Why Stack Exists and What Problems It Solves in DSA Python - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why do we use a stack in function calls?

Imagine you are cooking multiple dishes, and you need to remember which step to return to after finishing a sub-step. Which of these best explains why a stack is used in managing function calls in programming?

A Because a stack deletes all previous function calls immediately after a new call is made.

B Because a stack allows random access to any function call at any time.

C Because a stack remembers the last function called and returns to it first, like a stack of plates where you take the top plate first.

D Because a stack stores all function calls in a queue to process them in the order they were called.

💡 Hint

Think about how you would keep track of tasks that depend on finishing other tasks first.

A stack follows Last In First Out (LIFO) order, which matches how function calls return: the last function called must finish before returning to the previous one.

❓ Predict Output

intermediate

Output of Stack Operations

What is the printed output after running this Python code that uses a stack to reverse a list?

DSA Python

stack = []
items = [1, 2, 3, 4]
for item in items:
    stack.append(item)
reversed_list = []
while stack:
    reversed_list.append(stack.pop())
print(reversed_list)

A [4, 3, 2, 1]

B [1, 2, 3, 4]

C [1, 4, 2, 3]

D []

💡 Hint

Think about what happens when you add items to a stack and then remove them.

The stack stores items in order, but popping removes the last added item first, reversing the list.

🔧 Debug

advanced

Identify the Error in Stack Usage

This code tries to check if parentheses in a string are balanced using a stack. What error will it raise when run?

DSA Python

def check_parentheses(s):
    stack = []
    for char in s:
        if char == '(': 
            stack.append(char)
        elif char == ')':
            stack.pop()
    return len(stack) == 0

print(check_parentheses('())'))

A Returns False

B SyntaxError: invalid syntax

C Returns True

D IndexError: pop from empty list

💡 Hint

What happens if you try to pop from an empty stack?

If the string has more closing parentheses than opening ones, popping from an empty stack causes an IndexError.

🚀 Application

advanced

Using Stack to Evaluate Postfix Expression

Given the postfix expression '3 4 + 2 * 7 /', what is the final result after evaluating it using a stack?

A 2.0

B 14

C 1.0

D 7

💡 Hint

Evaluate step-by-step: push numbers, pop two for operation, push result back.

Postfix evaluation uses a stack to compute operations in order. The expression evaluates to 2.0.

🧠 Conceptual

expert

Why is Stack Preferred for Undo Operations?

In text editors, undo operations are often implemented using a stack. Why is a stack the best choice for this?

A Because stacks allow undoing actions in any random order.

B Because the last action performed should be the first one undone, matching the stack's Last In First Out behavior.

C Because stacks store all actions permanently without removing any.

D Because stacks process actions in the order they were first performed.

💡 Hint

Think about which action you want to undo first.

Undoing requires reversing the most recent action first, which fits the LIFO nature of stacks.