Program to evaluate s-expression as string in Python

Suppose we have a string s as S-expression. We have to evaluate that S-expression and return result as integer. As we know that the s-expression is an expression which is either one number, or a recursive expression wrapped in parentheses like (+ (- 3 2) (* 3 3)), which indicates (3 - 2) + (3 * 3) = 10. Here valid operators are +, -, *, and /.

So, if the input is like s = "(- (+ 3 2) 2)", then the output will be 3, as ((3 + 2) - 2) = 3.

To solve this, we will follow these steps −

• stack := a new stack

• remove the opening and closing parenthesis of s

• a := split s using spaces and make a list of partitions

• for each i in a in reverse order, do

  • if size of i > 1, then

    • if i[0] is same as "-", then

      • push i as integer into stack

      • go for next iteration

    • otherwise,

      • push i as integer into stack

  • otherwise when i is a digit, then

    • push i as integer into stack

  • otherwise,

    • if size of stack >= 2, then

      • num1 := pop from stack

      • num2 := pop from stack

      • if i is same as "+", then

        • perform num1 + num2 and push into stack

      • otherwise when i is same as "-", then

        • perform num1 - num2 and push into stack

      • otherwise when i is same as "*", then

        • perform num1 * num2 and push into stack

      • otherwise,

        • perform num1 / num2 and push into stack

• return top element from stack, and remove top element

Example  

Let us see the following implementation to get a better understanding −

 Live Demo

class Solution:
   def solve(self, s):
      stack = list()
      s = s.replace("(", "")
      s = s.replace(")", "")
      a = s.split()
      for i in a[::-1]:
         if len(i) > 1:
            if i[0] == "-":
               stack.append(int(i))
               continue
            else:
               stack.append(int(i))
         elif i.isdigit():
            stack.append(int(i))
         else:
            if len(stack) >= 2:
               num1 = stack.pop()
               num2 = stack.pop()
               if i == "+":
                  stack.append(int(num1 + num2))
               elif i == "-":
                  stack.append(int(num1 - num2))
               elif i == "*":
                  stack.append(int(num1 * num2))
               else:
                  stack.append(int(num1 / num2))
      return stack.pop()
ob = Solution()
s = "(- (+ 3 2) 2)"
print(ob.solve(s))

Input

s = "(- (+ 3 2) 2)"

Output

3