Question

#Write a function called check_formula. The check_formula #function should take as input one parameter, a string. It #should return True if the string holds a correctly #formatted arithmetic integer formula according to the rules #below, or False if it does not. # #For this problem, here are the rules that define a #correctly-formatted arithmetic string: # # - The only characters in the string should be digits or the five arithmetic operators: +, -, *, /, and =. Any other characters, including spaces, periods, commas, or any letters, are not permitted. # - There may not be any consecutive arithmetic operators. Any arithmetic operator must have a number on either side of it. # - There must be an equals sign in the formula. # #You do not need to worry about negative numbers or #parentheses, and you do not need to worry about whether #the equation is accurate. You may also assume all the #numbers in the string will be only one digit. # #Here are some examples of valid and invalid arithmetic #formulas: # # Valid Invalid 5*3=5+2 5*3+5+2 (no equals) 5=7 5= (equals sign isn't in the middle) 5=2-5 50=-5 (consecutive arithmetic operators) 6/2=5/2. a=51 (illegal character) -5=5+2 (starts with an operator) #Hint: Remember, as soon as you find *one* thing wrong #with the string, you know it's invalid and can return #False. So, go character-by-character through the string #checking everything that could be wrong. If you don't #find anything wrong, return True! #Write your function here!

Answer 1

We can use the below logic to implement our rules for the arithmetic string

• Look at each character in the string
  • if character is one of the allowed operator
    • return False if it is a consecutive operator
    • return False if it is at the start or at the end of the string
    • return False if there are no digits on either side of the character
    • set found_operator flag to True (to keep track of consecutive operators)
    • set found_equals flag to True if the character is = symbol
  • if character is a digit
    • set found_operator flag to False (as we did not have a consecutive operator)
  • return False (as the character is neither an allowed operator nor a digit)
• return False if found_equals flag is False
• return True (as all rules were satisfied)

Python code

def check_formula(expression):
   """returns True if expression holds a correctly formatted formula"""
   operators = ['+', '-', '*', '/', '='] # allowed operators
   found_equals = False
   found_operator = False
   for i in range(len(expression)):
       character = expression[i] # look at each character
       # checks if character is an allowed operator
       if character in operators:
           if found_operator: # checks if found_operator flag is already True
               return False # character is a consecutive operator
           if i == 0 or i == len(expression) - 1: # checks if operator is at the beginning or end
               return False
           # checks if there is a digit on either side of the operator
           if not expression[i-1].isdigit() or not expression[i+1].isdigit():
               return False
           found_operator = True # sets found operator flag as True
           if character == '=': # operator is an equals symbol
               found_equals = True
       # checks if character is a digit
       elif character.isdigit():
           found_operator = False
       # returns False if the character is neither an allowed operator nor a digit
       else:
           return False
   # returns False if an = symbol was not found in the expression
   if not found_equals:
       return False
   # all rules correct, so returns True
   return True

Screenshot (included for readability)

def check_formula(expression) : ""'"returns True if expression holds a correctly formatted formula" operators = ['+', '7', '='] # allowed operators found_equals = False found_operator = False for i in range(len(expression)): character = expression[i] # Look at each character # checks if character is an allowed operator if character in operators: if found_operator: # checks if found_operator flag is already True return false # character is a consecutive operator if i == 0 or i == len(expression) - 1: # checks if operator is at the beginning or end return false # checks if there is a digit on either side of the operator if not expression[i-1].isdigit() or not expression[i+1].isdigit(): return false found_operator = True # sets found operator flag as True if character == 'a': # operator is an equals symbol found_equals = True # checks if character is a digit elif character.isdigit(): found_operator = False # returns Fal se as the character is neither an allowed operator nor a digit else: return false # returns False as an = symbol was not found in the expression if not found equals: return false # all rules correct, so returns True return True

Output

print(check_formula("5*3=5+2")) print(check_formula("5=7")) print(check_formula("5=2-5")) print(check_formula("6/2=5/2")) print(check_formula("50*3+5+2")) print(check_formula ("5=")) print(check_formula("50=-5")) print(check_formula("a=51")) print(check_formula("-5=5+2")) True True True True False False False False False