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  <title>Python Chapter 6 - Functions</title>
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<header>
  <h1>Chapter 6: Functions in Python</h1>
  <p>Function banana, use karna, arguments dena, return lena – sab kuch simple Hinglish me!</p>
</header>

<div class="section" id="6python">
  <h2>Chapter 6: Functions in Python</h2>
  <p>Function ka matlab hota hai — ek reusable block of code jo baar-baar call kiya ja sakta hai. Isme hum arguments de sakte hain, return value le sakte hain, aur code ko modular aur readable bana sakte hain.</p>

  <h3>Function Define Karna:</h3>
  <div class="code-block">
    def greet():<br>
    &nbsp;&nbsp;print("Hello!")
  </div>
  <div class="output-block">Hello!</div>

  <h3>Function Call Karna:</h3>
  <div class="code-block">
    greet()
  </div>

  <h3>Function with Parameters:</h3>
  <div class="code-block">
    def greet(name):<br>
    &nbsp;&nbsp;print("Hello", name)<br><br>
    greet("Ravi")
  </div>
  <div class="output-block">Hello Ravi</div>

  <h3>Return Value:</h3>
  <div class="code-block">
    def add(a, b):<br>
    &nbsp;&nbsp;return a + b<br><br>
    result = add(2, 3)<br>
    print(result)
  </div>
  <div class="output-block">5</div>

  <h3>General Mistakes:</h3>
  <div class="code-block wrong">
    def add(a, b):<br>
    &nbsp;&nbsp;a + b   # ❌ Return nahi diya<br><br>
    print(add(2, 3))  # ❌ Output: None
  </div>
</div>

<!-- Recursion Section -->
<div class="section">
  <h2>Recursion in Python</h2>
  <p>Recursion matlab ek function khud ko call karta hai jab tak ek condition satisfy na ho jaaye. Isko step-by-step solve kiya jaata hai.</p>

  <h3>Structure of Recursive Function:</h3>
  <div class="code-block">
    def function_name():<br>
    &nbsp;&nbsp;if base_condition:<br>
    &nbsp;&nbsp;&nbsp;&nbsp;return result<br>
    &nbsp;&nbsp;else:<br>
    &nbsp;&nbsp;&nbsp;&nbsp;return function_name()
  </div>

  <h3>Example: Countdown</h3>
  <div class="code-block">
    def countdown(n):<br>
    &nbsp;&nbsp;if n == 0:<br>
    &nbsp;&nbsp;&nbsp;&nbsp;print("Liftoff!")<br>
    &nbsp;&nbsp;else:<br>
    &nbsp;&nbsp;&nbsp;&nbsp;print(n)<br>
    &nbsp;&nbsp;&nbsp;&nbsp;countdown(n - 1)<br><br>
    countdown(5)
  </div>
  <div class="output-block">5<br>4<br>3<br>2<br>1<br>Liftoff!</div>

  <h3>Rules of Recursion:</h3>
  <ul>
    <li>Base case zaroori hota hai – ye stopping point hai.</li>
    <li>Har recursive call input ko chhota banata hai.</li>
    <li>Python me ~1000 depth tak hi jaa sakte ho.</li>
  </ul>

  <h3>Common Mistake:</h3>
  <div class="code-block wrong">
    def bad(n):<br>
    &nbsp;&nbsp;return bad(n-1)  # ❌ No base case<br><br>
    bad(5)  # ❌ RecursionError: maximum depth exceeded
  </div>

  <h3>Example: Factorial</h3>
  <div class="code-block">
    def factorial(n):<br>
    &nbsp;&nbsp;if n == 1:<br>
    &nbsp;&nbsp;&nbsp;&nbsp;return 1<br>
    &nbsp;&nbsp;else:<br>
    &nbsp;&nbsp;&nbsp;&nbsp;return n * factorial(n-1)<br><br>
    print(factorial(5))
  </div>
  <div class="output-block">120</div>

  <h3>Advanced Example: Fibonacci</h3>
  <div class="code-block">
    def fib(n):<br>
    &nbsp;&nbsp;if n <= 1:<br>
    &nbsp;&nbsp;&nbsp;&nbsp;return n<br>
    &nbsp;&nbsp;return fib(n-1) + fib(n-2)<br><br>
    print(fib(6))
  </div>
  <div class="output-block">8</div>
</div>

<!-- Lambda Section -->
<div class="section">
  <h2>Lambda Functions</h2>
  <p>Lambda function ek short anonymous function hoti hai, jisme sirf ek expression hota hai. Mostly quick calculations, filter/map me use hoti hai.</p>

  <h3>Syntax:</h3>
  <div class="code-block">
    lambda arguments: expression
  </div>

  <h3>Simple Example:</h3>
  <div class="code-block">
    double = lambda x: x * 2<br>
    print(double(4))
  </div>
  <div class="output-block">8</div>

  <h3>With map():</h3>
  <div class="code-block">
    nums = [1, 2, 3, 4]<br>
    squares = list(map(lambda x: x*x, nums))<br>
    print(squares)
  </div>
  <div class="output-block">[1, 4, 9, 16]</div>

  <h3>With filter():</h3>
  <div class="code-block">
    nums = [1, 2, 3, 4, 5]<br>
    even = list(filter(lambda x: x % 2 == 0, nums))<br>
    print(even)
  </div>
  <div class="output-block">[2, 4]</div>

  <h3>With sorted():</h3>
  <div class="code-block">
    students = [('A', 50), ('B', 70), ('C', 40)]<br>
    students.sort(key=lambda x: x[1])<br>
    print(students)
  </div>
  <div class="output-block">[('C', 40), ('A', 50), ('B', 70)]</div>

  <h3>Common Mistake:</h3>
  <div class="code-block wrong">
    lambda x, y: x + y  # ❌ Not used or assigned<br><br>
    # ✔️ Correct:<br>
    add = lambda x, y: x + y<br>
    print(add(3, 4))
  </div>

  <h3>Lambda inside Function:</h3>
  <div class="code-block">
    def multiplier(n):<br>
    &nbsp;&nbsp;return lambda x: x * n<br><br>
    times3 = multiplier(3)<br>
    print(times3(5))
  </div>
  <div class="output-block">15</div>

  <h3>Lambda vs def:</h3>
  <ul>
    <li><b>Lambda:</b> One-line, quick use, anonymous</li>
    <li><b>def:</b> Multi-line, named, complex logic ke liye</li>
  </ul>
</div>

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