AtCoder vs Codeforces

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.

Codeforces is a programming-contest community and platform where regular contests give participants a way to solve problems and earn skill ratings. The site says it organizes approximately six contests each month, with free participation open to everybody. In contests following Codeforces rules, participants gain points for solving problems quickly and can test other contestants’ solutions through hacks. Ratings use a modified Elo system, and contestants are split into two divisions. The online judge checks submissions against prepared tests and tests suggested through hacks, then returns a verdict. Past contests are available for practice, while the site says learning content and training features are in development. Codeforces also offers an API that returns some data as machine-readable JSON over HTTP. Public data can be requested anonymously; private user data requires an API key. API requests are limited to one every two seconds, and faster calls receive a call-limit error. The service is free and available on web and through an API.