2D Lists

Objectives

• Define what a 2D list is in Python and explain its usefulness in representing tabular data with rows and columns.
• Recognize the relationship between 2D lists and tables.
• Demonstrate how to create a 2D list by encapsulating multiple lists within square brackets.
• Understand the structure of a 2D list with examples.
• Explain how to access elements in a 2D list using two indices, one for the row and one for the column.
• Emphasize the need for two indices when working with 2D lists.
• Use nested loops to iterate through the elements of a 2D list.

The examples thus far have only considered lists of one dimension but we can also create lists that look more like tables, with rows and columns, which give us two dimensions. Each element in a 2D list can be accessed using two indices: one for the row and one for the column. More dimensions are possible but not recommended as they can get tricky to handle.

Example 10

# Example program with a 2D list
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
print("Original matrix:")
for row in matrix:
    print(row)

element = matrix[1][2]
print("Element at row 2, column 3:", element)

matrix[0][0] = 10
print("Modified matrix:")
for row in matrix:
    print(row)

Predict what the output of the program will be:

• What will be printed for the original matrix?
• What will be the value of the element at row 2, column 3?
• How will the matrix look after modifying the first element?

Run the program in a Python environment and check your predictions, were they the same?

Creating 2D Lists

You can create a 2D list by enclosing multiple lists within square brackets []. Each inner list represents a row in the 2D list.

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

This is a list of lists! Think of it as being organised as a series of rows and columns, like a table.

Accessing Elements

To access an element in a 2D list, use two indices: one for the row and one for the column. Remember that Python uses 0-based indexing.

element = matrix[row_index][column_index]

Modifying 2D Lists

You can change the values of elements in a 2D list by assigning new values using the indices just as you would with 1D lists but now we need two indices.

matrix[1][2] = 42

Looping Through 2D Lists

You can use nested loops to iterate through the elements of a 2D list. The outer loop will work through each row, the inner loop will work through the columns in that row.

for row in matrix:
    for element in row:
        # code to process each element

Activity

Using the example program above, copy and paste into a Python IDE and make the following modifications:

• Change element at row 3, column 1 to 11
• Add a new row to the matrix
• Remove the second row

Example

We've been asked to create a simple address book of our friends with their name, email address and phone number. Our table of friends might look like this:

Name	Phone	Email
Alice	09876 444555	alice@example.com
Bob	01287 655444	bob@hotmail.co.uk
Charlie	09998 666555	charlie_is_great_@gmail.com
Davina	06667 543213	d_khan@myspace.tv
Edward	08889 765432	edward_davies@gmail.com

We currently have 5 rows and 3 columns, but will want to add more friends later.

First we initialise our empty table (2D list)

address_book = [][]

Or, we could initialise the table with the data we have already:

address_book = [
    ["Alice", "09876 444555","alice@example.com"],
    ["Bob","01287 655444","bob@hotmail.co.uk"],
    ["Charlie","09998 666555","charlie_is_great_@gmail.com"],
    ["Davina","06667 543213","d_khan@myspace.tv"],
    ["Edward","08889 765432","edward_davies@gmail.com"]
]

Our program will keep running, presenting us with the menu of options, until we enter quit (one of options):

while True:
    print("\nOptions:")
    print("1. Find friend's information")
    print("2. Add a new friend")
    print("3. Quit")

    choice = input("Enter your choice (1/2/3): ")

    if choice == "1":
        # find a friend
    elif choice == "2":
        # add a new friend
    elif choice == "3":
        print("Goodbye")
        break
    else:
        print("Invalid choice.  please enter 1,2 or 3")

Let's tackle option 2. We could create a separate function for this, which would be preferable if we had to consider validation and other factors such as saving and reading from file etc., but here we're only dealing with three items of data:

# previous code
elif choice == "2":
    name = input("Enter name: ")
    phone = input("Enter phone: ")
    email = input("Enter email: ")
    address_book.append([name, phone, email])   # add the list of data as a new row

Warning

Be careful with the append() operation, we have to append the list and not each individual item of data

Option 1, finding the friend, is suitable for a function. Here we could write a function that iterates through our table looking for a given name and then return the row when found or an error if not found. This could then be passed to another function to display. This would be best practice so that both functions e.g. find_friend(name) and display_friend(friend) would only be handling one task. However, here we'll combine the two for simplicity:

# Function to find and display friend information
def find_friend_info(name):
    for friend in address_book:                     # iterate over the list
        if friend[0] == name:
            print(f"Name: {friend[0]}")             # only need 1 index as it's the column we need
            print(f"Phone Number: {friend[1]}")
            print(f"Email: {friend[2]}")
            return
    print(f"{name} not found in your friends list.")

