can be tested by querying the
hash:

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:

hash[:foo] = "bar"
=> "bar"
hash.inspect
=> "{:foo=>\"bar\"}"

Now, inspect no longer shows an empty hash.  Hmmm...

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

Thanks!

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