Think like a programmer

Less magic. More logic.

The CODEX coding lab teaches programming with step-by-step Python and Java traces and debugging exercises, all on your device.

Step into your first program

Try a trace. Observe how a variable changes.

Interactive code tracing

discovery.py · learning snippet

1x = 3
2x = x + 2
3print(x)

0/3 steps

Step through the instructions to inspect the program state.

No arbitrary code is executed. These fixed Python and Java examples use a local tracing model; repair tests evaluate the selected rule locally.

Begin the programming path

Your programming toolkit

Exercise 1 · Trace & debug

Variables & state

A name for a changing value.

Learn

Exercise 2 · Trace & debug

Choose a branch

Teach a program to make decisions.

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Exercise 3 · Trace & debug

Make repetition work

Track one iteration at a time.

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Exercise 4 · Trace & debug

Inputs become outputs

Name a reusable transformation.

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Exercise 5 · Trace & debug

A place for every value

Meet indexed data structures.

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Exercise 6 · Trace & debug

Find it systematically

Trace a linear search.

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Exercise 7 · Trace & debug

Order through comparisons

Watch a bubble pass.

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A real free alternative to a remote code runner

Arbitrary Java and Python execution needs a secure runtime, which this app does not include. Instead, every trace follows fixed program semantics and debugging tests compute the actual results of your chosen repair locally. Java snippets are shown as method bodies, not standalone files. No API keys, servers, or usage charges.