Lists in Python

# Empty list - two ways
empty1 = []
empty2 = list()

# List with items
fruits = ["apple", "banana", "cherry"]
numbers = [1, 2, 3, 4, 5]

# Mixed types (allowed, but not recommended)
mixed = [1, "hello", 3.14, True, None]

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

# From other iterables
from_string = list("hello")  # ['h', 'e', 'l', 'l', 'o']
from_range = list(range(5))  # [0, 1, 2, 3, 4]

# Repeat elements
zeros = [0] * 5  # [0, 0, 0, 0, 0]

fruits = ["apple", "banana", "cherry", "date"]
#          0         1         2        3     (positive)
#         -4        -3        -2       -1     (negative)

# Indexing
fruits[0]    # "apple" (first)
fruits[-1]   # "date" (last)
fruits[2]    # "cherry"

# Slicing [start:end:step]
fruits[1:3]   # ["banana", "cherry"]
fruits[:2]    # ["apple", "banana"]
fruits[2:]    # ["cherry", "date"]
fruits[::2]   # ["apple", "cherry"] (every 2nd)
fruits[::-1]  # Reversed list

# Nested access
matrix = [[1, 2], [3, 4]]
matrix[0][1]  # 2

nums = [1, 2, 3]

# Change element
nums[0] = 10  # [10, 2, 3]

# Add elements
nums.append(4)           # [10, 2, 3, 4] - add to end
nums.insert(0, 0)        # [0, 10, 2, 3, 4] - add at index
nums.extend([5, 6])      # [0, 10, 2, 3, 4, 5, 6] - add multiple

# Remove elements
nums.pop()               # Remove and return last (6)
nums.pop(0)              # Remove and return at index (0)
nums.remove(10)          # Remove first occurrence of value
del nums[0]              # Delete by index
nums.clear()             # Remove all elements

# Difference: append vs extend
a = [1, 2]
a.append([3, 4])   # [1, 2, [3, 4]]

b = [1, 2]
b.extend([3, 4])   # [1, 2, 3, 4]

nums = [3, 1, 4, 1, 5, 9, 2, 6]

# Searching
nums.index(4)      # 2 (first occurrence)
nums.count(1)      # 2 (occurrences)
5 in nums          # True (membership)

# Sorting
nums.sort()                    # [1, 1, 2, 3, 4, 5, 6, 9]
nums.sort(reverse=True)        # [9, 6, 5, 4, 3, 2, 1, 1]

# sorted() returns new list (original unchanged)
original = [3, 1, 2]
new_sorted = sorted(original)  # [1, 2, 3]

# Custom sort
words = ["banana", "pie", "apple"]
words.sort(key=len)            # ['pie', 'apple', 'banana']

# Basic syntax: [expression for item in iterable]
squares = [x**2 for x in range(5)]
# [0, 1, 4, 9, 16]

# With condition: [expr for item in iter if condition]
evens = [x for x in range(10) if x % 2 == 0]
# [0, 2, 4, 6, 8]

# With if-else
labels = ["even" if x % 2 == 0 else "odd" for x in range(5)]
# ['even', 'odd', 'even', 'odd', 'even']

# Nested loops
pairs = [(x, y) for x in range(3) for y in range(3)]
# [(0,0), (0,1), (0,2), (1,0), (1,1), (1,2), (2,0), (2,1), (2,2)]

# Flatten nested list
matrix = [[1, 2], [3, 4], [5, 6]]
flat = [num for row in matrix for num in row]
# [1, 2, 3, 4, 5, 6]

# Shallow copy (only copies references)
original = [1, [2, 3]]
copy1 = original.copy()
copy2 = original[:]
copy3 = list(original)

# Deep copy (copies nested objects too)
import copy
deep = copy.deepcopy(original)

# Why it matters
original = [1, [2, 3]]
shallow = original.copy()
shallow[1][0] = 99
print(original)  # [1, [99, 3]] - nested list changed!

deep = copy.deepcopy([1, [2, 3]])
deep[1][0] = 99
print(original)  # [1, [99, 3]] - original unchanged

nums = [1, 2, 3, 4, 5]

# Length
len(nums)           # 5

# Sum, min, max
sum(nums)           # 15
min(nums)           # 1
max(nums)           # 5

# Check if all/any
all([True, True, False])  # False
any([True, False, False]) # True

# Enumerate (index + value)
for i, val in enumerate(nums):
    print(f"{i}: {val}")

# Zip (parallel iteration)
names = ["Alice", "Bob"]
ages = [25, 30]
for name, age in zip(names, ages):
    print(f"{name} is {age}")