In our main program loop we add the call to that function, once we're got the name to find from our user:

# previous code
if choice == "1":
    friend_name = input("Enter the name of your friend: ")
    find_friend_info(friend_name)
# subsequent code

Climate Quest Project

Throughout this topic we'll be working on a large scale project: Climate Quest. In this project a player embarks on a journey to combat the effects of climate change by making decisions that impact the environment. Each choice affects the outcome of the game, emphasizing the importance of individual actions in addressing climate change.

Go to task 9 - 2D Lists

Questions

1. What is a 2D list in Python?

A list containing other lists as its elements, representing rows and columns.

A list containing only numbers and strings.

A list where all elements must be of the same data type.

A list that can only hold two elements.

A 2D list in Python is a list of lists, where each inner list represents a row, allowing you to store data in a tabular format.

Submit

2. Which of the following represents the correct way to access the element in the second row and third column of a 2D list named 'matrix'?

matrix[2][3]

matrix[1, 2]

matrix[1][2]

matrix(1)(2)

In Python, indexing starts at 0. To access the element in the second row and third column, use 'matrix[1][2]'.

Submit

3. Given the following code, what will be the output?

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
element = matrix[2][0]
print(element)

2

4

7

9

The code accesses the first element (index 0) of the third row (index 2), which is 7.

Submit

4. What will be the output after running the following code?

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
matrix[0][0] = 10
for row in matrix:
    print(row)

[10, 2, 3] [4, 5, 6] [7, 8, 9]

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

[10, 2, 3] [4, 5, 10] [7, 8, 9]

[1, 2, 10] [4, 5, 6] [7, 8, 9]

The code modifies the first element of the first row from 1 to 10, resulting in the updated matrix being printed.

Submit

5. How would you correctly initialize an empty 2D list in Python?

matrix = [[]]

matrix = []

matrix = [[], []]

matrix = [(), ()]

An empty 2D list is initialized as an empty list '[]'. However, to prepare for a 2D structure, additional lists can be added later.

Submit

6. What is the purpose of using nested loops when working with 2D lists?

To initialize the list.

To iterate over each element in the 2D list, row by row.

To print the list in reverse order.

To add new rows to the list.

Nested loops allow you to iterate through each element of a 2D list by iterating over each row in the outer loop and each column in the inner loop.

Submit

7. Which of the following code snippets correctly modifies the element at the third row, first column, in the 2D list 'matrix'?

matrix[3][1] = 11

matrix[2][0] = 11

matrix[1][2] = 11

matrix[2, 0] = 11

The correct way to modify the element in the third row and first column is to use 'matrix[2][0] = 11', as indexing starts at 0.

Submit

8. What is the correct output of the following code snippet?

address_book = [
    ["Alice", "09876 444555", "alice@example.com"],
    ["Bob", "01287 655444", "bob@hotmail.co.uk"],
    ["Charlie", "09998 666555", "charlie_is_great_@gmail.com"]
]
address_book.append(["Davina", "06667 543213", "d_khan@myspace.tv"])
for friend in address_book:
    print(friend)

["Alice", "09876 444555", "alice@example.com"] ["Bob", "01287 655444", "bob@hotmail.co.uk"]

["Davina", "06667 543213", "d_khan@myspace.tv"]

["Alice", "09876 444555", "alice@example.com"] ["Bob", "01287 655444", "bob@hotmail.co.uk"] ["Charlie", "09998 666555", "charlie_is_great_@gmail.com"]

["Alice", "09876 444555", "alice@example.com"] ["Bob", "01287 655444", "bob@hotmail.co.uk"] ["Charlie", "09998 666555", "charlie_is_great_@gmail.com"] ["Davina", "06667 543213", "d_khan@myspace.tv"]

The code appends Davina's information to the 'address_book', so all four friends' information will be printed.

Submit

Programming Tasks

For each of the following tasks write a program using Python.

Seating Plan

Write a Python program to manage a simple seating plan. Initialise the seating plan as a 2D list with the first name of the student. Write code (using functions where necessary) to: print the initial seating plan, swap 2 students from one place to another and add a new row of students.

Read more ...

Matrix Addition

Write a Python program that adds two 2D matrices. Allow the user to input the dimensions and elements of the matrices and display the result.

Read more ...

Transpose a Matrix

Write a Python program that calculates and displays the transpose of a given 2D matrix. The transpose of a matrix swaps its rows and columns.

Read more ...

Matrix Multiplication

Write a Python program that multiplies two matrices and displays the result. Allow the user to input the dimensions and elements of the matrices.

Read more ...