One concept.
Nine angles.
Reading code, writing code, debugging, refactoring, reasoning about cost — Codyx drills every muscle with short, focused challenges.
Multiple Choice
Classic 4-option questions for concepts and quick recall.
Code Sorting
Reorder shuffled lines into a working program. Builds reading speed.
Find the Bug
Spot the off-by-one, the missing return, the wrong recursion.
Fill in the Blank
Type the missing token. Tests muscle memory for syntax & APIs.
True / False
Snap decisions on tricky behaviors and edge cases.
Output Prediction
Trace the code in your head and predict what it prints.
Variable State Tracker
Step through execution and track variable values turn-by-turn.
Refactoring Slider
Slide between versions of a function and pick the cleanest.
Big-O Complexity Matcher
Match snippets to time/space complexity. Builds intuition for scale.
Catch it in
under 90 seconds.
Every challenge is bite-sized. No setup, no scaffolding — just the code and the question.
fun fib(n: Int): Int {
if (n <= 1) return n
return fib(n) + fib(n - 2) // bug here
}