_Edmond_'s blog

WA in C++(64)
By _Edmond_, history, 2 years ago, translation, In English

Hello!

Today I submitted the exact same code in C++20(64) and C++ 17. In C++ 17 it passed but wrong answer in C++20(64).

173606485 C++ 17 — ACCEPTED. 173606464 C++ 20 (64) — WRONG ANSWAR.

How to understand this?

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2 years ago, # |
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Maybe it is because pow？
I tried 'GNU C++17 (64)'，and it returned WA too.

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2 years ago, # |
← Rev. 3   Vote: I like it +8 Vote: I do not like it

Since the pow function uses and returns double value, with big numbers it can have a slight error, which gave you the WA verdict (You can see in your C++20 submission, the answers are only off by 16: 101159538130177904 vs 101159538130177920). I converted the types to long double and it got accepted: 173634899

For why did the C++17 solution get accepted, I have no idea. Maybe it's because of their different ways of compiling code?

More on this, you shouldn't use the pow function for integers for the same reason: Instead of pow(a, 2), it's recommended to use a*a. Your same code using a*a: 173635181

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2 years ago, # |
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I think this is precisely what you are experiencing.

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    2 years ago, # ^ |
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    Yes. Floating point numbers work in mysterious ways in C++(32 bit). With C++(64 bit) they behave a lot more sane.

    For example submitting 

        double b = pow(a, 2);
    a = b / 120;
    cout << a;

    gives AC with C++17(32 bit) 173799443.

    While submitting 

        double b = pow(a, 2);
    pow(b, 10);
    a = b / 120;
    cout << a;

    gives WA with C++17(32 bit) 173799415.

    Dealing with floating point numbers in C++(32 bit) almost feels like quantum physics. Just looking at them can change the result.

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