app.rb

class Grade
  include Comparable
  attr_reader :grade

  def initialize(string)
    @grade = string.downcase
  end

  def ==(other_grade)
    @grade == other_grade.grade
  end

  def <=>(other_grade)
    letter = other_grade.grade.sub(/[-+]/, '')
    grade  = @grade.sub(/[-+]/, '')
    if grade == letter
      return 1 if other_grade.grade.include?("+")
      return -1 if other_grade.grade.include?('-')
      0
    elsif grade < letter
      -1
    elsif grade > letter
      1
    end
  end
end