Bài 10: Loops - Vòng Lặp

Mục Tiêu Bài Học

Sau khi hoàn thành bài này, bạn sẽ:

✅ Hiểu for và while loops
✅ Sử dụng range(), enumerate(), zip()
✅ Kiểm soát loops với break và continue
✅ Làm việc với nested loops
✅ Áp dụng loop patterns thực tế

For Loop

For loop dùng để iterate qua một sequence (list, tuple, string, etc.).

Cú Pháp Cơ Bản

# Syntax: for item in iterable:#             code block # Loop qua listfruits = ["apple", "banana", "orange"]for fruit in fruits:    print(fruit)# apple# banana# orange # Loop qua stringfor char in "Python":    print(char)# P, y, t, h, o, n # Loop qua tuplecolors = ("red", "green", "blue")for color in colors:    print(color)

range() Function

range() tạo sequence của numbers.

# range(stop) - từ 0 đến stop-1for i in range(5):    print(i, end=" ")# 0 1 2 3 4 # range(start, stop) - từ start đến stop-1for i in range(2, 6):    print(i, end=" ")# 2 3 4 5 # range(start, stop, step) - với bước nhảyfor i in range(0, 10, 2):    print(i, end=" ")# 0 2 4 6 8 # Đếm ngượcfor i in range(10, 0, -1):    print(i, end=" ")# 10 9 8 7 6 5 4 3 2 1 # Ứng dụng# Print multiplication tablen = 5for i in range(1, 11):    print(f"{n} x {i} = {n * i}")

enumerate() - Loop với Index

enumerate() trả về cả index và value.

fruits = ["apple", "banana", "orange"] # Với enumeratefor index, fruit in enumerate(fruits):    print(f"{index}: {fruit}")# 0: apple# 1: banana# 2: orange # Start index từ 1for index, fruit in enumerate(fruits, start=1):    print(f"{index}. {fruit}")# 1. apple# 2. banana# 3. orange # Ứng dụng: tìm vị trínumbers = [10, 20, 30, 40, 50]for i, num in enumerate(numbers):    if num == 30:        print(f"Found 30 at index {i}")

zip() - Loop Nhiều Iterables

zip() kết hợp nhiều iterables.

names = ["Alice", "Bob", "Charlie"]ages = [25, 30, 35] # Zip 2 listsfor name, age in zip(names, ages):    print(f"{name} is {age} years old")# Alice is 25 years old# Bob is 30 years old# Charlie is 35 years old # Zip 3 listscities = ["Hanoi", "HCMC", "Danang"]for name, age, city in zip(names, ages, cities):    print(f"{name}, {age}, lives in {city}") # Zip với enumeratefor i, (name, age) in enumerate(zip(names, ages), 1):    print(f"{i}. {name}: {age}") # ⚠️ Zip stops at shortestlist1 = [1, 2, 3, 4, 5]list2 = ["a", "b", "c"]for num, letter in zip(list1, list2):    print(num, letter)# 1 a# 2 b# 3 c

Loop qua Dictionary

person = {    "name": "Alice",    "age": 25,    "city": "Hanoi"} # Loop qua keys (default)for key in person:    print(key)# name, age, city # Loop qua keys (explicit)for key in person.keys():    print(f"{key}: {person[key]}") # Loop qua valuesfor value in person.values():    print(value) # Loop qua items (key-value pairs) - recommendedfor key, value in person.items():    print(f"{key}: {value}")

While Loop

While loop chạy khi condition còn True.

Cú Pháp Cơ Bản

# Syntax: while condition:#             code block # Đếm từ 1 đến 5count = 1while count <= 5:    print(count)    count += 1# 1, 2, 3, 4, 5 # Sum numberstotal = 0num = 1while num <= 10:    total += num    num += 1print(f"Sum: {total}")  # 55 # User input loopwhile True:    answer = input("Continue? (yes/no): ")    if answer.lower() == "no":        break    print("Continuing...")

While với Condition

# Đợi đến khi đủ điều kiệnpassword = ""while password != "secret":    password = input("Enter password: ")    if password != "secret":        print("Wrong password, try again")print("Access granted!") # Tìm số chia hếtn = 100while n % 7 != 0:    n += 1print(f"First number >= 100 divisible by 7: {n}") # Fibonaccia, b = 0, 1while a < 100:    print(a, end=" ")    a, b = b, a + b# 0 1 1 2 3 5 8 13 21 34 55 89

Loop Control - break và continue

break - Thoát Loop

break thoát khỏi loop ngay lập tức.

# Tìm số đầu tiênnumbers = [1, 3, 5, 8, 10, 12]for num in numbers:    if num % 2 == 0:        print(f"Found even number: {num}")        break# Found even number: 8 (dừng ngay) # Search in listnames = ["Alice", "Bob", "Charlie", "Diana"]search = "Charlie"found = False for name in names:    if name == search:        print(f"Found {search}!")        found = True        break if not found:    print(f"{search} not found") # Break khỏi whilecount = 0while True:    count += 1    if count > 5:        break    print(count)# 1, 2, 3, 4, 5

continue - Skip Iteration

continue bỏ qua iteration hiện tại, chuyển sang iteration tiếp theo.

