Find the missing value from the given equation a + b = c in Python

Suppose we have one equation in this form: a + b = c, now any one of the terms of a, b or c is missing. We have to find the missing one.

So, if the input is like ? + 4 = 9, then the output will be 5

To solve this, we will follow these steps −

• delete all blank spaces from the string and change (+ and = to comma ',')

• elements := a list of elements by splitting the string separated by comma

• idx := 0

• for i in range 0 to size of elements, do

  • if elements[i] is not numeric, then

    • idx := i

    • come out from the loop

• if last element is missing, then

  • return first element + second element

• otherwise when second element is missing, then

  • return last element - first element

• otherwise when first element is missing, then

  • return last element - second element

Example

Let us see the following implementation to get better understanding −

 Live Demo

def find_missing(string):
   string = string.strip().replace(' ', '')
   string = string.replace('=',',')
   string = string.replace('+',',')
   elements = string.split(',')
   idx = 0
   for i in range(len(elements)):
      if not elements[i].isnumeric():
         idx = i
         break
   if idx == 2:
      return int(elements[0]) + int(elements[1])
   elif idx == 1:
      return int(elements[2]) - int(elements[0])
   elif idx == 0:
      return int(elements[2]) - int(elements[1])

print(find_missing('6 + 8 = ?'))
print(find_missing('? + 8 = 20'))
print(find_missing('5 + ? = 15'))

Input

'6 + 8 = ?'
'? + 8 = 20'
'5 + ? = 15'

Output

14
12
10