Default Values for Ruby Hashes

Setting a default for ruby hashes which returns a new object.

I can't tell you how many times I've looked this up, so I'm scribbling it down here. Here we want a hash value which doesn't yet exist to be initialised to an empty array.

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

Now we can do:

langs[:billy] << 'ruby' << 'elixir'

Without having to set langs[:billy] to an empty array first.