# Skip số chẵnfor i in range(10):    if i % 2 == 0:        continue    print(i, end=" ")# 1 3 5 7 9 # Skip negativenumbers = [1, -2, 3, -4, 5, -6]for num in numbers:    if num < 0:        continue    print(num, end=" ")# 1 3 5 # Process valid itemsitems = ["apple", "", "banana", None, "orange", ""]for item in items:    if not item:  # Skip empty/None        continue    print(f"Processing: {item}")

else Clause trong Loop

else chạy khi loop kết thúc bình thường (không break).

# Tìm số nguyên tốnumber = 17for i in range(2, number):    if number % i == 0:        print(f"{number} is not prime")        breakelse:    print(f"{number} is prime") # Search với elsenames = ["Alice", "Bob", "Charlie"]search = "Diana" for name in names:    if name == search:        print(f"Found {search}")        breakelse:    print(f"{search} not found") # While với elsecount = 0while count < 5:    print(count)    count += 1else:    print("Loop completed normally")

Nested Loops

Nested loops là loops bên trong loops.

# Basic nested loopfor i in range(3):    for j in range(3):        print(f"({i}, {j})", end=" ")    print()# (0, 0) (0, 1) (0, 2)# (1, 0) (1, 1) (1, 2)# (2, 0) (2, 1) (2, 2) # Multiplication tablefor i in range(1, 6):    for j in range(1, 11):        print(f"{i}x{j}={i*j:2d}", end="  ")    print() # Pattern printing# *# **# ***# ****for i in range(1, 5):    for j in range(i):        print("*", end="")    print() # 2D list iterationmatrix = [    [1, 2, 3],    [4, 5, 6],    [7, 8, 9]] for row in matrix:    for item in row:        print(item, end=" ")    print()

Break trong Nested Loops

# Break chỉ thoát inner loopfor i in range(3):    for j in range(3):        if j == 1:            break        print(f"({i}, {j})")# (0, 0), (1, 0), (2, 0) # Break cả outer loop (dùng flag)found = Falsefor i in range(3):    for j in range(3):        if i == 1 and j == 1:            found = True            break    if found:        break    print(f"Row {i} complete")

List Comprehension vs Loop

# Traditional loopsquares = []for i in range(10):    squares.append(i ** 2) # List comprehension (preferred)squares = [i ** 2 for i in range(10)] # With condition# Traditionalevens = []for i in range(10):    if i % 2 == 0:        evens.append(i) # List comprehensionevens = [i for i in range(10) if i % 2 == 0]

Ví Dụ Thực Tế

1. Sum và Average

numbers = [85, 90, 78, 92, 88] # Calculate sumtotal = 0for num in numbers:    total += num average = total / len(numbers)print(f"Sum: {total}")print(f"Average: {average:.2f}") # Find max and minmax_val = numbers[0]min_val = numbers[0] for num in numbers:    if num > max_val:        max_val = num    if num < min_val:        min_val = num print(f"Max: {max_val}")print(f"Min: {min_val}") # Hoặc dùng built-inprint(f"Sum: {sum(numbers)}")print(f"Max: {max(numbers)}")print(f"Min: {min(numbers)}")

def show_menu():    print("\n=== Menu ===")    print("1. Add item")    print("2. View items")    print("3. Delete item")    print("4. Exit") items = [] while True:    show_menu()    choice = input("Choose option: ")        if choice == "1":        item = input("Enter item: ")        items.append(item)        print(f"Added: {item}")        elif choice == "2":        if items:            print("\nItems:")            for i, item in enumerate(items, 1):                print(f"  {i}. {item}")        else:            print("No items")        elif choice == "3":        if items:            for i, item in enumerate(items, 1):                print(f"  {i}. {item}")            idx = int(input("Delete index: ")) - 1            if 0 <= idx < len(items):                deleted = items.pop(idx)                print(f"Deleted: {deleted}")        else:            print("No items to delete")        elif choice == "4":        print("Goodbye!")        break        else:        print("Invalid choice")

3. Password Validator

def validate_password(password):    """Validate password strength"""        # Requirements    min_length = 8    has_upper = False    has_lower = False    has_digit = False    has_special = False        # Check length    if len(password) < min_length:        return False, "Too short (min 8 characters)"        # Check characters    for char in password:        if char.isupper():            has_upper = True        elif char.islower():            has_lower = True        elif char.isdigit():            has_digit = True        elif char in "!@#$%^&*":            has_special = True        # Validate    if not has_upper:        return False, "Need uppercase letter"    if not has_lower:        return False, "Need lowercase letter"    if not has_digit:        return False, "Need digit"    if not has_special:        return False, "Need special character"        return True, "Strong password" # Testpasswords = ["weak", "Password1", "Pass123!", "StrongP@ss1"]for pwd in passwords:    valid, message = validate_password(pwd)    status = "✓" if valid else "✗"    print(f"{status} {pwd:<15} - {message}")

