🎓 Ages 15–18 · Grades 9–12 · Computer Science

CodeForge

You don’t learn to read code by watching — you learn by predicting. Here you read a short program, say what it’ll print, and a real interpreter runs it right on your device to check you. Actual loops, functions, lists, and recursion; no jargon, nothing sent anywhere, and a wrong guess is just the fastest way to see how the machine really thinks.

The Trace Lab ★ the main event

Read a short program, predict its output, then watch a pure interpreter run it — line by line, on your device, no cloud. Two ways to play: Trace It (predict what it prints) and Debug It (find the one broken line and fix it). The engine computes the real answer every time, so it’s exact — and it can’t run forever or touch anything but this page.

Open the Trace Lab →

Who you’ll meet

Not mascots — everyday programmers, each one showing a single idea in action. You’ll run into them across the kits and the lab.

  • Kian — orders the steps so the program runs in the right order (sequencing)
  • Priyom — branches the path on a true/false test (conditionals & boolean logic)
  • Devi — repeats a block until a condition changes (loops)
  • Milo — names a reusable block and calls it (functions & abstraction)
  • Oskar — stores and retrieves with lists and maps (data structures)
  • Juno — picks search or sort and reasons about the cost (algorithms)
  • Lin — traces state line-by-line to find the fault (debugging)
  • Odile — solves a problem in terms of a smaller copy of itself (recursion)
  • Ferro — models a real thing as data + operations (data & abstraction)
  • Ekwe — the mentor — decompose, predict, run, debug, then reflect

Check what you know

Sixteen short kits — from sequencing and variables to loops, functions, lists, dictionaries, search, sorting, recursion, and debugging — anchored to AP Computer Science. Hints when you want them; your progress stays on this device.

1. Sequencing Sequencing — the order in which statements run · 25 questions 2. Variables & Types Variables and data types — assignment and values · 25 questions 3. Conditionals Conditionals — making decisions with if/else · 25 questions 4. Boolean Logic Boolean logic — and, or, not, and truth tables · 25 questions 5. Loops Loops — repeating actions with iteration · 25 questions 6. Functions & Abstraction Functions and abstraction — reusable named blocks · 25 questions 7. Lists Lists — ordered collections and indexing · 25 questions 8. Maps / Dictionaries Maps and dictionaries — key→value lookup · 25 questions 9. Strings Strings — text as data, slicing and formatting · 25 questions 10. Search Search algorithms — linear vs binary · 25 questions 11. Sort & Complexity Sorting and algorithmic complexity (Big-O intuition) · 25 questions 12. Recursion Recursion — solving a problem in terms of a smaller copy of itself · 25 questions 13. Debugging & Tracing Debugging and tracing — finding and fixing faults · 25 questions 14. Data & Abstraction Data and abstraction — modeling a thing as data · 25 questions 15. Algorithms Algorithms — combining constructs to solve problems · 25 questions 16. Capstone: Build a Program Capstone — decompose, build, and debug a real program · 25 questions

How it works

CodeForge is written to trust you — it’s about reasoning through what the code does, not memorizing syntax, and a wrong prediction is just a place to learn. The interpreter is a small, safe, hand-written one: no eval, no server, nothing sent anywhere. Your progress is saved only on this device: no accounts, no ads.