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