Python Program to Check Expression is Correctly Parenthesized


This is a Python program to check whether an expression is correctly parenthesized.

Problem Description

The program prompts the user for a string and determines whether it is a palindrome with the help of a stack.

Problem Solution

1. Create a class Stack with instance variable items initialized to an empty list.
2. Define methods push, pop and is_empty inside the class Stack.
3. The method push appends data to items.
4. The method pop pops the first element in items.
5. The method is_empty returns True only if items is empty.
6. Prompt the user for an expression.
7. Iterate through the characters of the expression and push to the stack if an open parenthesis is encountered and pop if a close parenthesis is encountered.
8. Determine whether the expression has balanced parentheses.

Program/Source Code

Here is the source code of a Python program to check for balanced parentheses using a stack. The program output is shown below.

class Stack:
    def __init__(self):
        self.items = []
    def is_empty(self):
        return self.items == []
    def push(self, data):
    def pop(self):
        return self.items.pop()
s = Stack()
exp = input('Please enter the expression: ')
for c in exp:
    if c == '(':
    elif c == ')':
        if s.is_empty():
            is_balanced = False
    if s.is_empty():
        is_balanced = True
        is_balanced = False
if is_balanced:
    print('Expression is correctly parenthesized.')
    print('Expression is not correctly parenthesized.')
Program Explanation

1. An instance of Stack is created.
2. The user is prompted to enter an expression.
3. Iterate through the expression and push to the stack for every open parenthesis and pop for every close parenthesis.
4. If a pop is required when the stack is empty or the stack is not empty when the expression has been iterated over, then the expression is not correctly parenthesized.
5. Display the result.

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Runtime Test Cases
Case 1:
Please enter the expression: (3 + 4 * (1 + (2))/(7 * (8 + 9)))
Expression is correctly parenthesized.
Case 2:
Please enter the expression: (a + b))(3)
Expression is not correctly parenthesized.
Case 3:
Please enter the expression: (4 + (3 * 2)
Expression is not correctly parenthesized.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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