Python Lists as a Collection: Mutability, Nesting, and Common Operations

# Direct literal
fruits = ["apple", "banana", "cherry"]

# List constructor from any iterable
numbers = list(range(1, 6))    # [1, 2, 3, 4, 5]
chars = list("Python")         # ['P', 'y', 't', 'h', 'o', 'n']
from_tuple = list((10, 20, 30))

# Repeat a pattern
zeros = [0] * 5    # [0, 0, 0, 0, 0]

# List comprehension
squares = [x**2 for x in range(1, 6)]   # [1, 4, 9, 16, 25]
evens = [x for x in range(20) if x % 2 == 0]

items = [3, 1, 4, 1, 5, 9]

# Add elements
items.append(2)           # adds to end: [3, 1, 4, 1, 5, 9, 2]
items.insert(0, 0)        # insert at index 0: [0, 3, 1, 4, 1, 5, 9, 2]
items.extend([6, 5, 3])   # add multiple: [..., 6, 5, 3]

# Remove elements
items.remove(1)           # removes first occurrence of 1
popped = items.pop()      # removes and returns last element
popped_at = items.pop(2)  # removes and returns element at index 2
del items[0]              # removes element at index 0

# Find and count
print(items.index(5))     # index of first occurrence of 5
print(items.count(3))     # how many times 3 appears

numbers = [5, 2, 9, 1, 7, 3]

# sort() modifies the list IN PLACE and returns None
numbers.sort()
print(numbers)          # [1, 2, 3, 5, 7, 9]

# sorted() returns a NEW sorted list — original unchanged
original = [5, 2, 9, 1, 7, 3]
new_sorted = sorted(original)
print(original)         # [5, 2, 9, 1, 7, 3]  — unchanged
print(new_sorted)       # [1, 2, 3, 5, 7, 9]

# Sort by a key
words = ["banana", "apple", "fig", "cherry"]
words.sort(key=len)     # sort by string length
print(words)            # ['fig', 'apple', 'banana', 'cherry']

# Reverse sort
numbers.sort(reverse=True)
print(numbers)          # [9, 7, 5, 3, 2, 1]

# Common mistake: assigning sort() to a variable
result = numbers.sort()   # result is None!

# 3×3 matrix
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
]

# Access element at row 1, column 2
print(matrix[1][2])   # 6

# Transpose: swap rows and columns
transposed = [[matrix[j][i] for j in range(3)] for i in range(3)]
print(transposed)
# [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

# Warning: avoid list multiplication for nested lists
wrong = [[0] * 3] * 3   # all three rows are the SAME object
wrong[0][0] = 99
print(wrong)   # [[99, 0, 0], [99, 0, 0], [99, 0, 0]] — unexpected!

correct = [[0] * 3 for _ in range(3)]   # each row is a separate list
correct[0][0] = 99
print(correct)  # [[99, 0, 0], [0, 0, 0], [0, 0, 0]]

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

# List comprehension
squared = [x**2 for x in numbers]

# map() — functional style, returns an iterator
squared_map = list(map(lambda x: x**2, numbers))

# Both produce [1, 4, 9, 16, 25]
# Comprehension is more Pythonic for simple transforms

# map() can be useful when the function already exists
import math
roots = list(map(math.sqrt, numbers))   # cleaner than [math.sqrt(x) for x in ...]

# With filter
evens = [x for x in numbers if x % 2 == 0]
evens_filter = list(filter(lambda x: x % 2 == 0, numbers))

data = [10, 50, 75, 83, 98, 84, 32, 10]

print(min(data))              # 10
print(max(data))              # 98
print(sum(data))              # 442
print(len(data))              # 8
print(sum(data) / len(data))  # 55.25 (mean)

# enumerate — iterate with index
for i, value in enumerate(data, start=1):
    print(f"{i}: {value}")

# zip — iterate multiple lists together
names = ["Alice", "Bob", "Carol"]
scores = [85, 92, 78]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

# any and all
print(any(x > 90 for x in scores))    # True — at least one
print(all(x >= 70 for x in scores))   # True — all pass

import sys

# Growing list via append
lst = []
for i in range(10):
    lst.append(i)
    print(f"len={len(lst)}, size={sys.getsizeof(lst)} bytes")

# Pre-allocated via comprehension — consistent memory from the start
lst2 = [i for i in range(10)]