Hash Modification Example

hash_modification.rb

#
# Hashes can be modified by a method they're passed in to, which would affect
# what they contain. This could be an unexpected side-effect.
# $ ruby hash-modification.rb
#   Hash before modification:
#   {:a=>1, :b=>2}
#   Hash after modification
#   {:a=>1, :b=>2, :c=>3, :d=>4}
#
original_hash = { a: 1, b: 2 }

puts "Hash before modification:"
puts original_hash

def hash_modifier(hash)
  hash.merge!(c: 3, d: 4)
end

hash_modifier(original_hash)

puts "Hash after modification"
puts original_hash