Strange ruby behavior when using Hash.new([])

Question

Consider this code:

h=Hash.new(0) #new hash pairs will by default have 0 as values
h[1]+=1 # {1=>1}
h[2]+=2 # {2=>2}

that's all fine, but:

h=Hash.new([]) #empty array as default value
h[1]<<=1 #{1=>[1]} - OK
h[2]<<=2 #{1=>[1,2], 2=>[1,2]} # why ?? 

At this point I expect the hash to be:

{1=>[1], 2=>[2]}

But something goes wrong.

Does anybody know what happens?

Answer 1

You're specifying that the default value for the hash is a reference to that particular (initially empty) array.

I think you want:

h = Hash.new { |hash, key| hash[key] = []; }
h[1]<<=1 
h[2]<<=2 

That sets the default value for each key to a new array.

Answer 2

When you call Hash.new([]), the default value for any key is not just an empty array, it's the same empty array.

To create a new array for each default value, use the block form of the constructor:

Hash.new { [] }