10_loops_tutorial.py

# ========== FOR LOOP ==========

# 1. Basic for loop (over a list)
fruits = ["apple", "banana", "orange"]

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

# 2. For loop with range()
# range(stop) - from 0 to stop-1
for i in range(5):
    print(f"i = {i}")  # 0, 1, 2, 3, 4

# range(start, stop) - from start to stop-1
for i in range(2, 6):
    print(f"i = {i}")  # 2, 3, 4, 5

# range(start, stop, step) - with step
for i in range(0, 10, 2):
    print(f"i = {i}")  # 0, 2, 4, 6, 8

# In reverse order
for i in range(10, 0, -1):
    print(f"i = {i}")  # 10, 9, 8, ..., 1

# 3. For loop with enumerate() (getting index and value)
fruits = ["apple", "banana", "orange"]

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

# With specified starting index
for index, fruit in enumerate(fruits, start=1):
    print(f"#{index}: {fruit}")

# 4. For loop over a dictionary
person = {"name": "Pavel", "age": 25, "city": "Moscow"}

# By keys (default)
for key in person:
    print(f"{key}: {person[key]}")

# By key-value pairs (recommended!)
for key, value in person.items():
    print(f"{key}: {value}")

# Only by values
for value in person.values():
    print(value)

# Only by keys (explicitly)
for key in person.keys():
    print(key)

# 5. For loop over a string
text = "Python"

for char in text:
    print(f"Character: {char}")

# 6. Nested loops
for i in range(3):
    for j in range(3):
        print(f"i={i}, j={j}")

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i * j}")

# 7. For loop with zip() (iterating over multiple lists simultaneously)
names = ["Pavel", "Ivan", "Maria"]
ages = [25, 30, 28]
cities = ["Moscow", "SPb", "Kazan"]

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

# ========== WHILE LOOP ==========

# 8. Basic while loop
count = 0

while count < 5:
    print(f"count = {count}")
    count += 1

# 9. Infinite loop with break
count = 0

while True:
    print(f"count = {count}")
    count += 1

    if count >= 5:
        break  # Exit the loop

# 10. While with input condition
# number = 0
# while number != 42:
#     number = int(input("Enter a number (42 to exit): "))
#     print(f"You entered: {number}")

# ========== LOOP CONTROL ==========

# 11. break - interrupt the loop
for i in range(10):
    if i == 5:
        print("Reached 5, exiting")
        break
    print(f"i = {i}")

# 12. continue - skip iteration
for i in range(10):
    if i % 2 == 0:  # Skip even numbers
        continue
    print(f"Odd number: {i}")

# 13. else in loops (executes if the loop finished WITHOUT break)
# With break
for i in range(5):
    if i == 3:
        print("Found 3, interrupting")
        break
else:
    print("Loop completed fully")  # Will NOT execute

# Without break
for i in range(5):
    pass
else:
    print("Loop completed fully")  # Will execute

# Practical application: element search
numbers = [1, 2, 3, 4, 5]
search = 3

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

# 14. pass - empty operation (placeholder)
for i in range(5):
    pass  # Do nothing (used as a temporary placeholder)

# ========== PRACTICAL EXAMPLES ==========

# 15. Sum of numbers
numbers = [1, 2, 3, 4, 5]
total = 0

for num in numbers:
    total += num

print(f"Sum: {total}")

# 16. Finding maximum
numbers = [3, 7, 2, 9, 1, 5]
max_num = numbers[0]

for num in numbers[1:]:
    if num > max_num:
        max_num = num

print(f"Maximum: {max_num}")

# 17. Counting even numbers
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_count = 0

for num in numbers:
    if num % 2 == 0:
        even_count += 1

print(f"Even numbers: {even_count}")

# 18. Creating a new list based on the old one
numbers = [1, 2, 3, 4, 5]
squared = []

for num in numbers:
    squared.append(num ** 2)

print(f"Squares: {squared}")

# 19. Nested lists (matrix)
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

for row in matrix:
    for element in row:
        print(element, end=" ")
    print()  # New line after each matrix row

# 20. Iteration with list modification (be careful!)
# Wrong: modifying list during iteration
numbers = [1, 2, 3, 4, 5]
# for num in numbers:
#     if num % 2 == 0:
#         numbers.remove(num)  # May skip elements!

# Correct: create a copy or use list comprehension
numbers = [1, 2, 3, 4, 5]
numbers = [num for num in numbers if num % 2 != 0]
print(f"Odd: {numbers}")