Lists

A list is an ordered, mutable collection that can store multiple items. Lists are one of the most useful and flexible data types in Python.

Why Lists?

Store multiple related items together
Grow and shrink dynamically
Support powerful operations like slicing, filtering, and transformations

Creating Lists

# Empty list
empty_list = []
print(empty_list)

# With initial items
fruits = ["apple", "banana", "cherry"]
print(fruits)

# Mixed types
mixed = ["Alice", 30, 5.7, True]
print(mixed)

# Nested list (list of lists)
matrix = [[1, 2], [3, 4], [5, 6]]
print(matrix)

[]
['apple', 'banana', 'cherry']
['Alice', 30, 5.7, True]
[[1, 2], [3, 4], [5, 6]]

Exercise: Create a list of three cities and print it.

cities = ["Tokyo", "Paris", "New York"]
print(cities)

['Tokyo', 'Paris', 'New York']

Types of Lists

Homogeneous Lists (same type)

numbers = [1, 2, 3, 4]
names = ["Alice", "Bob", "Cara"]

Heterogeneous Lists (mixed types)

mixed = ["Alice", 30, 5.7, True]

Nested Lists (lists inside lists)

matrix = [[1, 2], [3, 4], [5, 6]]

Exercise: Create a nested list representing a 2×3 grid, then access the item in row 2, column 3.

grid = [
    [1, 2, 3],   # row 1
    [4, 5, 6]    # row 2
]
print(grid[1][2])  # row 2, col 3 -> 6

Indexing & Slicing

fruits = ["apple", "banana", "cherry", "date", "elderberry"]
print(fruits[0])    # first item
print(fruits[-1])   # last item

# Slicing: list[start:stop]
print(fruits[1:4])  # items at indexes 1, 2, 3
print(fruits[:3])   # first three items
print(fruits[2:])   # from index 2 to the end
print(fruits[-2:])  # last two items
print(fruits[::2])  # step of 2

apple
elderberry
['banana', 'cherry', 'date']
['apple', 'banana', 'cherry']
['cherry', 'date', 'elderberry']
['date', 'elderberry']
['apple', 'cherry', 'elderberry']

Exercise: Given nums = [10, 20, 30, 40, 50, 60], slice to get:

The first three numbers
The last two numbers
Every other number (step=2)

nums = [10, 20, 30, 40, 50, 60]
first_three = nums[:3]
last_two = nums[-2:]
every_other = nums[::2]
print(first_three, last_two, every_other)

[10, 20, 30] [50, 60] [10, 30, 50]

Adding Elements

fruits = ["apple", "banana"]
fruits.append("cherry")         # add to end
fruits.insert(1, "mango")       # insert at index 1
fruits.extend(["date", "fig"]) # extend with another list
print(fruits)

['apple', 'mango', 'banana', 'cherry', 'date', 'fig']

Exercise: Start with colors = ["red", "green"] and:

Insert "blue" at index 1
Append "yellow"
Extend with ["black", "white"] Print the result.

colors = ["red", "green"]
colors.insert(1, "blue")
colors.append("yellow")
colors.extend(["black", "white"])
print(colors)

['red', 'blue', 'green', 'yellow', 'black', 'white']

Removing Elements

fruits = ["apple", "banana", "cherry", "banana"]
fruits.remove("banana")  # removes first occurrence
popped = fruits.pop(1)    # removes by index, returns item
del fruits[0]             # delete by index (no return)
print("After removals:", fruits)
print("Popped item:", popped)

After removals: ['banana']
Popped item: cherry

Exercise 5: Given nums = [5, 10, 15, 20, 25], remove the middle value using pop() and print the updated list and the removed value.

nums = [5, 10, 15, 20, 25]
mid_index = len(nums)//2
removed = nums.pop(mid_index)
print(nums, removed)

[5, 10, 20, 25] 15

Updating Elements

prices = [9.99, 14.99, 4.99]
prices[0] = 8.99
prices[1:3] = [12.99, 3.99]  # slice assignment
print(prices)

[8.99, 12.99, 3.99]

Exercise 6: Replace the last two elements of letters = ['a', 'b', 'c', 'd'] with ['x', 'y'] using slice assignment.

