Balanced Parentheses

Question

Someone can help me?

I don't understand why this code doesn't run ok for all the inputs.

Thaks!!

class Stack:
def __init__(self):
self.items = []

def is_empty(self):
return self.items == []

def push(self, item):
self.items.insert(0, item)

def pop(self):
return self.items.pop(0)

def print_stack(self):
print(self.items)

def is_match(p1, p2):
if p1 == "(" and p2 == ")":
return True
else:
return False

def balanced(expression):
s = Stack()
balanced = True
index = 0

while index < len(expression) and balanced:
paren = expression[index]
if paren in "(":
s.push(paren)
else:
if paren in ")":
s.pop(paren)
else:
if s.is_empty():
balanced = False
else:
top = s.pop()
if not is_match(top, paren):
balanced = False

index += 1

if s.is_empty and balanced:
return True
else:
return False

print(balanced(input()))

Answer 1

I think, logic is_match() function can never return True.. So it may causing loop stop.
What is the need of it? Remove it and use s.pop() instead of s.pop(paren). it works then..

Answer 2

Thank you for your answer!

I delete the match function and use s.pop() but it doesn't work fine.

For this input:

(x+y)*(x-z)-9

the output is: False

for this input:

(x-8*(2*5+(2)

the output is:

True.

The code now is like this:

class Stack:
def __init__(self):
self.items = []

def is_empty(self):
return self.items == []

def push(self, item):
self.items.insert(0, item)

def pop(self):
return self.items.pop(0)

def print_stack(self):
print(self.items)

def balanced(expression):
s = Stack()
balanced = True
index = 0

while index < len(expression) and balanced:
paren = expression[index]
if paren in "(":
s.push(paren)
else:
if paren in ")":
s.pop()
else:
if s.is_empty():
balanced = False

index += 1

if s.is_empty and balanced:
return True
else:
return False

print(balanced(input()))

Answer 3

In first case : when it read '*' then it is not "(", or ")" So else part s.is_empty() makes balanced= false. Loop

In second case wong because of : in last if condition you are using s.is_empty instead of s.is_empty() , paranthesis missing...

Check is empty before poping item.. May it need some more modification, hope you can find it.. .