NM_Mehedy's blog

Is cerr evil?
By NM_Mehedy, history, 3 years ago, In English

I solved this problem in CF #705. Screenshot-450

But it failed system testing showing Screenshot-451

TLE code

Accepted code after contest

If we compare this two submissions-

Screenshot-452

This is the only difference.

Definition of see(...) in my code

The main thing is I used cerr for debugging and it got TLE.

Can anyone explain Why did it happen?

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3 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Great

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3 years ago, # |
  Vote: I like it +44 Vote: I do not like it

Nothing is absolutely free. Your debugging print code consumes precious CPU cycles.

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    3 years ago, # ^ |
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    If I recall correctly, on some online judges cerr is sent to /dev/null, and "writing" there is very fast. Sadly, not on Codeforces.

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      3 years ago, # ^ |
        Vote: I like it +14 Vote: I do not like it

      Even if cerr is redirected to /dev/null, the useless output strings are still being created in huge amounts. The blog author's main loop contains only a tiny amount of arithmetic operations. But each of these loop iterations also tries to at least create one useless std::string and print it to cerr with a ton of overhead.

      The failing testcase prints 211.5k lines to stderr and 100 lines to stdout. But I agree that this shouldn't have been fatal. On my computer it takes just slightly more than one second to run if I redirect stderr to /dev/null. And my computer is slower than the codeforces judge.

      It's safer to put the debugging print code inside of an #ifndef ONLINE_JUDGE block either way.

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    3 years ago, # ^ |
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    This is why some people define a macro with a local flag instead of a function for debugging: if the local flag is not defined, then the macro will become nothing, and nothing will be run.

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3 years ago, # |
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Very evil. Accepted: 117533891. Idleness limit exceeded: 117532620. It's not even an interactive problem.

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3 years ago, # |
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solution
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