Push Operation on Stack in DSA Python - Build from Scratch

Push Operation on Stack

📖 Scenario: Imagine you have a stack of books. You want to add a new book on top of the stack. This is called the push operation in stacks.

🎯 Goal: You will create a stack using a list, set a new book to add, push it onto the stack, and then print the stack to see the new book on top.

📋 What You'll Learn

Create a list called stack with these exact books: 'Math', 'Science', 'History'

Create a variable called new_book and set it to 'English'

Use the append() method on stack to push new_book onto the stack

Print the stack list to show the books after pushing

💡 Why This Matters

🌍 Real World

Stacks are used in many places like undo features in apps, browser history, and managing tasks in order.

💼 Career

Understanding stack operations is important for software development, especially in areas like parsing, memory management, and algorithm design.

1

Create the initial stack

Create a list called stack with these exact books: 'Math', 'Science', 'History'

DSA Python

# Create the stack list with the books
# Your code here

Hint

Use square brackets [] to create a list and separate items with commas.

2

Set the new book to push

Create a variable called new_book and set it to the string 'English'

DSA Python

stack = ['Math', 'Science', 'History']
# Create the variable new_book with value 'English'
# Your code here

Hint

Use the equals sign = to assign the string 'English' to new_book.

3

Push the new book onto the stack

Use the append() method on the list stack to add the new_book on top

DSA Python

stack = ['Math', 'Science', 'History']
new_book = 'English'
# Push new_book onto stack using append()
# Your code here

Hint

Call stack.append(new_book) to add the new book to the end of the list.

4

Print the stack after push

Print the list stack to show the books after pushing the new book

DSA Python

stack = ['Math', 'Science', 'History']
new_book = 'English'
stack.append(new_book)
# Print the stack list
# Your code here

Hint

Use print(stack) to display the current stack.

Practice

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

easy

A. Checks if the stack is empty

B. Removes the top element from the stack

C. Returns the bottom element of the stack

D. Adds an element to the top of the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, so new elements are added on top.

2. Step 2: Define push operation

   Push means adding a new element on the top of the stack.

3. Final Answer:

   Adds an element to the top of the stack -> Option D

4. Quick Check:

   Push = Add top element [OK]

Hint: Push always adds element on top of stack [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Mixing stack with queue operations

2. Which of the following is the correct syntax to push an element 5 into a stack named stack implemented as a Python list?

easy

A. stack.append(5)

B. stack.push(5)

C. stack.add(5)

D. stack.insert(0, 5)

Solution

1. Step 1: Recall Python list methods for stack

   Python lists use append() to add elements at the end, which acts as top of stack.

2. Step 2: Check each option

   push() and add() are not list methods; insert(0, 5) adds at bottom, not top.

3. Final Answer:

   stack.append(5) -> Option A

4. Quick Check:

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

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

Common Mistakes:

• Using push() which is not a Python list method
• Using insert(0, value) which adds at bottom
• Confusing add() with append()

3. What will be the content of the stack after executing the following code?

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

medium

A. [20, 10, 30]

B. [30, 20, 10]

C. [10, 20, 30]

D. [10, 30, 20]

Solution

1. Step 1: Trace each append operation

   Start with empty list. Append 10 -> [10], append 20 -> [10, 20], append 30 -> [10, 20, 30].

2. Step 2: Understand stack order

   Elements are added at the end; top is last element (30).

3. Final Answer:

   [10, 20, 30] -> Option C

4. Quick Check:

   Stack after pushes = [10, 20, 30] [OK]

Hint: Append adds at end; last element is top [OK]

Common Mistakes:

• Reversing the order of elements
• Confusing stack top with bottom
• Assuming append adds at start

4. Identify the error in the following code that tries to push an element 15 into a stack implemented as a list:

stack = [5, 10]
stack.push(15)

medium

A. Missing import statement for stack

B. Using push() method which does not exist for list

C. Stack should be initialized as a tuple

D. Using append() instead of push()

Solution

1. Step 1: Check list methods for adding elements

   Python lists do not have a push() method; they use append().

2. Step 2: Analyze the code error

   Calling stack.push(15) will cause an AttributeError because push is undefined.

3. Final Answer:

   Using push() method which does not exist for list -> Option B

4. Quick Check:

   List has append(), not push() [OK]

Hint: Use append() for list push, not push() [OK]

Common Mistakes:

• Assuming push() exists for Python lists
• Thinking stack must be tuple
• Ignoring AttributeError on push()

5. Given a stack implemented as a list, how would you push multiple elements [1, 2, 3] onto the stack so that 3 is on top after all pushes?

hard

A. Use a loop to append elements in order: 1, then 2, then 3

B. Use a loop to append elements in reverse order: 3, then 2, then 1

C. Use extend() method with reversed([1, 2, 3])

D. Insert elements at index 0 in order: 1, then 2, then 3

Solution

1. Step 1: Understand stack push order

   To have 3 on top, push 1 first, then 2, then 3 so 3 is last pushed.

2. Step 2: Analyze each option

   Loop append in order 1,2,3 puts 3 on top. Reverse order puts 1 on top. extend(reversed([1,2,3])) adds 3,2,1 so 1 on top. Insert at index 0 in order ends with [3,2,1], 1 on top.

3. Final Answer:

   Use a loop to append elements in order: 1, then 2, then 3 -> Option A

4. Quick Check:

   Push in order to get last element on top [OK]

Hint: Push elements in order to have last on top [OK]

Common Mistakes:

• Pushing in reverse order causing wrong top
• Using insert(0, val) which adds at bottom
• Using extend(reversed([1, 2, 3])) which puts wrong order on top