ruby - Improving calculate function where input string does not follow normal computational order -

i working through sample interview questions , came across problem have write calculate function. caveat input string isn't written in computational order - instead numbers first , operators come after.

test cases:

calculate("2 3 +") # => 5

calculate("12 2 /") #=> 6

calculate("48 4 6 * /") # => 2

def calculate(string)   numbers = string.scan(/\d+/)   operators = string.scan(/\d\w/).map{|o|o.gsub(" ","")} #todo better regex remove space without having map    formatted_string = numbers.inject(""){|string, b| string+"#{b}#{operators.shift}" }    eval(formatted_string) end 

i able come solution, wondering if there more efficient/better way solve problem. don't come programming background not familiar tools/algorithms may help.

any feedback appreciated.

ooh, fun! syntax called reverse polish notation (rpn), aka "postfix notation" , still used powerful calculators (the hp-48g made me fall in love programming!).

the typical way parse , calculate rpn split input string whitespace-separated tokens, , each token, push onto stack if it's integer, otherwise perform arithmetic operation designated token using last 2 items on stack operands , push result on.

here solution came quickly:

def calculate(str)   stack = []   tokens = str.split(/\s+/)           # split tokens whitespace.   tokens.each |token|     if token =~ /^\d+$/               # it's number       stack.push(token.to_i)          # ...push on stack.     elsif %w(+ - * /).include? token  # it's operator       a, b = stack.pop(2)             # ...grab last 2 numbers       c = a.send(token, b)            # ...perform operation       stack.push(c)                   # ...and push resulting value.     else       raise "invalid token: #{token}" # punt!     end   end   stack.pop # return last value pushed stack. end  calculate("2 3 +")      # => 5, since      2+3 = 5 calculate("12 2 /")     # => 6, since     12/2 = 6 calculate("48 4 6 * /") # => 2, since 48/(4*6) = 2