Stack operations (push, pop, peek) in Data Structures Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Data Structures Theory

stack.[1](element)

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

Apeek

Bpop

Cremove

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using 'peek' which only views the top element without removing or adding.

The push operation adds an element to the top of the stack.

2fill in blank

medium

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

Data Structures Theory

top_element = stack.[1]()

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

Apeek

Bpush

Cpop

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'push' which adds instead of removing.

Using 'peek' which only views the top element without removing it.

The pop operation removes and returns the top element of the stack.

3fill in blank

hard

Fix the error in the code to view the top element without removing it.

Data Structures Theory

top_element = stack.[1]()

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

Apeek

Bpop

Cpush

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' which removes the element instead of just viewing it.

Using 'push' which adds an element instead of viewing.

The peek operation returns the top element without removing it from the stack.

4fill in blank

hard

Fill both blanks to add an element and then view the top element.

Data Structures Theory

stack.[1](new_item)
top = stack.[2]()

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

Apush

Bpop

Cpeek

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' instead of 'peek' to view the top element.

Using 'remove' which is not a standard stack operation.

First, push adds the new item to the stack. Then, peek shows the top element without removing it.

5fill in blank

hard

Fill all three blanks to push an element, pop the top, and then peek at the new top.

Data Structures Theory

stack.[1](item)
popped = stack.[2]()
current_top = stack.[3]()

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

Apop

Bpush

Cpeek

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up push and pop operations.

Using 'remove' which is not a stack operation.

First, push adds the item. Then, pop removes the top element. Finally, peek shows the new top element.

Practice

1. What does the pop operation do in a stack?

easy

A. Shows the bottom item without removing it

B. Adds a new item to the bottom of the stack

C. Removes and returns the top item from the stack

D. Clears all items from the stack

Solution

1. Step 1: Understand the pop operation

   The pop operation removes the last item added to the stack, which is the top item.

2. Step 2: Confirm the behavior of pop

   It returns this top item and removes it from the stack, following last-in, first-out order.

3. Final Answer:

   Removes and returns the top item from the stack -> Option C

4. Quick Check:

   pop = remove top item [OK]

Hint: Pop always removes the last item added [OK]

Common Mistakes:

• Thinking pop adds an item
• Confusing pop with peek
• Assuming pop removes bottom item

2. Which of the following is the correct way to add an item 5 to a stack named s using the push operation?

easy

A. s.pop(5)

B. s.push(5)

C. s.peek(5)

D. s.add(5)

Solution

1. Step 1: Identify the push syntax

   Push adds an item to the top of the stack, usually called with push(item).

2. Step 2: Match the correct method call

   Only s.push(5) correctly adds 5 to the stack.

3. Final Answer:

   s.push(5) -> Option B

4. Quick Check:

   push = add item [OK]

Hint: Push means add, so look for push() method [OK]

Common Mistakes:

• Using pop instead of push
• Using peek to add items
• Using add which is not standard for stacks

3. Given the following stack operations:

s.push(10)
s.push(20)
s.pop()
s.push(30)
top = s.peek()

What is the value of top after these operations?

medium

A. 30

B. 20

C. None

D. 10

Solution

1. Step 1: Trace the stack changes step-by-step

   After push(10), stack = [10]. After push(20), stack = [10, 20]. After pop(), top 20 is removed, stack = [10]. After push(30), stack = [10, 30].

2. Step 2: Determine the top item with peek

   Peek returns the top item without removing it, so top = 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed after pop is 30 [OK]

Hint: Track stack after each operation carefully [OK]

Common Mistakes:

• Forgetting pop removes top
• Assuming peek removes item
• Mixing order of operations

4. Consider this code snippet for a stack:

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

What is the error in this code?

medium

A. stack is not initialized correctly

B. pop() does not return the top item

C. stack.pop() removes the bottom item, not top

D. push() is not a method of list in Python

Solution

1. Step 1: Check stack initialization

   The stack is initialized as an empty list, which is correct in Python.

2. Step 2: Verify method usage on list

   Python lists use append() to add items, not push(). Using push() causes an error.

3. Final Answer:

   push() is not a method of list in Python -> Option D

4. Quick Check:

   Python list uses append(), not push() [OK]

Hint: Remember Python list uses append(), not push() [OK]

Common Mistakes:

• Assuming push() works on Python lists
• Confusing pop() behavior
• Thinking stack initialization is wrong

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

hard

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

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

C. Peek all items, then push them again

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

Solution

1. Step 1: Understand stack reversal logic

   Pushing items in original order stores them with last item on top. Popping all items removes them in reverse order.

2. Step 2: Apply push then pop to reverse list

   Push 1, then 2, then 3. Popping returns 3, 2, 1, which is the reversed list.

3. Final Answer:

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

4. Quick Check:

   Push then pop reverses order [OK]

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

Common Mistakes:

• Pushing in reverse order first
• Using peek to reverse
• Popping before pushing