array as value in hash in ruby

Question

I want to have a hash whose key is a string and the value is an array. I tried it the following way:

h = Hash.new([]) # => {} 
h["one"]         # => [] 
h["two"]         # => [] 
h["one"].push 1  # => [1] 
h["one"]         # => [1] 
h["two"]         # => [1] //why did it assign it to h["two"] also??

What is the correct way to do it?

Answer 1

You get this behavior because [] that you passed into new method isn't copied but referenced in all unset hash keys. So h['one'] references the same object as h['two'] . So if you modify object referenced by h['one'] (using push method), h['two'] will also be modified.

The correct way to set default value which will be initialized for every single hash key is to use block:

h = Hash.new { |hash, key| hash[key] = [] }

Answer 2

I usually do it like this:

h = Hash.new { |h,k| h[k] = [] }
h['one']
h
# => { 'one' => [] }
h['two'] << 12
h
# => { 'one' => [], 'two' => [12] }

Which is more verbose and (IMO) reads nicer.