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

When you write,

h = Hash.new([])

you pass default reference of array to all elements in hash. because of that all elements in hash refers same array.

if you want each element in hash refer to separate array, you should use

h = Hash.new{[]} 

for more detail of how it works in ruby please go through this: http://ruby-doc.org/core-2.2.0/Array.html#method-c-new