逆ポーランド記法の練習

rpn.rb

require 'set'

class RPN
  def initialize(exp)
    @exp = exp.split(/\s+/)
    @stack = []
    @operators = Set['+', '-', '*', '/', '%'].freeze
  end

  def eval
    while v = @exp.shift
      if @operators.include? v
        n2, n1 = @stack.pop, @stack.pop
        @stack << n1.send(v.to_sym, n2)
      else
        @stack << v.to_i
      end
    end
    @stack.last
  end
end

DATA.read.lines.map(&:chomp).each { |exp|
  puts "(#{exp}) = #{RPN.new(exp).eval}"
}

__END__
3 4 +
3 4 + 1 2 - *
1 5 + 2 3 + *