Exercise 06-18:
Make a program that reads an expression with parentheses. Using stacks, verify that the parentheses have been opened and closed in the correct order. Example:
(())
OK
()()(()())
OK
())
Error
You can add elements to the stack whenever you find an open parenthesis and unstack it whenever you find a closed one. When unstacking, make sure that the top of the stack is an open parenthesis. If the expression is correct, your stack will be empty at the end.
Answer:
##############################################################################
# Python From Scratch
# Author: Nilo Ney Coutinho Menezes
# Editora Novatec (c) 2010-2025 - LogiKraft 2025
# Site: https://pythonfromscratch.com
# ISBN: 978-85-7522-949-1 (Paperback), 978-85-7522-950-7 (hardcover), 978-85-7522-951-4 (ebook)
#
# File: chapter 06/exercise-06-07.py.py
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expression = input("Enter the sequence of parentheses to validate:")
x = 0
stack = []
while x < len(expression):
if expression[x] == "(":
stack.append("(")
if expression[x] == ")":
if len(stack) > 0:
top = stack.pop(-1)
else:
stack.append(")") # Forces error message
break
x += 1
if len(stack) == 0:
print("OK")
else:
print("Error")