http://en.wikipedia.org/wiki/Reverse_Polish_notation

rpn.rb

# http://en.wikipedia.org/wiki/Reverse_Polish_notation

# {method => arity} lookup hash of Float intance methods
METHOD_ARITY = Float.instance_methods(false).inject({}) {|ma, (m, _)|
  ma[m] = 0.0.method(m).arity; ma}

def rpn str
  str.split(/\s+/).inject([]) do |s, t|
    m = t.to_sym
    s << case METHOD_ARITY[m]
         when 0, -1 # no or optional parameter
           x = s.pop
           x.send(m)
         when 1     # infix operations like +, *
           y, x = s.pop, s.pop
           x.send(m, y)
         else       # it's a float number
           t.to_f
         end
  end.pop           # last member is the calculation
end

rpns = [
  '10 4 3 + 2 * -',  # -4.0
  '5 1 2 + 4 * + 3', # 3.0
  '43.2425 0.5 **',  # 6.575902979819578
  '-3 5 / abs'       # 0.6
]

ARGV.empty? ? rpns.each {|x| puts rpn(x)} : ARGV.each {|x| puts rpn(x)}