I'm sure we won't need this post after Codeforces comes out of beta, but meanwhile it could be useful :) Feel free to suggest what should be added here.
Q. How do I hack other's solutions?
A. First lock your own solution in the main ('problems') tab. Then in your room double click or Ctrl+click the solution you want to hack. Check this post for complete rules concerning hacking.
Q. How is rating calculated?
A. The rating system is described in these two posts. Note that even for a joined contest the rating updates are calculated separately for the first and the second divisions. Therefore, a situation is possible, when a div2-coder performed worse than a div1-coder in the same contest, but gained more rating points.
Q. How do I read/write 64bit integers in C/C++?
A. If you submit under "GNU C++" or "GNU C" option, use this:
printf("%I64d\n",n);
but NOT this:
printf("%lld\n",n);
If you submit under "MS C++" option, both variants should work.
And of course, with C++ you can always write
std::cout << n << "\n";
Q. How can my program determine whether it runs in Codeforces server?
A. For most programming languages Codeforces environment defines the symbol ONLINE_JUDGE. Again, see this post for details.
Q. How do I read other people's code?
A. After the contest is over, go to its page (for example, http://codeforces.com/contest/15 ) and click on this picture:
. You will see a page with several submits (currently 100, I think). You can sort and filter the solutions there. To see the fastest solutions, you need to manually paste a URL like this: http://codeforces.com/contest/15/status/A?order=BY_ARRIVED_ASC
(currently there's no way to do this via UI)
Competitions: general
Q. What are the rules of these contests?A. Read this.
Q. What languages are supported? What are the compiler options and run command lines? What is the configuration of judge servers?
A. The judge machines are Core 2 Duo, 2.67 Ghz (E6750). The full list of supported languages and compiler options can be found in this postQ. How do I hack other's solutions?
A. First lock your own solution in the main ('problems') tab. Then in your room double click or Ctrl+click the solution you want to hack. Check this post for complete rules concerning hacking.
Q. How is rating calculated?
A. The rating system is described in these two posts. Note that even for a joined contest the rating updates are calculated separately for the first and the second divisions. Therefore, a situation is possible, when a div2-coder performed worse than a div1-coder in the same contest, but gained more rating points.
Competitions: writing code
Q. How do I read/write 64bit integers in C/C++?
A. If you submit under "GNU C++" or "GNU C" option, use this:
printf("%I64d\n",n);
but NOT this:
printf("%lld\n",n);
If you submit under "MS C++" option, both variants should work.
And of course, with C++ you can always write
std::cout << n << "\n";
Q. How can my program determine whether it runs in Codeforces server?
A. For most programming languages Codeforces environment defines the symbol ONLINE_JUDGE. Again, see this post for details.
Using the site
Q. How do I read other people's code?
A. After the contest is over, go to its page (for example, http://codeforces.com/contest/15 ) and click on this picture:
. You will see a page with several submits (currently 100, I think). You can sort and filter the solutions there. To see the fastest solutions, you need to manually paste a URL like this: http://codeforces.com/contest/15/status/A?order=BY_ARRIVED_ASC(currently there's no way to do this via UI)
Q. During the practice, can I see the test case that my program failed on?
A.No. YES! Go to 'My submissions' tab and click the link showing the ID of your submission.
Q. How can I find some useful information on this site?
Q. Why are my comments / post empty?
Q. Can I post a highlighted code sample?
A. Currently the best way is to submit the code to a service like codepad.org and post only links to the submit.
Q. I've found a bug! When I upvote a post, its rating increases by 2 points, but when I downvote a post, its rating decreases only by 1 point.
A. It's not a bug, it's a feature.
A.
Q. How can I find some useful information on this site?
A. Try google or this query: "http://codeforces.com/search?query=<what do you want>".
Q. Why are my comments / post empty?
A. If you used copy-paste, your comment may have some incorrect tags. Try to look at the HTML code of your message (button "< >") and erase all charset tags. If the system tells you "Rendering to html failed: ....", you used "\$" symbol. Codeforces supports 
formulas which are enabled by this symbol. Try to avoid it.
formulas which are enabled by this symbol. Try to avoid it.Q. Can I post a highlighted code sample?
A. Currently the best way is to submit the code to a service like codepad.org and post only links to the submit.
Miscellaneous
Q. I've found a bug! When I upvote a post, its rating increases by 2 points, but when I downvote a post, its rating decreases only by 1 point.
A. It's not a bug, it's a feature.
A3. Yes, you should. Open all problems in different browser tabs as soon as possible.
There is a good chance you won't be able to do that later, because of the server's occasional shutdowns.
:)
But seriously, it happened only twice, I think. (Although I usually try to follow this advice anyway :) )
Q2. How do I read/write 64bit integers in C (GNU/ mingw)?
Q3. How do I read/write 64bit integers in C (MSVC)?
Q4. How do I read/write 64bit integers in C++?Wrong.
Q2. How do I read/write 64bit integers in C (GNU/ mingw)?
Q3. How do I read/write 64bit integers in C (MSVC)?
Q4. How do I read/write 64bit integers in C++?For GNU only I64d.
std::cout << std::endl;
and
std::cout << "\n";
The first statement flushes output buffers to disk, so it is much slower. It matters if output is huge.
[rus_spoiler]Думаю ещё стоит описать особенности системы комментирования: символ ^ и backspace (browser.back) для возврата на предыдущее сообщение.[/rus_spoiler]
Месяц мучался, а потом меня научили. :-)
What is the Codeforces scoring system? What are contests rules?
«http://www.google.ru/search?sclient=psy&hl=ru&newwindow=1&qscrl=1&q=site:codeforces.com&qscrl=1&aq=f&aqi=&aql=&oq=&pbx=1&cad=h»
стоит заменить на
«http://www.google.com/search?q=i%27m+searching+for...+site%3Acodeforces.com»
A. Here is some tactic:
1. Solve the Problemset starting from most solvable - seems to be a more simple problem.
2. Wait for a registration for a next contest.
3. Register, check yourself in registrants (
4. Wait for the contest starts, read the original interesting contest rules, don't get confused with them, read the rules again.
5. Write the contest.
6. If you are not in «Top rated», goto step 1.
7. ???????
8. PROFIT
A. Yes (?), but you should better post your code on one of the services like codepad.org and have just one post, containing your templates, e.g.: