from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

# rect = Rectangle(5, 3)
# print(rect.area())  # 15
# Shape()  # Error: Can't instantiate abstract class

# Basic for loop over iterable (list)
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:  # Iterates each element directly
    print(fruit)      # Output: apple \n banana \n cherry

# For loop with range() for numeric sequences
for i in range(3):    # Generates 0, 1, 2 (start=0, stop=3, step=1)
    print(i)          # Output: 0 \n 1 \n 2

for i in range(1, 6, 2):  # Start=1, stop=6, step=2
    print(i)              # Output: 1 \n 3 \n 5

# Index-aware with enumerate() (gets both index and value)
for index, fruit in enumerate(fruits, start=1):  # start=1 for 1-based indexing
    print(f"{index}: {fruit}")                   # Output: 1: apple \n 2: banana \n 3: cherry

# Parallel iteration with zip() (pairs multiple iterables)
names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
for name, age in zip(names, ages):  # Stops at shortest iterable
    print(f"{name} is {age} years old")  # Output: Alice is 25 years old \n Bob is 30 years old \n Charlie is 35 years old

# Nested for loops (outer for rows, inner for columns; e.g., matrix)
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for row in matrix:         # Outer: each sublist
    for num in row:        # Inner: each element in row
        print(num, end=' ')  # Output: 1 2 3 4 5 6 7 8 9 (space-separated)

# For loop in list comprehension (concise iteration with optional condition)
squares = [x**2 for x in range(5)]  # Basic comprehension
print(squares)  # Output: [0, 1, 4, 9, 16]

evens_squared = [x**2 for x in range(10) if x % 2 == 0]  # With condition (if)
print(evens_squared)  # Output: [0, 4, 16, 36, 64]

# Nested comprehension (flattens 2D list)
flattened = [num for row in matrix for num in row]  # Equivalent to nested for
print(flattened)  # Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]

# For loop: Use for fixed iterations over iterables (e.g., processing lists)
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:  # Iterates each element
    print(fruit)      # Output: apple \n banana \n cherry

for i in range(3):    # Numeric sequence (start=0, stop=3)
    print(i)          # Output: 0 \n 1 \n 2

# While loop: Use when iterations depend on a dynamic condition (e.g., user input, convergence)
count = 0
while count < 3:      # Runs as long as condition is True
    print(count)
    count += 1       # Increment to avoid infinite loop! Output: 0 \n 1 \n 2

# Nested loops: Use for 2D data (e.g., matrices, grids); outer for rows, inner for columns
matrix = [[1, 2], [3, 4]]
for row in matrix:    # Outer: each sublist
    for num in row:   # Inner: elements in row
        print(num)    # Output: 1 \n 2 \n 3 \n 4

# Control statements: break (exit loop), continue (skip iteration)
for i in range(5):
    if i == 2:
        continue    # Skip 2
    if i == 4:
        break       # Exit at 4
    print(i)        # Output: 0 \n 1 \n 3

# List comprehension: Concise for loop alternative (use for simple transformations/filtering)
squares = [x**2 for x in range(5) if x % 2 == 0]  # Even squares
print(squares)  # Output: [0, 4, 16]

# For loop: Use for fixed iterations over iterables (e.g., processing lists)
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:  # Iterates each element
    print(fruit)      # Output: apple \n banana \n cherry

for i in range(3):    # Numeric sequence (start=0, stop=3)
    print(i)          # Output: 0 \n 1 \n 2

# While loop: Use when iterations depend on a dynamic condition (e.g., user input, convergence)
count = 0
while count < 3:      # Runs as long as condition is True
    print(count)
    count += 1       # Increment to avoid infinite loop! Output: 0 \n 1 \n 2

# Nested loops: Use for 2D data (e.g., matrices, grids); outer for rows, inner for columns
matrix = [[1, 2], [3, 4]]
for row in matrix:    # Outer: each sublist
    for num in row:   # Inner: elements in row
        print(num)    # Output: 1 \n 2 \n 3 \n 4

# Control statements: break (exit loop), continue (skip iteration)
for i in range(5):
    if i == 2:
        continue    # Skip 2
    if i == 4:
        break       # Exit at 4
    print(i)        # Output: 0 \n 1 \n 3

# List comprehension: Concise for loop alternative (use for simple transformations/filtering)
squares = [x**2 for x in range(5) if x % 2 == 0]  # Even squares
print(squares)  # Output: [0, 4, 16]