It shows how to enhance ruby class by define a new method to String class. This is just for study.

solution_1.rb

class String
  def sex_code
    if ["male", "男"].include? self
      0
    elsif self == "女"
      1
    else
      self
    end
  end  
end

p String.instance_methods.grep(/^sex/) # => [:sex_code]

p "male".sex_code  # => 0
p "女".sex_code  # => 1
p "other".sex_code # => "other"

solution_2.rb

String.class_eval do
  def sex_code
    if ["male", "男"].include? self
      0
    elsif self == "女"
      1
    else
      self
    end
  end  
end

p String.instance_methods.grep(/^sex/) # => [:sex_code]

solution_mistake.rb

def String.hello
  "hello from String class methods"
end

p String.singleton_methods.grep(/hello/)
p String.hello  # => "hello from String class methods"