Question

dup is shallow copy, so when doing this:

h = {one: {a:'a', b: 'b'}}
h_copy = h.dup
h_copy[:one][:b] = 'new b'

now h and h_copy is same: {:one=>{:a=>"a", :b=>"new b"}}
yes, that right.

But when h is a one dimension hash:

h = {a:'a', b: 'b'}
h_copy = h.dup
h_copy[:b] = 'new b'
h still is: {a:'a', b: 'b'}
h_copy is {a:'a', b: 'new b'}

Why?

Answer 1

So after 1 hour of brainstorming..I have come to the conclusion that in the multi dimensional hashes, the dup generates the same object_id for each key which is in turn referring to the hash whereas in the single dimensional hash, the object_ids are similar initially but when we make any changes to the object the Ruby would assign new object_id to the hash keys..

Look at the following code

h = { :a => "a", :b => "b" } # => {:a=>"a", :b=>"b"} 
h_clone = h.dup #=> {:a=>"a", :b=>"b"} 
h.object_id #=> 73436330 
h_clone.object_id #=> 73295920 
h[:a].object_id #=> 73436400 
h_clone[:a].object_id #=> 73436400 
h[:b].object_id #=> 73436380 
h_clone[:b].object_id #=> 73436380 
h_clone[:b] = "New B" #=> "New B" 
h_clone[:b].object_id #=> 74385280 
h.object_id #=> 73436330 
h_clone.object_id #=> 73295920 
h[:a].object_id #=> 73436400 
h_clone[:a].object_id #=> 73436400

Look the following code for the multidimensional array

h = { :one => { :a => "a", :b => "b" } } #=> {:one=>{:a=>"a", :b=>"b"}} 
h_copy = h.dup #=> {:one=>{:a=>"a", :b=>"b"}} 
h_copy.object_id #=> 80410620 
h.object_id #=> 80552610 
h[:one].object_id #=> 80552620 
h_copy[:one].object_id #=> 80552620 
h[:one][:a].object_id #=> 80552740 
h_copy[:one][:a].object_id #=> 80552740 
h[:one][:b].object_id #=> 80552700 
h_copy[:one][:b].object_id #=> 80552700 
h_copy[:one][:b] = "New B" #=> "New B" 
h_copy #=> {:one=>{:a=>"a", :b=>"New B"}} 
h #=> {:one=>{:a=>"a", :b=>"New B"}} 
h.object_id #=> 80552610 
h_copy.object_id #=> 80410620 
h[:one].object_id #=> 80552620 
h_copy[:one].object_id #=> 80552620 
h[:one][:b].object_id #=> 81558770 
h_copy[:one][:b].object_id #=> 81558770