Python Balanced Parentheses Data Structures

Question

Hi, everyone.

I am playing with the code in this exercise but it does not work for every case scenario.

My code is this:

def balanced(expression):
lst = []
for i in expression:
if i == '(' or i == '{' or i == '[':
lst.append(i)
elif i == ')' or i == '}' or i == ']':
lst.pop(0)
if len(lst) == 0:
return True
else:
return False

print(balanced(input()))

What could be my error?

Thanks in advance!

Answer 1

Because there could be an input wherein there is a closing bracket but no opening bracket before it.

Example:
INPUT: )Hello(

And in first iteration, this will cause an error since the lst is still empty, i.e. no '(' or items to 'pop' .

So if there is ')' but no '(' was found like in this example ")Hello(", it is automatically unbalanced therefore return False.

TO SOLVE:
---> Check first if there are opening brackets in lst before removing a bracket.
---> If there is a ')' but no '(' before it, return False.

Additional:
The problem is just looking for brackets/parentheses, so you don't need those other types of brackets.

https://code.sololearn.com/cW0z4pz669Kw/?ref=app

Answer 2

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

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

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

s = Stack()
inp = input()
def balanced(inp):
try:
for i in inp:
if i == str("("):
s.push('(')
elif i == str(")"):
s.pop()
else:
continue
if len(s.items) == 0:
print(" True")
else:
print("False")
except:
print("False")
balanced(inp)

Answer 3

Another point of view;
lista = list()
def balanced(expression):
#your code goes here
item = list(expression)
for i in item:
if i == '(':
lista.insert(0, i)
elif i == ')':
if lista == []:
return False
else:
lista.pop(0)
if lista == []:
return True
else:
return False

print(balanced(input()))

Answer 4

Hello , every one . Please i am having troubles with the exercise the the code keep giving me the same feedback that my expression is not defined

For i in expression :
It keeps telling me everytime that the term expression is not defined in this line

Answer 5

Oh, so I just missed the condition to find out if the first parentheses was an opening one. Thank you!!!

Answer 6

Javier Muñoz
What your solution?