erik_calc.rb

class Menu
  attr_reader :first, :last

  def initialize
    @first, @last = 2.times.collect{|i| prompt(i + 1)}
  end

  def prompt(number)
    print "Enter number #{number}:\t"
    gets.chomp.to_i
  end

  def display_result(operation, result)
    puts "Result of #{operation} is:\t#{result}"
  end

  def clear(size)
    puts "-" * (16 + size)
  end
end

class Calculator
  OPERATIONS = ['+', '-', '/', '*']

  def initialize(menu)
    @menu = menu
    calculate
  end

  def calculate
    for operation in OPERATIONS
      result = eval("#{@menu.first} #{operation} #{@menu.last}") rescue "undefined"
      @menu.display_result(operation, result)
    end
    @menu.clear(result.to_s.size.to_i)
  end
end

loop do
  Calculator.new(Menu.new)
end