Stacks (last-in, first-out) in Intro to Computing - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an item to the top of the stack.

Intro to Computing

stack = []
stack.[1]('apple')

Drag options to blanks, or click blank then click option'

Aremove

Bpop

Cappend

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' which removes an item instead of adding.

Using 'remove' which deletes a specific item.

Using 'insert' which adds at a specific position, not necessarily the top.

To add an item to the top of a stack, we use the append method, which adds the item to the end of the list, representing the top of the stack.

2fill in blank

medium

Complete the code to remove the top item from the stack.

Intro to Computing

stack = ['apple', 'banana', 'cherry']
top_item = stack.[1]()

Drag options to blanks, or click blank then click option'

Apop

Bremove

Cappend

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'append' which adds an item instead of removing.

Using 'remove' which deletes a specific item by value.

Using 'insert' which adds an item at a position.

The pop() method removes and returns the last item from the list, which represents the top of the stack.

3fill in blank

hard

Fix the error in the code to check if the stack is empty.

Intro to Computing

stack = []
if len(stack) [1] 0:
    print('Stack is empty')

Drag options to blanks, or click blank then click option'

A!=

B==

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' which checks if stack is not empty.

Using '>' which checks if stack has items.

Using '<=' which includes zero and negative numbers (not relevant here).

To check if the stack is empty, we compare the length of the stack to zero using ==. If length equals zero, the stack is empty.

4fill in blank

hard

Fill both blanks to create a stack with numbers 1 to 5 and remove the top item.

Intro to Computing

stack = [[1] for num in range(1, 6)]
top = stack.[2]()

Drag options to blanks, or click blank then click option'

Anum

Bpop

Cappend

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'append' instead of 'pop' to remove the top item.

Using 'number' which is not defined.

Using 'num' incorrectly outside the comprehension.

The list comprehension uses num to add numbers 1 to 5 to the stack. Then, pop() removes the top item.

5fill in blank

hard

Fill all three blanks to create a stack from letters, add 'z', and then remove the top item.

Intro to Computing

stack = [[1] for [2] in ['a', 'b', 'c']]
stack.[3]('z')
top = stack.pop()

Drag options to blanks, or click blank then click option'

Aletter

Bx

Cappend

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the comprehension.

Using 'pop' instead of 'append' to add an item.

Using undefined variable names.

The list comprehension uses letter to loop through the list. Then append adds 'z' to the top of the stack.

Practice

1. What does the term last-in, first-out (LIFO) mean in the context of a stack?

easy

A. The last item added is the first one removed.

B. The first item added is the first one removed.

C. Items are removed in random order.

D. Items are removed based on their size.

Solution

1. Step 1: Understand the order of operations in a stack

   A stack stores items so that the last item you put in is the first one you take out.

2. Step 2: Match the definition to the options

   The last item added is the first one removed correctly describes this behavior as "last item added is first removed".

3. Final Answer:

   The last item added is the first one removed. -> Option A

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember: last added is first removed in stacks [OK]

Common Mistakes:

• Confusing LIFO with FIFO (queue behavior)
• Thinking items are removed in the order they were added
• Assuming random removal order

2. Which of the following is the correct way to add an item to a stack?

easy

A. Use the pop operation.

B. Use the peek operation.

C. Use the push operation.

D. Use the enqueue operation.

Solution

1. Step 1: Recall stack operations

   Stacks use push to add items and pop to remove items.

2. Step 2: Identify the correct operation for adding

   Only push adds an item to the stack, so Use the push operation is correct.

3. Final Answer:

   Use the push operation. -> Option C

4. Quick Check:

   Push adds items to stack [OK]

Hint: Push adds, pop removes from stack [OK]

Common Mistakes:

• Confusing pop as adding instead of removing
• Using queue terms like enqueue
• Thinking peek adds items

3. Consider this sequence of stack operations starting with an empty stack:

push(5)
push(3)
pop()
push(2)
pop()
pop()

What is the result of the last pop() operation?

medium

A. 5

B. 3

C. 2

D. Stack is empty

Solution

1. Step 1: Trace each operation on the stack

   Start empty: []
   push(5) -> [5]
   push(3) -> [5, 3]
   pop() removes 3 -> [5]
   push(2) -> [5, 2]
   pop() removes 2 -> [5]
   pop() removes 5 -> []

2. Step 2: Identify the last pop result

   The last pop removes 5, so the result is 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Last pop removes 5 [OK]

Hint: Follow push/pop step-by-step to track top item [OK]

Common Mistakes:

• Forgetting the order of pop removes last pushed
• Mixing up which item is on top
• Assuming pop returns the first pushed item

4. The following code tries to pop an item from an empty stack. What is the likely problem?

stack = []
item = stack.pop()

medium

A. It will silently ignore the pop.

B. It will return None.

C. It will add an item instead of removing.

D. It will raise an error because the stack is empty.

Solution

1. Step 1: Understand pop behavior on empty stack

   Calling pop on an empty list (stack) causes an error because there is no item to remove.

2. Step 2: Identify the error type

   In Python, this raises an IndexError indicating the stack is empty.

3. Final Answer:

   It will raise an error because the stack is empty. -> Option D

4. Quick Check:

   Pop on empty stack causes error [OK]

Hint: Pop on empty stack causes error, never returns None [OK]

Common Mistakes:

• Assuming pop returns None if empty
• Thinking pop adds items
• Ignoring runtime errors

5. You want to reverse the order of a list [1, 2, 3, 4] using a stack. Which sequence of operations correctly reverses the list?

hard

A. Pop all items first, then push them back in order.

B. Push all items in order, then pop all items to get reversed list.

C. Push items in reverse order, then pop all items.

D. Push and pop items randomly to reverse.

Solution

1. Step 1: Understand stack reverses order by LIFO

   Pushing items in original order puts last item on top. Popping all items returns them in reverse order.

2. Step 2: Match the correct sequence

   Push all items in order, then pop all items to get reversed list describes pushing all items then popping all to reverse the list correctly.

3. Final Answer:

   Push all items in order, then pop all items to get reversed list. -> Option B

4. Quick Check:

   Stack reverses order by push then pop [OK]

Hint: Push original order, pop all to reverse list [OK]

Common Mistakes:

• Pushing in reverse order then popping returns original order
• Popping before pushing causes error
• Random push/pop does not guarantee reversal