Working with Lists

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

Creating Lists

# Empty list
empty = []

# List with values
numbers = [1, 2, 3, 4, 5]
fruits = ["apple", "banana", "cherry"]
mixed = [1, "hello", 3.14, True]

# Using list() constructor
letters = list("hello")  # ['h', 'e', 'l', 'l', 'o']

Accessing Elements

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

# Positive indexing (from start)
print(fruits[0])   # "apple"
print(fruits[1])   # "banana"

# Negative indexing (from end)
print(fruits[-1])  # "date"
print(fruits[-2])  # "cherry"

Slicing

Get a portion of a list:

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

print(numbers[2:5])    # [2, 3, 4]
print(numbers[:3])     # [0, 1, 2]
print(numbers[7:])     # [7, 8, 9]
print(numbers[::2])    # [0, 2, 4, 6, 8] (every 2nd)
print(numbers[::-1])   # [9, 8, 7, ...] (reversed)

Modifying Lists

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

# Change an element
fruits[1] = "blueberry"
print(fruits)  # ["apple", "blueberry", "cherry"]

# Add to end
fruits.append("date")
print(fruits)  # ["apple", "blueberry", "cherry", "date"]

# Insert at position
fruits.insert(1, "apricot")
print(fruits)  # ["apple", "apricot", "blueberry", "cherry", "date"]

# Remove by value
fruits.remove("cherry")

# Remove by index
del fruits[0]
# or
popped = fruits.pop(0)  # Returns the removed item

List Methods

Method	Description	Example
`append(x)`	Add item to end	`lst.append(4)`
`insert(i, x)`	Insert at index	`lst.insert(0, "first")`
`remove(x)`	Remove first occurrence	`lst.remove("apple")`
`pop(i)`	Remove and return item	`lst.pop()` or `lst.pop(0)`
`clear()`	Remove all items	`lst.clear()`
`index(x)`	Find index of item	`lst.index("apple")`
`count(x)`	Count occurrences	`lst.count(5)`
`sort()`	Sort in place	`lst.sort()`
`reverse()`	Reverse in place	`lst.reverse()`
`copy()`	Create a copy	`new = lst.copy()`

List Comprehensions

A concise way to create lists:

# Traditional way
squares = []
for x in range(5):
    squares.append(x ** 2)

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

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

Common Operations

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

# Length
print(len(numbers))  # 8

# Sum
print(sum(numbers))  # 31

# Min/Max
print(min(numbers))  # 1
print(max(numbers))  # 9

# Check membership
print(5 in numbers)  # True
print(7 in numbers)  # False

# Concatenation
a = [1, 2]
b = [3, 4]
c = a + b  # [1, 2, 3, 4]

Practice

Try solving the Two Sum problem after this lesson - it's a classic list manipulation challenge!