hash-wth-default-value.rb

# Consider a hash with a default value:

hash = Hash.new([])
p hash[:anything] # => []

hash[:my_array] << :value
p hash[:my_array] # => [:value]
p hash[:anything] # => [:value] o_O ??!?!?!?!

=begin
Explanation:

We pass an object to `Hash.new()`, that object becomes default value of any key of the hash.
But it is *the same object*.

Example:
=end

object = Object.new
hash = Hash.new(object)
p hash[1].object_id # => 19868300
p hash[2].object_id # => 19868300  (same object, not a duplicate)


# Smart people would immediately point out that Hash.new accepts lambda which is called to obtain a default value:

hash = Hash.new { Object.new }
p hash[1].object_id # => 18889740
p hash[2].object_id # => 18875020


# Let's try:

hash = Hash.new { [] }
hash[:my_array] << :value
p hash[:my_array] # => [] o_O ???!?!!!

=begin
Explanation:

Default value will be returned for any hash key which does not yet has a value.
But we don't set a value for `:my_array`. If we unfold `hash[:my_array] << :value` we will get:

```
array = hash[:my_array]
array << :value
```
no value has been set for hash[:my_array]

What to do then?
=end

hash = Hash.new { |hash, key| hash[key] = [] }
hash[:my_array] << :value
p hash[:my_array] # => [:value]
p hash[:anything_else] # => []