Stacks (last-in, first-out) in Intro to Computing - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ trace

intermediate

Trace the stack operations and find the top element

Consider a stack initially empty. The following operations are performed in order:

1. Push 5
2. Push 10
3. Pop
4. Push 7
5. Push 3
6. Pop

What is the top element of the stack after these operations?

A 3

B 10

C 5

D 7

💡 Hint

Remember, pop removes the last pushed item, and push adds to the top.

Step-by-step stack content:
Push 5 → [5]
Push 10 → [5, 10]
Pop → removes 10, stack is [5]
Push 7 → [5, 7]
Push 3 → [5, 7, 3]
Pop → removes 3, stack is [5, 7]
Top element is 7.

🧠 Conceptual

intermediate

Identify the correct property of a stack

Which of the following best describes the behavior of a stack data structure?

A First-in, first-out (FIFO)

B Last-in, first-out (LIFO)

C Random access to any element

D Sorted order insertion

💡 Hint

Think about which element is removed first when you pop from a stack.

A stack follows Last-in, first-out (LIFO) behavior, meaning the last item added is the first to be removed.

❓ Comparison

advanced

Compare stack and queue behaviors

Which statement correctly compares a stack and a queue?

A Stacks remove elements in LIFO order; queues remove elements in FIFO order.

B Stacks and queues both remove elements in the order they were added.

C Stacks remove elements in FIFO order; queues remove elements in LIFO order.

D Stacks allow random access; queues only allow access to the last element.

💡 Hint

Recall the order in which elements are removed from each data structure.

Stacks remove the last added element first (LIFO). Queues remove the first added element first (FIFO).

❓ identification

advanced

Identify the error in stack operation code

Given the following pseudocode for stack operations:

stack = []
stack.push(1)
stack.push(2)
stack.pop()
stack.pop()
stack.pop()

What error will occur when this code runs?

Intro to Computing

stack = []
stack.push(1)
stack.push(2)
stack.pop()
stack.pop()
stack.pop()

A AttributeError: 'list' object has no attribute 'push'

B SyntaxError: invalid syntax

C IndexError: popping from empty stack

D No error, code runs successfully

💡 Hint

Check if the method names used for list operations are correct in Python.

In Python, lists do not have a 'push' method. The correct method to add an element is 'append'. Using 'push' causes an AttributeError.

🚀 Application

expert

Determine the output of stack-based expression evaluation

A stack is used to evaluate the postfix expression: 3 4 2 * 1 5 - 2 3 ^ ^ / +

What is the final result after evaluating this expression using a stack?

A 5.5

B Error: invalid expression

C 3.0001220703125

D 7.0

💡 Hint

Evaluate step-by-step using stack: push numbers, pop two for operators, apply operator, push result back.

Step-by-step evaluation:
3 push
4 push
2 push
* pop 2 and 4 → 4*2=8 push 8
1 push
5 push
- pop 5 and 1 → 1-5=-4 push -4
2 push
3 push
^ pop 3 and 2 → 2^3=8 push 8
^ pop 8 and -4 → (-4)^8=65536 push 65536
/ pop 65536 and 8 → 8/65536=0.0001220703125 push 0.0001220703125
+ pop 0.0001220703125 and 3 → 3 + 0.0001220703125 = 3.0001220703125
Final result: 3.0001220703125