Stack Concept and LIFO Principle in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Python

stack = []
stack.[1](5)
print(stack)

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

Aappend

Bpop

Cremove

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of append removes an element instead of adding.

Using remove or insert changes the wrong part of the list.

Use append to add an element to the top of the stack (end of the list).

2fill in blank

medium

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

DSA Python

stack = [1, 2, 3]
top = stack.[1]()
print(top, stack)

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

Apop

Binsert

Cappend

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using append adds an element instead of removing.

Using remove deletes a specific value, not the last element.

Use pop to remove and return the last element, which is the top of the stack.

3fill in blank

hard

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

DSA Python

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

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

A>

B==

C<

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' checks for non-empty instead of empty.

Using '>' or '<' may give wrong results for empty stack.

Check if length equals zero to confirm the stack is empty.

4fill in blank

hard

Fill both blanks to create a stack and add two elements.

DSA Python

stack = [1]
stack.[2](10)
stack.append(20)
print(stack)

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

A[]

B{}

Clist()

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using {} creates a dictionary, not a list.

Using set() creates an unordered collection, not suitable for stack.

Use [] to create an empty list as a stack and append to add elements.

5fill in blank

hard

Fill all three blanks to pop the top element and print the stack.

DSA Python

stack = [5, 10, 15]
top = stack.[1]()
print('Popped:', top)
print('Stack now:', stack[2]stack[3])

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

Apop

B+

C[::-1]

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' causes an error.

Using other methods than pop to remove the top element.

Use pop() to remove the top element. The print shows the stack plus its reversed version using + [::-1].

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the first item added is the first to be removed

B. Last In, First Out - the last item added is the first to be removed

C. First In, First Out - the first item added is the first to be removed

D. First In, Last Out - the first item added is the last to be removed

Solution

1. Step 1: Understand LIFO meaning

   LIFO means the last element added to the stack is the first one to be removed.

2. Step 2: Match with options

   Last In, First Out - the last item added is the first to be removed correctly describes this behavior as "Last In, First Out".

3. Final Answer:

   Last In, First Out - the last item added is the first to be removed -> Option B

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking first added is removed first
• Mixing order of insertion and removal

2. Which of the following is the correct way to push an element '5' onto a stack named stack in Python?

easy

A. stack.append(5)

B. stack.add(5)

C. stack.push(5)

D. stack.insert(5)

Solution

1. Step 1: Recall Python list methods for stack

   Python uses list's append() method to add elements to the top of the stack.

2. Step 2: Check options for correct method

   Only stack.append(5) uses append(5), which is correct for pushing onto a stack.

3. Final Answer:

   stack.append(5) -> Option A

4. Quick Check:

   Push in Python stack = append() [OK]

Hint: Use append() to push in Python stack [OK]

Common Mistakes:

• Using push() which is not a Python list method
• Using add() which is for sets
• Using insert() which inserts at specific index

3. What will be the output of the following Python code?

stack = []
stack.append(10)
stack.append(20)
stack.append(30)
print(stack.pop())
print(stack.pop())
print(stack)

medium

A. 30
10
[20]

B. 10
20
[30]

C. 20
30
[10]

D. 30
20
[10]

Solution

1. Step 1: Trace stack after each append

   After append(10): [10], append(20): [10, 20], append(30): [10, 20, 30]

2. Step 2: Trace pop operations

   pop() removes last element: first pop() returns 30, second pop() returns 20, stack left is [10]

3. Final Answer:

   30 20 [10] -> Option D

4. Quick Check:

   Stack pops last in first out [OK]

Hint: Pop removes last pushed element first [OK]

Common Mistakes:

• Thinking pop removes first element
• Confusing order of popped elements
• Not updating stack after pop

4. Identify the error in the following code that implements a stack using a Python list:

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

medium

A. Using push() method which does not exist for Python lists

B. Using pop() method incorrectly

C. Stack should be initialized as a tuple, not list

D. Missing import statement for stack

Solution

1. Step 1: Check methods used on list

   Python lists do not have a push() method; append() is used to add elements.

2. Step 2: Verify pop() usage

   pop() is valid for lists and correctly used here.

3. Final Answer:

   Using push() method which does not exist for Python lists -> Option A

4. Quick Check:

   Python list push() does not exist [OK]

Hint: Use append(), not push(), to add in Python list [OK]

Common Mistakes:

• Assuming push() is valid for Python lists
• Thinking pop() needs arguments
• Confusing list with stack module

5. Given a stack implemented as a Python list, which sequence of operations will reverse the order of elements in the stack?
Assume initial stack is [1, 2, 3] where 3 is the top.
Choose the correct sequence:

hard

A. Append elements to the stack again without popping

B. Use pop() repeatedly and append to the same stack without extra storage

C. Pop all elements into a new stack

D. Sort the stack list in ascending order

Solution

1. Step 1: Understand reversing stack by popping

   Pop elements from original stack and push them into a new stack reverses order.

2. Step 2: Confirm other options

   Appending without popping or sorting does not reverse stack order correctly.

3. Final Answer:

   Pop all elements into a new stack -> Option C

4. Quick Check:

   Two stacks pop/push reverses order [OK]

Hint: Use two stacks to reverse order [OK]

Common Mistakes:

• Trying to reverse by sorting
• Appending without popping
• Using one stack without extra storage