Python — Arrays

What are Lists?

Lists are ordered, mutable collections that can hold any type of data:

fruits = ["apple", "banana", "cherry"]
numbers = [1, 2, 3, 4, 5]
mixed = ["hello", 42, True, 3.14]

Creating Lists

# Empty list
empty = []

# List literal
colors = ["red", "green", "blue"]

# Using list() constructor
numbers = list(range(1, 6))  # [1, 2, 3, 4, 5]

# List comprehension
squares = [x**2 for x in range(5)]  # [0, 1, 4, 9, 16]

Accessing Elements

fruits = ["apple", "banana", "cherry", "date"]

fruits[0]     # "apple" — first element
fruits[-1]    # "date" — last element
fruits[1:3]   # ["banana", "cherry"] — slice
fruits[:2]    # ["apple", "banana"] — from start
fruits[2:]    # ["cherry", "date"] — to end
fruits[::2]   # ["apple", "cherry"] — every other

Modifying Lists

fruits = ["apple", "banana"]

# Change element
fruits[0] = "mango"

# Add to end
fruits.append("grape")

# Insert at index
fruits.insert(1, "orange")

# Extend with another list
fruits.extend(["kiwi", "peach"])

# Remove element
fruits.remove("banana")

# Remove by index
fruits.pop(0)

# Remove last
fruits.pop()

# Delete by index
del fruits[0]

# Clear all
fruits.clear()

List Methods

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

nums.sort()          # [1, 1, 3, 4, 5, 9] — modifies in place
nums.reverse()       # [9, 5, 4, 3, 1, 1]
nums.count(1)        # 2
nums.index(4)        # 2
nums.copy()          # creates a copy

Iterating Over Lists

fruits = ["apple", "banana", "cherry"]

# Simple loop
for fruit in fruits:
    print(fruit)

# With index
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

# With enumerate (Pythonic)
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# With index and value
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")

List Comprehensions

# Basic
squares = [x**2 for x in range(10)]

# With condition
evens = [x for x in range(20) if x % 2 == 0]

# With transformation
names = ["alice", "bob"]
upper_names = [name.upper() for name in names]  # ["ALICE", "BOB"]

# Nested
matrix = [[i*3+j+1 for j in range(3)] for i in range(3)]
# [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

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

Useful Operations

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

# Length
len(nums)  # 5

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

# Check if exists
3 in nums      # True
6 in nums      # False

# Concatenation
[1, 2] + [3, 4]  # [1, 2, 3, 4]

# Repetition
[0] * 5  # [0, 0, 0, 0, 0]

# Sorting
sorted(nums)         # returns new sorted list
sorted(nums, reverse=True)  # [5, 4, 3, 2, 1]

Unpacking

first, second, *rest = [1, 2, 3, 4, 5]
# first = 1, second = 2, rest = [3, 4, 5]

first, *middle, last = [1, 2, 3, 4, 5]
# first = 1, middle = [2, 3, 4], last = 5

Common Patterns

# Remove duplicates
unique = list(set([1, 1, 2, 3, 3]))  # [1, 2, 3]

# Filter
even = list(filter(lambda x: x % 2 == 0, range(10)))

# Map
doubled = list(map(lambda x: x * 2, range(5)))

# Zip two lists
names = ["Alice", "Bob"]
scores = [95, 87]
paired = list(zip(names, scores))  # [("Alice", 95), ("Bob", 87)]

# Reverse
reversed_list = nums[::-1]

Mini Practice

1. Create a list of 5 favorite movies
2. Add and remove items from the list
3. Sort the list alphabetically
4. Use a list comprehension to filter movies by length
5. Reverse the list using slicing

Up Next

Next: Tuples →