Basic Calculator II in Python

Python Server Side Programming Programming

Suppose we have to implement a basic calculator to evaluate a simple expression string. The expression string will hold only non-negative integers, some operators like, +, -, *, / and empty spaces. The integer division should take the quotient part only.

So if the input is like “3+2*2”, then the output will be 7.

To solve this, we will follow these steps −

define a stack s, i := 0, x := an empty string
for each character j in s
- if j is not a blank character
  - append j into x
s := x, n := length of x
while i < n
- if s[i] is ‘/’, then
  - increase i by 1
  - num := number starting from ith index, then update i as the last character of the number
  - if stack top element < 0, then update stack top element as -(stack top / num), otherwise update stack top element as (stack top / num)
- otherwise when s[i] = “*”
  - increase i by 1
  - num := number starting from ith index, then update i as the last character of the number
  - stack top := num * stack top
- otherwise when s[i] = ‘-’
  - increase i by 1
  - num := number starting from ith index, then update i as the last character of the number
  - insert –num into stack
- otherwise
  - num := number starting from ith index, then update i as the last character of the number
  - insert num into stack
  - increase i by 1
return sum of the elements of the stack

Let us see the following implementation to get better understanding −

Example

class Solution(object):
   def calculate(self, s):
      """
      :type s: str
      :rtype: int
      """
      stack = []
      i = 0
      x=""
      for j in s:
      if j !=" ":
      x+=j
      s = x
      n = len(s)
      while i<n:
         if s[i] == '/':
            i+=1
         num,i = self.make_num(s,i)
         if stack[-1]<0:
            stack[-1] = -1*(abs(stack[-1])/num)
         else:
            stack[-1] = stack[-1]/num
      elif s[i] == '*':
         i+=1
         num,i = self.make_num(s,i)
         stack[-1] = stack[-1]*num
      elif s[i] == '-':
         i+=1
         num,i = self.make_num(s,i)
         stack.append(-num)
      elif s[i] =='+':
         i+=1
         num,i = self.make_num(s,i)
         stack.append(num)
      else:
         num,i = self.make_num(s,i)
         stack.append(num)
         i+=1
         return sum(stack)
   def make_num(self,s,i):
      start = i
      while i<len(s) and s[i]!= '/' and s[i]!= '*' and s[i]!= '-' and s[i]!='+':
      i+=1
   return int(s[start:i]),i-1

Input

"3+2*2"

Output

Arnab Chakraborty

Updated on: 04-Mar-2020

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