letters = ['a', 'b', 'c', 'd']
letters[-2:] = ['x', 'y']
print(letters)

['a', 'b', 'x', 'y']

Looping Through Lists

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# With index using enumerate
for idx, fruit in enumerate(fruits):
    print(idx, fruit)

apple
banana
cherry
0 apple
1 banana
2 cherry

Exercise: Loop through names = ['Ann', 'Ben', 'Cara'] and print each name with its index (starting from 1).

names = ['Ann', 'Ben', 'Cara']
for i, name in enumerate(names, start=1):
    print(i, name)

1 Ann
2 Ben
3 Cara

Useful List Methods

nums = [4, 1, 3, 2]
nums.sort()            # sort ascending (in place)
print("sorted:", nums)
nums.reverse()         # reverse order (in place)
print("reversed:", nums)
print("count of 2:", nums.count(2))
print("index of 3:", nums.index(3))

sorted: [1, 2, 3, 4]
reversed: [4, 3, 2, 1]
count of 2: 1
index of 3: 1

Exercise: Given nums = [10, 5, 8, 5, 3], sort the list ascending, count how many 5s there are, and find the index of 8.

nums = [10, 5, 8, 5, 3]
nums.sort()
count_5 = nums.count(5)
idx_8 = nums.index(8)
print(nums, count_5, idx_8)

[3, 5, 5, 8, 10] 2 3

List Comprehensions

# Square numbers 0..5
squares = [x**2 for x in range(6)]
print(squares)

# Filter even numbers
evens = [x for x in range(10) if x % 2 == 0]
print(evens)

# Transform strings
names = ["alice", "bob", "cara"]
caps = [n.title() for n in names]
print(caps)

[0, 1, 4, 9, 16, 25]
[0, 2, 4, 6, 8]
['Alice', 'Bob', 'Cara']

Exercise: Using nums = [1, 2, 3, 4, 5, 6], build a new list of squares only for the odd numbers.

nums = [1, 2, 3, 4, 5, 6]
odd_squares = [x**2 for x in nums if x % 2 == 1]
print(odd_squares)

[1, 9, 25]

Summary

Lists are ordered and mutable
Support indexing, slicing, adding/removing, updating, and iterating
Powerful methods and list comprehensions make data processing concise

Practice: Consolidated Exercises

Create a list pets with three animal names. Append one more.
From grades = [60, 70, 80, 90, 100], slice the last three grades.
Replace the first two items in tools = ['hammer', 'screwdriver', 'wrench'] with ['saw', 'drill'].
Use a loop to print each item in colors = ['red', 'green', 'blue'] with its index.
Use a comprehension to create [x*10 for x in [1,2,3,4]].

# 1) pets list
pets = ["cat", "dog", "hamster"]
pets.append("parrot")
print(pets)

# 2) slice last three grades
grades = [60, 70, 80, 90, 100]
print(grades[-3:])

# 3) replace first two items
tools = ['hammer', 'screwdriver', 'wrench']
tools[:2] = ['saw', 'drill']
print(tools)

# 4) loop with index
colors = ['red', 'green', 'blue']
for i, c in enumerate(colors):
    print(i, c)

# 5) comprehension * 10
print([x*10 for x in [1,2,3,4]])

['cat', 'dog', 'hamster', 'parrot']
[80, 90, 100]
['saw', 'drill', 'wrench']
0 red
1 green
2 blue
[10, 20, 30, 40]

Working with Nested Lists (Matrices)

This notebook demonstrates how to access elements in a nested list (list of lists), like a matrix or table.

Example: A 3x2 Matrix

matrix = [
    [1, 2],    # row 0
    [3, 4],    # row 1
    [5, 6]     # row 2
]

print(matrix)

[[1, 2], [3, 4], [5, 6]]

Accessing Specific Elements

print(matrix[0][0])  # 1 (row 0, column 0)
print(matrix[0][1])  # 2 (row 0, column 1)
print(matrix[2][0])  # 5 (row 2, column 0)

1
2
5

Looping Through a Matrix

for row in matrix:
    for item in row:
        print(item, end=' ')
    print()  # new line after each row