Code Runner

What runs, and where

An editor on a phone is only useful if it is honest about what happens when you press run, because the six languages work in three completely different ways. Python executes on your device. HTML, CSS and JavaScript render on your device. C and C++ are compiled and run on a public server that is not ours.

That last sentence would be easier to leave out. It is here because the alternative is a student pasting a lab program into it and never learning where their code went — or typing a C program with no network and concluding the app is broken.

Python genuinely runs on the phone

Python runs through a WebAssembly build of CPython: a real interpreter, on the device, not a code-completion trick and not a request to a server. It supports an on-screen input box for programs that call input(), and it works out whether your program reads input at all, so the box stays out of the way on the programs that never ask for anything.

The runtime is about ten megabytes and it is fetched the first time you open Python, then kept alive and reused for the rest of the session. That download is why Python needs a connection once; it is not re-fetched on every run.

C and C++ go to a public compile service

C and C++ are sent to Wandbox, a free public compilation service, compiled there, and the output comes back. It is a real compiler and it reports real errors — but it is somebody else's computer. Your code leaves your phone, and it needs a working connection before anything happens at all.

This is the one place in the app where that is true. Results, attendance, marks, flashcards and the conversations you have with your own notes all stay on the device. The C and C++ path is the exception, which is why it is stated here rather than buried in a settings screen.

The editor is the part you use most

Syntax highlighting covers all six languages, and each one opens with a short runnable template instead of a blank page — enough to watch the language work before you have typed anything. The templates print, loop and define a function, because those are the three things you check first in a language that is new to you.

HTML, CSS and JavaScript render in a preview on the device. A full document is served exactly as you wrote it; a bare fragment is wrapped in a page first, so a single div still renders without you scaffolding a document around it.

Where it fits

This is the place to try a twenty-line idea from a lab or a lecture. When you need the concept behind it explained, the AI Professor answers from your own notes, and the formula library covers the equations you are coding up.