infix to postfix

infix2postfix.rb

require "awesome_print"

exp = "1 + 2 * 3"
exp = "9 + 24 / (7 - 3)"
exp = "3 * atan(2/3)"
exp = "(12.3mm - 1')*52 - 473 mm/(4 + 2^3)"


def prec(op)
   case op
   when "+", "-"
      return 10
   when "*", "/"
      return 20
   when "^"
      return 30
   when "(", ")"
      return 40
   else
      return nil
   end
end

def is_operator(t)
   operators = ["-", "+", "*", "/", "^"]
   operators.include?(t)
end

# Anything that is not an operator, parenthesis, or space is a operand
def tokenize(exp)
   operand = ""
   tokens = []
   exp.each_char do |c|
      if is_operator(c) || c == "(" || c == ")"
         if operand != ""
            tokens << operand
            operand = ""
         end
         tokens << c
      else
         operand << c unless c == " "
      end
   end
   tokens << operand unless operand.empty?
   return tokens
end

def infix2postfix(tokens)
   output = []
   stack = []
   tokens.each do |token|
      if is_operator(token)
         while (!stack.empty? && is_operator(stack[-1]) && prec(stack[-1]) >= prec(token))
            output << stack.pop
         end
         stack << token
         next
      end
      if token == "("
         stack.push token
         next
      end
      if token == ")"
         while stack[-1] && stack[-1] != "("
            output << stack.pop
         end
         stack.pop
         next
      end
      output << token
   end # do loop
   while (!stack.empty?)
      output << stack.pop
   end
   return output
end

def eval_postfix(arr)
   stack = []
   arr.each do |elm|

   end
end

puts exp
tokens = tokenize exp
puts "Tokens:"
ap tokens

postfix = infix2postfix(tokens)
puts 
puts "Postfix:"
ap postfix