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kakhatake's blog

Simple code not working properly on codeforces side
By kakhatake, history, 6 years ago, In English

Hey there, I was solving 789B and I had this very simple and short code but it just fails on test 3 although it works perfectly on my machine. I have read that undefined behavior can induce this but I can't recognize what's wrong with what I typed. Here's the code:

code

Thank you!!

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6 years ago, # |
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I've added outputting an empty string before the if and got AC. I don't know why it works.

code

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6 years ago, # |
  Vote: I like it +13 Vote: I do not like it

You compare doubles using ==. Don't do that.

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    6 years ago, # ^ |
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    In general, yes, you're right. But why do r1 and r2 differ in this case (for the input data 6 6)?

    If x is 6 and y is 6, then surely r1 == r2 is true:

    r1 = x*log10(y);
r2 = y*log10(x);
if(r1 == r2) cout << "=";

    Why adding cout<<""; changes the result of the test (see my comment above)?

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      6 years ago, # ^ |
        Vote: I like it +13 Vote: I do not like it

      In short, r1 == r2 is true if they are both 64bit floats, compiler may calculate and store one of them in 80bit register (for optimization), and 80bit logarithm != 64bit logarithm. And cout disables this optimization.

      This issue was discussed here (and not here) million times, but every time there is a person who didn't read these discussions.

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      6 years ago, # ^ |
        Vote: I like it +16 Vote: I do not like it

      Actually, not. Even for the following example:

      double x, y;
cin >> y;
x = y;
... // x and y are not changed
if (x == y) // there is no guarantees that it's true
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6 years ago, # |
← Rev. 2   Vote: I like it -24 Vote: I do not like it

[Deleted] Wrong Info

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