hashmap - Setting ruby hash .default to a list

Question

(This question already has answers here): question from:https://stackoverflow.com/questions/190740/setting-ruby-hash-default-to-a-list

Answer 1

This is a very useful idiom:

(myhash[key] ||= []) << value

It can even be nested:

((myhash[key1] ||= {})[key2] ||= []) << value

The other way is to do:

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

But this has the significant side-effect that asking about a key will create it, which renders has_key? fairly useless, so I avoid this method.