🔍 Day 5: Functions and Recursion in Python

📌 What is a Function?

A function is a block of organized, reusable code used to perform a single, related action.

🛠️ How to Define and Call a Function

def greet():
    print("Hello, World!")
greet()

🧩 Types of Functions

• Built-in functions (e.g., print(), len())
• User-defined functions
• Lambda functions (anonymous functions)

⚙️ Function Arguments

• Default arguments: def greet(name="Guest")
• Keyword arguments: greet(name="Alice")
• Variable-length: *args, **kwargs

🔄 Return Statement

Use return to send a result back from the function.

⚡ Lambda Functions

One-line anonymous function: square = lambda x: x*x

🧠 What is Recursion?

Recursion is a process in which a function calls itself directly or indirectly.

✅ Base Case & Recursive Case

def factorial(n):
    if n == 0:
        return 1  # base case
    return n * factorial(n - 1)  # recursive case

🆚 Recursion vs Iteration

• Recursion is elegant and closer to mathematical logic.
• Iteration uses loops and is generally more memory efficient.

💡 Common Recursion Examples

• Factorial of a number
• Fibonacci series
• Sum of digits
• GCD calculation

🧪 Interview Questions on Functions & Recursion

1. What is the difference between a function and a method?
2. How does Python handle default arguments?
3. What are *args and **kwargs?
4. Explain recursion with an example.
5. How would you prevent a recursion from running infinitely?
6. What are the advantages of using recursion?
7. Write a recursive function to calculate the factorial of a number.
8. Compare recursion with iteration. Which is better and why?
9. What is a lambda function and where would you use it?
10. How do you return multiple values from a function?