Balanced Parentheses-How do you think I could write this code better?

Question

##Balanced Parentheses
##
##Parentheses are balanced, if all opening parentheses have their corresponding closing parentheses.
##
##Given an expression as input, we need to find out whether the parentheses are balanced or not.
##For example, "(x+y)*(z-2*(6))" is balanced, while "7-(3(2*9))4) (1" is not balanced.
##
##The problem can be solved using a stack.
##
##Push each opening parenthesis to the stack and pop the last inserted opening parenthesis
##whenever a closing parenthesis is encountered.
##
##If the closing bracket does not correspond to the opening bracket, then stop and
##say that the brackets are not balanced.
##
##Also, after checking all the parentheses, we need to check the stack to be empty
##-- if it's not empty, then the parentheses are not balanced.
##
##Implement the balanced() function to return True if the parentheses in the given expression are balanced,
##and False if not.

def balanced(expression):

while "(" not in expression or ")" not in expression:
print("Please enter a in parentheses expression : ", end="")
expression = input()

lst = []
for i in expression:
if i == "(" or i == ")":
lst.insert(0, i)

if lst[-1] == ")":
return False

lst = []
for i in expression:
if i == "(":
lst.insert(0, i)
try:
if i == ")":
lst.pop(0)
except IndexError:
return False

if len(lst) == 0 :
return True

else:
return False

print(balanced(input("Please enter a in parentheses expression : ")))

Answer 1

Yup, your program looks good in my opinion, nice job :)

Well actually, I don't think you need the first loop at all. I think you are trying to check that the last parenthesis is a closing one, but it's not needed. What if it isn't?

example: ())(

Here in the third letter, you would be trying to pop from an empty stack which raises an IndexError.

example: (())(

Here, the balance at the end would not be 0. In other words, `len(lst) == 1`.

So your program already does these checks, like whether it starts with a "(" and ends with a ")". The extra loop at the top is superfluous :)

Answer 2

If you only want to check for "(" and ")" you don't even need a stack.

Set i = 0.
If you read "(", then `i+=1`. This is like `lst.insert`.
If you read ")", then `i-=1`. This is like `lst.pop`.
If `i<0`, return false. This is like `IndexError`.

At the end of the program, if `i != 0`, return False. This is like `len(lst) != 0`.

else return True.

Using a stack is important if you want to match many different pairs of parentheses like (), [], {}, <>, all at once.

Answer 3

Schindlabua you are right but the project is about stack so i coded it like this. but alternatively it's so cool in your opinion..I also need to check if the opening parenthesis precedes the closing parenthesis in the expression.

Answer 4

Schindlabua You're right. The first "for" loop was unnecessary. I also saw that in the "while" loop it should be "and" instead of "or". Even if the code works, I always need other ideas to come up with a better one. Thanks...