Chapter 3: Stacks in Python 🚀¶

Prerequisites: Python Lists, Python Functions

Mentor's Note: A Stack is one of the most fundamental structures in computer science. Every time you press "Undo" (Ctrl+Z) or hit the "Back" button in your browser, you are using a Stack! 💡

🌟 The Scenario: The Pizza Box Stack 🍕¶

Imagine you are at a pizza delivery hub in Surat.

The Logic: As new pizzas come out of the oven, they are stacked one on top of the other. 📦
The Rule: You can only take the Top box. To get to the bottom box, you must first remove all the ones above it. 📦
The Result: This is LIFO (Last In, First Out). The last pizza placed on the stack is the first one to be delivered. ✅

📖 Concept Explanation¶

1. What is a Stack?¶

A stack is a linear data structure that follows the principle of Last In, First Out (LIFO).

2. Core Operations 🛠️¶

Operation	Logic	Python Method
PUSH	Add an item to the top.	`.append()`
POP	Remove the top item.	`.pop()`
PEEK	Look at the top item without removing it.	`stack[-1]`
isEmpty	Check if the stack is empty.	`len(stack) == 0`

🎨 Visual Logic: LIFO in Action¶

graph TD
    A[Bottom] --- B[Middle]
    B --- C[TOP 🍕]

    D[New Item] -- PUSH --> C
    C -- POP --> E[Removed Item]

💻 Implementation: The Stack Lab¶

Python (CBSE Style)

# 🛒 Scenario: Managing a stack of boxes
# 🚀 Action: Implementing PUSH and POP

def push(stack, item):
    stack.append(item)
    print(f"Pushed: {item} 📥")

def pop(stack):
    if len(stack) == 0:
        print("Underflow! Stack is empty. ❌")
        return None
    return stack.pop()

# 🏗️ Testing the stack
my_stack = []
push(my_stack, "Box 1")
push(my_stack, "Box 2")

removed = pop(my_stack)
print(f"Removed: {removed} 📤")

# 🛍️ Outcome: Prints "Removed: Box 2" (LIFO)

📊 Sample Dry Run¶

Step	Operation	Stack State	Result
1	`push(10)`	`[10]`	10 is at Top
2	`push(20)`	`[10, 20]`	20 is at Top
3	`pop()`	`[10]`	20 is removed! ✅

📈 Technical Analysis¶

Underflow: Trying to pop from an empty stack. ⚠️
Overflow: Trying to push into a stack that has no more memory (rare in Python).

🎯 Practice Lab 🧪¶

Task: The String Reverser

Task: Use a stack to reverse a string (e.g., "HELLO" -> "OLLEH"). Hint: Push each letter into a stack, then pop them all out! 💡

💡 Board Exam Focus (CBSE Class 12) 👔¶

Important: You will likely have to write a complete Python script including push() and pop() functions for 3-5 marks. Practice the "Underflow" check!

💡 Pro Tip: "A stack is not just a data structure; it's a way of looking at the world. Last in, first out!" - Anonymous