Matrices in Python using NumPy

NumPy arrays are used to represent matrices in Python. Matrices, which are rectangular grids of numbers with rows and columns, naturally map to two-dimensional (2D) NumPy arrays. Similarly, one-dimensional (1D) NumPy arrays are suitable for representing vectors.

Creating Matrices with numpy.array

The most straightforward way to create a matrix is by passing a list of lists to the numpy.array() function. Each inner list represents a row in the matrix.

Let's say we want to represent the following $2 \times 3$ matrix (2 rows, 3 columns):

$$A = \begin{bmatrix} 1 & 2 & 3 \\ 4 & 5 & 6 \end{bmatrix}$$

In Python using NumPy, we would create it like this:

import numpy as np

# Create a 2x3 matrix from a list of lists
A = np.array([
    [1, 2, 3],  # First row
    [4, 5, 6]   # Second row
])

print(A)

The output will show the NumPy array representation:

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

Notice how the structure directly mirrors the mathematical notation: the outer list contains two inner lists, corresponding to the two rows, and each inner list contains three elements, corresponding to the three columns.

Checking Matrix Dimensions

Just like with vectors, understanding the dimensions (or "shape") of a matrix is important, especially when we get to operations like matrix multiplication. NumPy arrays have a convenient .shape attribute that returns a tuple representing the dimensions. For a 2D array (matrix), the tuple will be (number_of_rows, number_of_columns).

Let's check the shape of the matrix A we just created:

# Get the dimensions (shape) of matrix A
print(A.shape)

The output will be:

(2, 3)

This confirms that A is indeed a $2 \times 3$ matrix.

Another Example: A Square Matrix

Let's create a $3 \times 3$ square matrix (where the number of rows equals the number of columns):

$$B = \begin{bmatrix} 9 & 8 & 7 \\ 6 & 5 & 4 \\ 3 & 2 & 1 \end{bmatrix}$$

Using NumPy:

# Create a 3x3 matrix
B = np.array([
    [9, 8, 7],
    [6, 5, 4],
    [3, 2, 1]
])

print("Matrix B:")
print(B)

# Check its shape
print("\nShape of B:")
print(B.shape)

Output:

Matrix B:
[[9 8 7]
 [6 5 4]
 [3 2 1]]

Shape of B:
(3, 3)

As expected, the shape is (3, 3).

Representing Vectors as Matrices

It's worth noting that while we often represent vectors using 1D NumPy arrays (like np.array([1, 2, 3]) with shape (3,)), they can also be explicitly represented as matrices with either one column (a column vector) or one row (a row vector).

• Column Vector (e.g., $3 \times 1$):

  col_vector = np.array([
      [1],
      [2],
      [3]
  ])
  print("Column Vector:\n", col_vector)
  print("Shape:", col_vector.shape) # Output: (3, 1)
  

• Row Vector (e.g., $1 \times 3$):

  row_vector = np.array([
      [1, 2, 3]
  ])
  print("\nRow Vector:\n", row_vector)
  print("Shape:", row_vector.shape) # Output: (1, 3)
  

This distinction between a 1D array (shape (n,)) and a 2D array with one dimension being 1 (shape (n, 1) or (1, n)) becomes significant in certain NumPy operations, particularly those involving matrix multiplication. For now, the main takeaway is that general matrices are represented as 2D NumPy arrays, created typically from lists of lists.

With this ability to represent matrices in Python, we are ready to explore different types of specialized matrices you'll often encounter.