calculator.rb

def calculate(first_number, operator, second_number)
  case operator
  when '+' then first_number + second_number
  when '-' then first_number - second_number
  when '*' then first_number * second_number
  when '/' then first_number / second_number.to_f
  else "Invalid operator"
  end
end

interface.rb

require_relative 'calculator'

puts "Welcome to the calculator"

loop_input = 'y'
until loop_input == 'n'
  puts "What's the first number you want to operate on?"
  first_number = gets.chomp.to_i

  puts "Which operator? (+|/|*|-)"
  operator = gets.chomp

  puts "What's the second number you want to operate on?"
  second_number = gets.chomp.to_i

  puts "The result is:"
  result = calculate(first_number, operator, second_number)
  puts result

  puts "Do you want to calculate again? (y/n)"
  loop_input = gets.chomp
end