Python Loops
From for to mastery

def basic_for_loop():
    fruits = ["apple", "banana", "cherry"]

    for fruit in fruits:
        print(f"I like {fruit}")

    print("Done!")

basic_for_loop()

def string_iteration():
    word = "Python"

    for char in word:
        print(f"Char: {char} → uppercase: {char.upper()}")

string_iteration()

def range_stop_demo():
    print("Squares of 0-4:")
    for i in range(5):
        print(f"{i}² = {i ** 2}")

    print(f"\nRange as list: {list(range(5))}")

range_stop_demo()

def range_full_demo():
    print("Even numbers 2-8:")
    for i in range(2, 10, 2):
        print(i, end=" ")
    print()

    print("\nCountdown 10-1:")
    for i in range(10, 0, -1):
        print(i, end=" ")
    print()

    print(f"\nSum 1-100: {sum(range(1, 101))}")

range_full_demo()

def list_with_index():
    colors = ["red", "green", "blue", "yellow"]

    print("Using enumerate():")
    for idx, color in enumerate(colors):
        print(f"{idx}: {color}")

    print("\nUsing range(len()):")
    for i in range(len(colors)):
        print(f"{i}: {colors[i]}")

list_with_index()

def dict_iteration():
    person = {"name": "Alice", "age": 30, "city": "Paris"}

    print("Keys:")
    for key in person:
        print(f"  {key}")

    print("\nValues:")
    for value in person.values():
        print(f"  {value}")

    print("\nKey-Value pairs:")
    for key, value in person.items():
        print(f"  {key}: {value}")

dict_iteration()

def while_count():
    count = 0
    while count < 5:
        print(f"Count: {count}")
        count += 1
    print("Loop finished!")

while_count()

def while_user_input():
    valid = False
    while not valid:
        try:
            num = int(input("Enter a number: "))
            valid = True
        except ValueError:
            print("That's not a valid number. Try again.")
    print(f"Valid number: {num}")

while_user_input()

def break_demo():
    for i in range(1, 11):
        if i == 6:
            print("Breaking!")
            break
        print(i)
    print("Loop exited.")

break_demo()

def continue_demo():
    for i in range(1, 11):
        if i % 2 == 0:
            print(f"Skipping {i} (even)")
            continue
        print(f"Odd number: {i}")

continue_demo()

def pass_demo():
    print("Looping with pass:")
    for i in range(5):
        pass
    print("Loop done!")

    print("\nUsing pass in an if statement:")
    x = 10
    if x > 5:
        pass
    else:
        print("x is not greater than 5")

pass_demo()

def multiplication_table():
    for i in range(1, 5):
        for j in range(1, 5):
            print(f"{i * j:4}", end="")
        print()

multiplication_table()

def matrix_iteration():
    matrix = [
        [1, 2, 3],
        [4, 5, 6],
        [7, 8, 9]
    ]

    for row in matrix:
        for element in row:
            print(element, end=" ")
        print()

matrix_iteration()

def list_comp_demo():
    numbers = [1, 2, 3, 4, 5]
    squares = [x**2 for x in numbers]
    print(f"Numbers: {numbers}")
    print(f"Squares: {squares}")

    print(f"Squares (loop): {squares_loop}")

list_comp_demo()

def comp_with_condition():
    numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    evens = [x for x in numbers if x % 2 == 0]
    print(f"All: {numbers}")
    print(f"Evens: {evens}")

    print(f"Evens (loop): {evens_loop}")

comp_with_condition()

def for_else_demo():
    numbers = [1, 3, 5, 7, 9]
    search = 5

    for num in numbers:
        if num == search:
            print(f"Found {search}!")
            break
    else:
        print(f"{search} not found.")

for_else_demo()

def while_else_demo():
    password = "secret"
    attempts = 0

    while attempts < 3:
        guess = input("Enter password: ")
        attempts += 1
        if guess == password:
            print("Access granted!")
            break
    else:
        print("Access denied. Too many attempts.")

while_else_demo()

def enumerate_demo():
    fruits = ["apple", "banana", "cherry", "date"]

    for idx, fruit in enumerate(fruits):
        print(f"{idx}: {fruit}")

    print("\nStarting at 1:")
    for idx, fruit in enumerate(fruits, start=1):
        print(f"{idx}: {fruit}")

enumerate_demo()

def zip_demo():
    names = ["Alice", "Bob", "Charlie", "Diana"]
    ages = [30, 25, 35, 28]

    for name, age in zip(names, ages):
        print(f"{name} is {age} years old.")

    print()

    cities = ["New York", "London", "Paris", "Tokyo"]
    for name, age, city in zip(names, ages, cities):
        print(f"{name} ({age}) lives in {city}")

zip_demo()

def infinite_with_break():
    print("Type 'quit' to exit.")
    while True:
        user_input = input("> ")
        if user_input.lower() == "quit":
            print("Goodbye!")
            break
        print(f"You typed: {user_input}")

infinite_with_break()

def safe_infinite_loop():
    print("Press Ctrl+C to stop.")
    count = 0
    try:
        while True:
            print(f"Running... {count}")
            count += 1
            import time
            time.sleep(0.5)
    except KeyboardInterrupt:
        print("\nStopped by user. Goodbye!")

safe_infinite_loop()

def process_file():
    try:
        with open("sample.txt", "r", encoding="utf-8") as f:
            line_count = 0
            word_count = 0
            char_count = 0

            for line in f:
                line_count += 1
                word_count += len(line.split())
                char_count += len(line)

            print(f"Lines: {line_count}")
            print(f"Words: {word_count}")
            print(f"Characters: {char_count}")
    except FileNotFoundError:
        print("File not found.")

process_file()

def filter_transform():
    raw_data = [1, "hello", 3, "world", 5, 6.5, 7, "python"]

    cleaned = []
    for item in raw_data:
        if isinstance(item, (int, float)):
            cleaned.append(item ** 2)

    print(f"Raw: {raw_data}")
    print(f"Cleaned and squared: {cleaned}")

filter_transform()

def flatten_matrix():
    matrix = [
        [1, 2, 3],
        [4, 5, 6],
        [7, 8, 9]
    ]

    flat = []
    for row in matrix:
        for element in row:
            flat.append(element)

    print(f"Matrix: {matrix}")
    print(f"Flattened: {flat}")

flatten_matrix()