How to use these pages
Answer the question first, on paper. Then click the answer to unfold it.
Reading a worked answer feels like learning and mostly is not — recognising an answer is a completely different skill from producing one, and only one of them is any use in an exam.
Try it: click this line
That is what every answer on these pages looks like. Nothing is hidden from you — it is hidden from your eye, so that a page of questions stays a page of questions until you have had a go.
Each set below follows a concept page — build the idea there first, then use these questions to make it dependable. Every set ramps from gentle to genuinely chewy, most answers are code you can type in and check for yourself, and none of these sets are timed. Ever.
Stuck partway through a set? Good — that is the useful part. Start with Getting Unstuck, and remember that in this subject being stuck is not an emergency — Debugging Is the Job.