Can't use an array as default values for Ruby Hash? [duplicate]

Question

I was adding items to a Hash key. I was expecting to get a structure like this:

{ 
  'a' : [1],
  'b' : [2, 3, 4]
}

I used an Array to initialize the Hash.

irb> hash = Hash.new([])
 => {} 

Then started using it:

irb> hash['a'] << 1
 => [1] 
irb> hash['b'] << 2
 => [1, 2] 

But it turns out:

irb> hash
 => {}

Answer 1

Try the following instead:

hash = Hash.new{|h, k| h[k] = []}
hash['a'] << 1 # => [1]
hash['b'] << 2 # => [2]

The reason you got your unexpected results is that you specified an empty array as default value, but the same array is used; no copy is done. The right way is to initialize the value with a new empty array, as in my code.