4. Pattern Printing

# Trianglen = 5print("Right Triangle:")for i in range(1, n + 1):    print("*" * i) # Inverted Triangleprint("\nInverted Triangle:")for i in range(n, 0, -1):    print("*" * i) # Pyramidprint("\nPyramid:")for i in range(1, n + 1):    spaces = " " * (n - i)    stars = "*" * (2 * i - 1)    print(spaces + stars) # Diamondprint("\nDiamond:")# Upper halffor i in range(1, n + 1):    print(" " * (n - i) + "*" * (2 * i - 1))# Lower halffor i in range(n - 1, 0, -1):    print(" " * (n - i) + "*" * (2 * i - 1)) # Number patternprint("\nNumber Pattern:")for i in range(1, 6):    for j in range(1, i + 1):        print(j, end=" ")    print()

5. Prime Number Finder

def is_prime(n):    """Check if number is prime"""    if n < 2:        return False    for i in range(2, int(n ** 0.5) + 1):        if n % i == 0:            return False    return True # Find all primes up to 100primes = []for num in range(2, 101):    if is_prime(num):        primes.append(num) print(f"Primes up to 100: {len(primes)}")print(primes) # Print in rows of 10print("\nFormatted:")for i, prime in enumerate(primes):    print(f"{prime:3d}", end=" ")    if (i + 1) % 10 == 0:        print()

6. Shopping Cart

cart = [] while True:    print("\n=== Shopping Cart ===")    print("1. Add item")    print("2. View cart")    print("3. Calculate total")    print("4. Checkout")    print("5. Exit")        choice = input("Choose: ")        if choice == "1":        name = input("Product name: ")        price = float(input("Price: "))        qty = int(input("Quantity: "))        cart.append({"name": name, "price": price, "qty": qty})        print(f"Added {qty}x {name}")        elif choice == "2":        if not cart:            print("Cart is empty")        else:            print("\nYour Cart:")            for i, item in enumerate(cart, 1):                subtotal = item["price"] * item["qty"]                print(f"{i}. {item['name']:<15} ${item['price']:.2f} x{item['qty']} = ${subtotal:.2f}")        elif choice == "3":        total = sum(item["price"] * item["qty"] for item in cart)        print(f"\nTotal: ${total:.2f}")        elif choice == "4":        if cart:            print("\n=== Checkout ===")            total = 0            for item in cart:                subtotal = item["price"] * item["qty"]                total += subtotal                print(f"{item['name']:<15} ${subtotal:.2f}")                        print(f"\nTotal: ${total:.2f}")            print("Thank you for shopping!")            cart.clear()        else:            print("Cart is empty")        elif choice == "5":        print("Goodbye!")        break

Performance Tips

# 1. Avoid modifying list while iterating# ❌ Saiitems = [1, 2, 3, 4, 5]for item in items:    if item % 2 == 0:        items.remove(item)  # Dangerous! # ✅ Đúngitems = [item for item in items if item % 2 != 0] # 2. Use enumerate thay vì range(len())# ❌ Không tốtfruits = ["apple", "banana", "orange"]for i in range(len(fruits)):    print(f"{i}: {fruits[i]}") # ✅ Tốt hơnfor i, fruit in enumerate(fruits):    print(f"{i}: {fruit}") # 3. Use built-in functions# ❌ Chậmtotal = 0for num in numbers:    total += num # ✅ Nhanh hơntotal = sum(numbers)

Bài Tập Thực Hành

Bài 1: Factorial

Tính n! (factorial) bằng loop.

Bài 2: Fibonacci

In n số Fibonacci đầu tiên.

Bài 3: Palindrome

Check string có phải palindrome không (đọc xuôi = đọc ngược).

Bài 4: Guess Number

Game đoán số:

Random số từ 1-100
User đoán
Hint: too high/too low
Count attempts

Bài 5: Pattern

Print pattern:

    *   ***  ***** ****************

Tóm Tắt

✅ For loop: for item in iterable
✅ range(): range(start, stop, step)
✅ enumerate(): Loop với index
✅ zip(): Kết hợp nhiều iterables
✅ While loop: while condition:
✅ break: Thoát loop
✅ continue: Skip iteration
✅ else: Chạy khi loop kết thúc bình thường
✅ Nested loops: Loop trong loop

Bài Tiếp Theo

Bài 11: Functions - Định nghĩa functions, parameters, return values, scope, và lambda functions.

Remember:

For loop cho iterables, while loop cho conditions
enumerate() tốt hơn range(len())
break thoát loop, continue skip iteration
Avoid modifying list while iterating
List comprehension thường tốt hơn loop đơn giản
Practice để master loops!

#break-continue

Loops - Vòng Lặp Trong Python