calculate expression

postfix.rb

# '( 2 + ( ( 4 + 6 ) * (9 * 2) - ( 5 - 1)))'

class Postfix
  include Math

  def initialize expression
    @expression = expression.gsub(' ', '')
  end

  def process
    postf = transform
    print postf
    cal postf
  end

private

  def cal(s)
    operand = {nil => 0, '*' => 2, '+' => 2, '-' => 2, '/' => 2, 'sqrt' => 1}
    stack = []
    size = s.size
    for i in 0..size-1 do
      if is_digit(s[i])
        stack.push s[i]
      else
        if operand[s[i]] == 2
          num2, num1 = stack.pop(2)
          a = eval "#{num2} #{s[i]} #{num1}"
        elsif operand[s[i]] == 1
          num = stack.pop
          a = eval "#{s[i]} #{num}"
        end
        stack.push(a.to_i)
      end
    end
    if stack.count == 1
      stack.pop()
    else
      -1
    end
  end

  def is_digit(ele)
    begin
      return true if Integer(ele)
    rescue
      return false
    end
  end

  def transform
    precedence = {nil => 0, '' => 0, '(' => 0, '*' => 2, '+' => 1, '-' => 1, '/' => 2, 'sqrt' => 3}
    output = Array.new
    stack = Array.new
    temp = ''
    @expression.each_char do |asc|
      case asc
        when ')'
          if temp.length > 0
            if is_digit(temp)
              output.push(temp)
            else
              stack.push(temp)
            end
            temp = ''
          end

          loop{
            if stack.empty? == false
              popped = stack.pop
              if popped == '('
                break
              else
                output.push(popped)
              end
            else
              raise 'Missing "("!'
            end
          }
        when '(', '+', '-', '*', '/'
          if temp.length > 0
            if is_digit(temp)
              output.push(temp)
            else
              stack.push(temp)
            end
            temp = ''
          end

          unless asc == '('
            p_asc = precedence[asc]
            loop{
              if (p_asc > precedence[stack.last])
                break
              else
                output.push(stack.pop)
              end
            }
          end
          stack.push(asc)
        else
          temp += asc
      end
    end
    stack.each{|asc| output.push(asc)}
    !output.include?('(') or raise 'Missing ")"!'
    output.delete('')
    output
  end
end