02_while_loop.py

"""
================================================================================
File: 02_while_loop.py
Topic: While Loops in Python
================================================================================

This file demonstrates the 'while' loop in Python, which repeatedly executes
a block of code as long as a condition remains True. Unlike for loops,
while loops are typically used when the number of iterations is not known
in advance.

Key Concepts:
- Basic while loop syntax
- Counter-controlled loops
- Sentinel-controlled loops (input validation)
- Infinite loops and how to handle them
- While-else construct

================================================================================
"""

# -----------------------------------------------------------------------------
# 1. Basic While Loop
# -----------------------------------------------------------------------------
# Executes as long as condition is True

print("--- Basic While Loop ---")

count = 1

while count <= 5:
    print(f"Count: {count}")
    count += 1  # Don't forget to update the condition variable!

print("Loop finished!")

# -----------------------------------------------------------------------------
# 2. Counter-Controlled While Loop
# -----------------------------------------------------------------------------
# When you know how many iterations you need

print("\n--- Counter-Controlled Loop ---")

# Countdown
countdown = 5

print("Rocket Launch Countdown:")
while countdown > 0:
    print(f"T-minus {countdown}...")
    countdown -= 1
print("Liftoff! 🚀")

# -----------------------------------------------------------------------------
# 3. Summing Numbers with While
# -----------------------------------------------------------------------------
# Accumulator pattern

print("\n--- Sum of Numbers ---")

# Sum numbers from 1 to 10
n = 1
total = 0

while n <= 10:
    total += n
    n += 1

print(f"Sum of 1 to 10 = {total}")

# -----------------------------------------------------------------------------
# 4. Sentinel-Controlled Loop (Input Validation)
# -----------------------------------------------------------------------------
# Loop until a specific condition is met

print("\n--- Sentinel-Controlled Loop ---")

# Simulating password validation (in practice, use input())
attempts = 0
max_attempts = 3
correct_password = "secret123"

# Simulated user inputs
user_inputs = ["wrong1", "wrong2", "secret123"]

while attempts < max_attempts:
    # In real code: password = input("Enter password: ")
    password = user_inputs[attempts]
    print(f"Attempt {attempts + 1}: Entered '{password}'")
    
    if password == correct_password:
        print("Access granted! ✓")
        break
    else:
        print("Incorrect password.")
        attempts += 1
else:
    # This runs if the loop completes without break
    print("Too many failed attempts. Account locked.")

# -----------------------------------------------------------------------------
# 5. While Loop with User Simulation
# -----------------------------------------------------------------------------
# Menu-driven program example

print("\n--- Menu-Driven Example ---")

# Simulated menu choices
menu_selections = [1, 2, 4, 3]
selection_index = 0

running = True
while running:
    # Simulating menu selection
    choice = menu_selections[selection_index]
    selection_index += 1
    
    print(f"\nMenu: 1=Add, 2=View, 3=Exit | Choice: {choice}")
    
    if choice == 1:
        print("  ➤ Adding item...")
    elif choice == 2:
        print("  ➤ Viewing items...")
    elif choice == 3:
        print("  ➤ Exiting program...")
        running = False
    else:
        print("  ➤ Invalid choice, try again.")

# -----------------------------------------------------------------------------
# 6. Infinite Loops (and how to avoid them)
# -----------------------------------------------------------------------------
# A loop that never ends - usually a bug

print("\n--- Avoiding Infinite Loops ---")

# BAD: This would run forever (commented out)
# i = 0
# while i < 5:
#     print(i)
#     # Missing: i += 1

# GOOD: Always update the condition variable
i = 0
while i < 5:
    print(f"i = {i}")
    i += 1  # This is crucial!

# -----------------------------------------------------------------------------
# 7. While-Else Construct
# -----------------------------------------------------------------------------
# The else block executes if the loop completes normally (no break)

print("\n--- While-Else Construct ---")

# Example 1: Loop completes normally
n = 5
while n > 0:
    print(n, end=" ")
    n -= 1
else:
    print("\n  ↳ Countdown complete! (else block executed)")

# Example 2: Loop exits via break
print("\nSearching in list:")
items = ["a", "b", "c", "d"]
target = "b"
index = 0

while index < len(items):
    if items[index] == target:
        print(f"  Found '{target}' at index {index}")
        break
    index += 1
else:
    print(f"  '{target}' not found")

# -----------------------------------------------------------------------------
# 8. Nested While Loops
# -----------------------------------------------------------------------------
# A while inside another while

print("\n--- Nested While Loops ---")

# Create a simple pattern
row = 1
while row <= 4:
    col = 1
    while col <= row:
        print("*", end=" ")
        col += 1
    print()  # New line
    row += 1

# -----------------------------------------------------------------------------
# 9. While with Break and Continue
# -----------------------------------------------------------------------------
# Control flow within loops (preview - detailed in next file)

print("\n--- Break and Continue in While ---")

# Break example
print("Break example (stop at 5):")
i = 0
while True:  # Intentional infinite loop
    i += 1
    if i > 5:
        break
    print(i, end=" ")
print()

# Continue example
print("\nContinue example (skip even numbers):")
i = 0
while i < 10:
    i += 1
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i, end=" ")
print()

# -----------------------------------------------------------------------------
# 10. Practical Examples
# -----------------------------------------------------------------------------

print("\n--- Practical Examples ---")

# Fibonacci sequence
print("Fibonacci sequence (first 10 numbers):")
a, b = 0, 1
count = 0
while count < 10:
    print(a, end=" ")
    a, b = b, a + b
    count += 1
print()

# Finding digits in a number
print("\nDigits of 12345:")
number = 12345
while number > 0:
    digit = number % 10
    print(f"  Digit: {digit}")
    number //= 10

# Guessing game logic
print("\n Guess the Number Game Logic:")
secret = 7
guesses = [3, 8, 5, 7]
guess_index = 0

while guess_index < len(guesses):
    guess = guesses[guess_index]
    print(f"  Guess: {guess}", end=" ")
    
    if guess < secret:
        print("- Too low!")
    elif guess > secret:
        print("- Too high!")
    else:
        print("- Correct! 🎉")
        break
    
    guess_index += 1