Hands-On Machine Learning with Scikit-Learn and PyTorch

Chapter 3. Classification

In Chapter 1 I mentioned that the most common supervised learning tasks are regression (predicting values) and classification (predicting classes). In Chapter 2 we explored a regression task, predicting housing values, using various algorithms such as linear regression, decision trees, and random forests (which will be explained in further detail in later chapters). Now we will turn our attention to classification systems.

MNIST

In this chapter we will be using the MNIST dataset, which is a set of 70,000 small images of digits handwritten by high school students and employees of the US Census Bureau. Each image is labeled with the digit it represents. This set has been studied so much that it is often called the “hello world” of machine learning: whenever people come up with a new classification algorithm they are curious to see how it will perform on MNIST, and anyone who learns machine learning tackles this dataset sooner or later.

Scikit-Learn provides many helper functions to download popular datasets. MNIST is one of them. The following code fetches the MNIST dataset from OpenML.org:⁠¹

from sklearn.datasets import fetch_openml

mnist = fetch_openml('mnist_784', as_frame=False)

The sklearn.datasets package contains mostly three types of functions: fetch_* functions such as fetch_openml() to download real-life datasets, load_* functions to load small toy datasets bundled with Scikit-Learn (so they don’t need to be downloaded over the internet), and ...