Question

hash = Hash.new(Hash.new([]))
hash[1][2] << 3

hash[1][2] # => [3]
hash # => {}
hash.keys # => []
hash.values # => []

What’s going on? Ruby’s hiding data (1.9.3p125)

Answer 1

What’s going on? Ruby’s hiding data (1.9.3p125)

Ruby hides neither data nor its docs.

Default value you pass into the Hash constructor is returned whenever the key is not found in the hash. But this default value is never actually stored into the hash on its own.

To get what you want you should use Hash constructor with block and store default value into the hash yourself (on both levels of your nested hash):

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

hash[1][2] << 3

p hash[1][2]  #=> [3]
p hash        #=> {1=>{2=>[3]}}
p hash.keys   #=> [1]
p hash.values #=> [{2=>[3]}]