ClosetNarcissist's blog

WA in C++17 but AC in C++23?
By ClosetNarcissist, history, 12 months ago, In English

So I was solving this problem https://mirror.codeforces.com/gym/105723/problem/F

and wrote this solution https://ideone.com/SNfGJ6

It works fine in my laptop and ideone C++ 14 but when I submitted it in C++17 (GCC 7-32) I got WA on test 1. It did not make sense to me how I got a vastly different result on the test case in my laptop and CF. I suspected some type of RTE but I could not find any. Then I just submitted in C++23 (GCC 14-64, msys2) and got AC even though I did not change anything in the code.

Anyone knows why this happened. I feel like knowing it would kind of help me avoid it in contests. idk -_-

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12 months ago, hide # |
 
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I suspect this has something to do with floating point precision error. G++23 has a 64 bit judge, therefore the floats are more accurately represented. Try to avoid floats as much as possible if you can

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    12 months ago, hide # ^ |
     
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    I also suspected something similar but it got the right answer in C++14 on ideone, which was 261798.698746. But C++17 on CF got me 361929.351562. It's insane how much the precision error blew up if that's the case.

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      12 months ago, hide # ^ |
       
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      The thing is that with c++17 you get inside the if (rt > rb) but with c++20 you don't

      Comparing doubles with some eps value could help probably

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        12 months ago, hide # ^ |
         
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        Damn how did i miss that. I added precision handling on the if(len <= ab) statement later but completely forgot about the other if statement. Thanks

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