Program to evaluate one mathematical expression without built-in functions in python

Suppose we have a string that represents a mathematical expression with (+, -, *, /) Here / is representing integer division, we have to evaluate and return the result without using any built-in function.

So, if the input is like s = "2+3*5/7", then the output will be 4, as 2 + ((3 * 5) / 7) = 4

To solve this, we will follow these steps −

• s := reverse the given string
• Define a function get_value().
• sign := 1
• if s is not empty and last element of s is same as "-", then
  • delete last element from s
  • sign := -1
• value := 0
• while s is not empty and last element of s is a digit, do
  • value := value * 10
  • value := value + numeric value of last element from s, and delete last element of s
• return sign * value
• Define a function get_term()
• term := get_value()
• while s is not empty and last element of s is either * or /, do
  • op := last element of s and delete last element from s
  • value := get_value()
  • if op is same as "*", then
    • term := term * value
  • otherwise,
    • term := the floor of (1.0 * term / value)
• return term
• From the main method do the following:
• ans := get_term()
• while s is not empty, do
  • op := last element of s, and delete it from s
  • term := get_term()
  • if op is same as "+", then
    • ans := ans + term
  • otherwise,
    • ans := ans - term
• return ans

Let us see the following implementation to get better understanding −

Example 

Live Demo

from math import floor, trunc
class Solution:
   def solve(self, s):
      s = list(s[::-1])

      def get_value():
         sign = 1
         if s and s[-1] == "-":
            s.pop()
            sign = -1
         value = 0
         while s and s[-1].isdigit():
            value *= 10
            value += int(s.pop())
         return sign * value

      def get_term():
         term = get_value()
         while s and s[-1] in "*/":
            op = s.pop()
            value = get_value()
            if op == "*":
               term *= value
            else:
               term = floor(1.0 * term / value)
         return term

      ans = get_term()
      while s:
         op, term = s.pop(), get_term()
         if op == "+":
            ans += term
         else:
            ans -= term
      return ans

ob = Solution()
s = "2+3*5/7"
print(ob.solve(s))

Input

"2+3*5/7"

Output

4