ruby calculator

Purpose

I thought about a calculator algorithm that can perform + (addition),-(subtraction), * (multiplication),/(division) with ruby. The most famous way to make a calculator algorithm is (1) Receive the given formula → (2) Convert to reverse Polish notation → (3) Calculate and return the calculation result. This time, I will introduce my own way of thinking from ② to ③. As a matter of fact, I am a beginner myself, so please point out any mistakes.

Algorithm concept and its code

1. Receive in the standard input of the formula expressed in Reverse Polish Notation.

input = gets.chomp.split(" ")

Reverse Polish notation (postfix notation) is a formula that puts an operator after a number, such as 1 2 + or 2 6 *. By the way, what we usually see is infix notation, such as 1 + 2 or 2 * 6 with the operator in the middle.

2. Create an array of operators in advance to determine whether it is an operator later. At the same time, create a stack to store numerical values.

operator = ['+','-','*','/']
num_stack = []

2. Loop the received calculation formula, push it to the stack when a numerical value arrives, pop two numerical values in the stack when an operator arrives, perform each operation, and push the calculation result to the stack.

input.each do |s|
  #If you get a number, put it on the stack (using a regular expression).
  if s =~ /[0-9]/
    num_stack << s
  end
  #If an operator comes, take two numbers from the stack and calculate according to each operator.
  if operator.include?(s)
    a = num_stack.pop
    b = num_stack.pop
    a = a.to_i
    b = b.to_i
    case s
    when '+'    
      num_stack << b + a   
    when '-'  
      num_stack << b - a
    when '*'   
      num_stack << b * a
    when '/'   
      num_stack << b / a
    end
  end
end

3. The last remaining number on the stack is the calculation result.

ans = num_stack.pop
puts ans