Matrices for Organizing Data

In the previous chapter, we saw how vectors serve as fundamental building blocks, often representing a single data point or a set of features for one observation in a machine learning context. For instance, a vector could hold the pixel values for one image, the word counts for one document, or the features (like square footage, number of bedrooms) for one house.

However, real-world machine learning tasks almost always involve working with collections of data points, not just one. We need a way to organize and manipulate these collections efficiently. This is where matrices come in.

A matrix is a rectangular grid of numbers, arranged in rows and columns. If a matrix has $m$ rows and $n$ columns, we say it has dimensions $m \times n$. You can think of it as a generalization of a vector; a vector is essentially a matrix with only one column (a column vector) or one row (a row vector).

From Datasets to Matrices

The most common way matrices arise in machine learning is by representing entire datasets. Imagine a typical dataset stored in a table, like a spreadsheet or a CSV file:

Size (sq ft)	Bedrooms	Price ($1000s)
1500	3	300
1200	2	250
1800	4	380
1350	3	290

We can directly translate this tabular data into a matrix. Conventionally, each row of the matrix corresponds to a single sample or data point (like one house), and each column corresponds to a specific feature or attribute (like size, number of bedrooms, or price).

For the housing data above, if we are using 'Size' and 'Bedrooms' as our input features ($X$) to predict 'Price' ($y$), we can represent the features as a matrix:

$$X = \begin{bmatrix} 1500 & 3 \\ 1200 & 2 \\ 1800 & 4 \\ 1350 & 3 \end{bmatrix}$$

This matrix $X$ has dimensions $4 \times 2$ because there are 4 samples (houses) and 2 features. Each row, like $[1500, 3]$, is a feature vector for one sample. The target variable, price, would typically be stored as a separate vector:

$$y = \begin{bmatrix} 300 \\ 250 \\ 380 \\ 290 \end{bmatrix}$$

This matrix $X$ is often called the feature matrix or design matrix. It's a standard structure used in many machine learning algorithms.

This diagram shows how a dataset table, where rows are samples and columns are features, is commonly represented as a feature matrix $X$. Each row in the matrix is a feature vector for a single sample.

Other Data Types as Matrices

Matrices aren't limited to simple tabular data. Consider other data types common in machine learning:

• Images: A grayscale image can be directly represented as a matrix where each element corresponds to the intensity value of a pixel. A $28 \times 28$ pixel grayscale image becomes a $28 \times 28$ matrix. For color images (e.g., RGB), you might use three separate matrices (one for Red, one for Green, one for Blue) or stack them into a 3-dimensional structure called a tensor (which extends the matrix concept to more dimensions).
• Time Series: A sequence of measurements over time can sometimes be structured into a matrix, perhaps by creating overlapping windows of observations, where each row represents a window.
• Graphs/Networks: Adjacency matrices represent connections between nodes in a graph, where the entry $A_{ij}$ indicates if there's an edge between node $i$ and node $j$.

Why Use Matrices?

Organizing data into matrices provides several significant advantages:

1. Compact Representation: Matrices offer a concise way to store and refer to large amounts of structured data.
2. Standardized Format: This structure is universally understood and forms the basis for many algorithms and software libraries.
3. Foundation for Operations: As we'll see shortly, defining operations like matrix multiplication allows us to perform complex data transformations (like rotations or scaling) and calculations (like solving systems of equations) efficiently across the entire dataset simultaneously.
4. Computational Efficiency: Libraries like NumPy are highly optimized for numerical operations on matrices (often called ndarrays or N-dimensional arrays in NumPy). Performing an operation on a whole matrix using NumPy is drastically faster than looping through individual data points in Python. This "vectorization" is essential for performance in machine learning.

By representing collections of data points as rows (or sometimes columns) within a matrix, we create a structured format that is ideal for mathematical manipulation and computational processing, creating the way for the linear transformations and system solving we'll cover next. We'll rely heavily on NumPy to create and work with these matrix representations in Python.