calculator.rb

def process_calculation(first_number, second_number, operator)
  first_number.send(operator, second_number)
end

interface.rb

require_relative "calculator"

operators = ["+", "-", "*", "/", "%", "**"]
user_input = "Y"

while user_input == "Y"
  puts "First number?"
  first_number = gets.to_i

  puts "Choose operator: #{operators.join(', ')}"
  operator = gets.chomp

  puts "Second number?"
  second_number = gets.to_i

  if operators.include? operator
    result = process_calculation(first_number, second_number, operator)
    puts "Result: #{result}"
  else
    puts "ERROR: You can use only #{operators.join(', ')}"
  end

  puts "Do you want to continue? (Y/N)"
  user_input = gets.upcase[0]
end

puts "Goodbye"