zero_init.rb(hash)

This hash is initialized with a default value of 0 for any key that doesn’t exist. The next line increments the value associated with the key ‘key’ by 1. If the key ‘key’ doesn’t exist in the hash, it is created with an initial value of 1. So, this code is effectively counting the occurrences of the ‘key’ in the hash.

Execution:

hash = Hash.new(0)
#=> {}
hash['key'] += 1
#=> 1

Executed with Ruby 3.3.6