AtCoder vs CodingBat

AtCoder is a free web service for competitive programming contests and practice. Users solve programming problems in Algorithm Contest and Heuristic Contest formats, and can run source code in a browser rather than setting up a local programming environment. Its practice library contains about 5,000 past problems with explanations, and it supports nearly all major programming languages, including C++, Python, Rust, Java, and C#. Beginner Contests take place weekly, Regular Contests about monthly, Grand Contests five to six times a year, and heuristic contests about monthly. Real-time participation in Algorithm or Heuristic contests changes a user's AtCoder rating. AtCoder Jobs lets users apply to companies using their rating for roles including internships and experienced positions. PAST is an online algorithm skills test available in Japanese and English, with problems updated every six months. AtCoder says it has about 200,000 registered users and about 10,000 participants in real-time contests weekly. Its terms limit members to one account and prohibit account sharing, lending, or transfer.

CodingBat is a free browser-based site for practicing Java and Python through live coding problems. Users enter code in a text area and click Go to save, compile, run it, and see the result immediately; nothing needs to be installed. Problems in each section become harder toward the end, and some include hints or solution code. Every Warmup section has solution code available. The exercises are intended for homework, self-study, labs, and live lecture examples. An account saves code and completion checkmarks, lists completed problems, and provides progress graphs and code badges. Accounts can share information with a teacher account, whose report lists students and their completed-problem counts. Without an account, work is forgotten after a session ends following two hours without activity. Users can also create custom pages and author Java or Python problems. CodingBat says cookies maintain login sessions and that it does not use third-party cookies. It collects regular web-server logs and may use submitted code and data for teaching-technology research.