The title of the book includes "from scratch" for a reason -- it's from "first principles" where you learn about something by building it up from scratch rather than using an implementation. At the end of each chapter, Joel points out the existing resources you can use after learning about the topic.

makes sense. I took "from scratch" from an understanding perspective rather than implementation. Thanks for clarifying it. Looks like it'll be a great resource.

Sorry, I downvoted you by accident, I have a habit of clicking randomly while reading :/

This has been the most common question I've gotten. I'd say partially it's my bias as someone who works in recommender systems a lot. Many/most common statistical problems have a compact matrix representation (e.g., systems of linear equations). Finally, everyday matrix decomposition tasks like SVD.

Thanks for the response. That gives me something new to learn/look into.