Dfyjkt

I wanted to pre-populate a Hash, given an array of keys and a default value (an empty array). I attempted to do this using the #product method of Array.

> hash = Hash[[:foo, :bar].product([])] # => :foo=>, :bar=>
> hash[:foo].push(:baz) # => :foo=>[:baz], :bar=>[:baz]

I don't understand why the value is being applied to all keys in the hash. If instead, I use the returned value of product and populate the hash directly from that, I get expected behavior.

> [:foo, :bar].product([]) # => [[:foo, ], [:bar, ]]
> hash = Hash[[[:foo, ], [:bar, ]]] # => :foo=>, :bar=>
> hash[:foo].push(:baz) # => :foo=>[:baz], :bar=>

I am using ruby 2.3.6

It's because the arrays that you pass to your hash initializer are the same object, so if you modify said object, the changes will be present everywhere it is used:

> hash = Hash[[:foo, :bar].product([])]
 # => :foo=>, :bar=> 
> hash[:foo].object_id
 # => 47106586247680 
> hash[:bar].object_id
 # => 47106586247680

If you copy-paste the output of your product, you're using 2 different arrays as they get instantiated separately.