Strange, unexpected behavior (disappearing/changing values) when using Hash default value, e.g. 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}

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.