DarkZero's blog

Different results given by GNU C++ 4.7 and Microsoft VC++ 2010
By DarkZero, 10 years ago, In English

In CodeForces #243 Div 2 B (something to do with mirroring), I submitted a solution (number is 6543367), and on the 10th test it gives 2 with GNU C++ 4.7 compiler, but on my own computer (Windows 8, gdb 6.8) it gives the correct answer 1.

And then I tried to switch to Microsoft VC++ 2010, and got AC.....

I checked my code twice and did not discover any hazard parts. Is there anyone could explain why? Thanks!

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10 years ago, # |
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I've edited your solution a bit. 6543599

Problem in

double tmp = log((double)(n/x)) / log(2.0);
	if( tmp != int(tmp))
		return false;
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    10 years ago, # ^ |
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    Thank you very much for your attention! Does the solution imply that I should avoid using "log" when compiling with GNU C++?

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      10 years ago, # ^ |
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      Code that looks like:

      double a;
if(a != int(a))

      usually bad idea (imho).

      Look at this 6543990

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        10 years ago, # ^ |
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        Thank you, I will use 

        double tmp;
if( tmp != round(tmp) )

        instead.

        Learnt a lesson today, thank you!

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          10 years ago, # ^ |
            Vote: I like it +31 Vote: I do not like it

          It's also a bad idea. You should avoid using operators == and != together with floating points variables. For example, instead of the code you've provided in the last comment one can write the following:

          double a;
if (abs(a - round(a)) > eps) ...

          Here eps should be small enough, e.g. 10 - 9. Actually, its value depends on the problem you are solving.

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            10 years ago, # ^ |
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            Thank you, I have known this trick but in that round I was too lazy to write in that way, and it came back to me = =

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    10 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    (Deleted)

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