rpn.rb

class Stack
  class UnderFlowError < StandardError;end

  def initialize
    @stack = []
  end

  def empty?
    @stack.empty?
  end

  def push(val)
    @stack.push(val)
  end

  def pop
    raise UnderFlowError, "Stack is empty" if empty?
    @stack.pop
  end

  def peek
    @stack.last
  end
end

class RPNExpression
  def initialize(expr)
    @expr = expr
  end

  def evaluate
    stack = Stack.new

    tokens.each do |token|
      case 
      when numeric?(token)
        stack.push(token.to_f) # Casting to a float instead of integer
      when token.match(/^[\+\-\*]$/)  # Just check for an operator
        rhs = stack.pop
        lhs = stack.pop
        stack.push(lhs.send(token, rhs))
      else
        raise "whaaaat I don't know this token? #{token}"
      end
    end

    stack.pop
  end

  private

  def tokens
    @expr.split(" ")
  end

  def numeric?(token)
    Float(token) rescue false
  end
end

puts RPNExpression.new("4 7 3 + 8 * 16.2 + *").evaluate