calculator.rb

# def compute(a, b, operator)
#   case operator
#   when "+" then a + b
#   when "-" then a - b
#   when "*" then a * b
#   when "/" then a / b
#   when "%" then a % b
#   else "Not implemented"
#   end
# end

def compute(a, b, operator)
  return "Not implemented" unless POSSIBLE_OPERATORS.include? operator
  a.send(operator, b)
end

interface.rb

require_relative "calculator"

def ask_user_for_number
  print "number> "
  gets.chomp.to_i
end

POSSIBLE_OPERATORS = ["+", "-", "/", "*", "%"]

user_answer = "Y"

until user_answer.downcase.start_with? "n"
  first_number = ask_user_for_number
  second_number = ask_user_for_number

  print "operator #{POSSIBLE_OPERATORS.join(', ')}> "
  operator = gets.chomp

  print "#{first_number} #{operator} #{second_number} = "
  puts compute(first_number, second_number, operator)

  puts "Do another calculation?"
  print "Y/N> "
  user_answer = gets.chomp
end