postfix.rb

class Parser
  def initialize
    @ops = ['+', '-', '/', '*']
    @position = 0
    @terms = []
  end

  def parse(expr)
    @terms = expr.split(' ')
    if @terms.length > 2 then
      @position = 0
      scan_for_op 
      op = @terms[@position]
      apply_op(op)
    else
      @terms.first.to_f
    end
  end
  
  def apply_op(op)
    result = @terms[@position-2].to_f.send(op.to_sym, @terms[@position-1].to_f)
    @terms.delete_at(@position-1)
    @terms.delete_at(@position-2)
    @position -= 2
    @terms[@position] = result.to_s
    parse(@terms * " ")
  end

  def scan_for_op
    while (not @ops.member? @terms[@position]) 
      @position += 1
    end
  end
end