_Comfortably_Numb__'s blog

When should we use ios_base::sync_with_stdio(0)?
By _Comfortably_Numb__, history, 7 years ago, In English

In problem 220B - Little Elephant and Array my submission 36377029 got WA. Then I comment ios_base::sync_with_stdio(0) and submit the same code 36377330 again. It got accepted. Now I wonder why my first code got WA and when we should use ios_base::sync_with_stdio(0) and when we shouldn't.

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7 years ago, # |
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I think you only use it if you are using cin and cout. Don't mix cin/scanf/cout/printf or weird things can happen.

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    7 years ago, # ^ |
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    I submitted this 36376045 with ios_base::sync_with_stdio(0) , cin and cout. No scanf, printf. But it got TLE. And in this 36378988 submission I add cout.tie(0) with them. But
    it got WA. :(

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      7 years ago, # ^ |
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      On your second sumbission you forget to delete scanf and printf, that's why you got WA.

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        7 years ago, # ^ |
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        Thanks. cin.tie(0) works 36386259.

        But still wonder why ios_base::sync_with_stdio(0) without cin.tie(0) gives WA in this problem.

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7 years ago, # |
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I would also add that this optimisation only makes sense if your input or output have more than a constant amount of characters (e.g. if there is a valid test in which any of these reach at least a thousand), because otherwise the time you spend on executing this function overweights the time you save from input/output.

You can also use cin.tie(0) and cout.tie(0) in addition to this, they as well speed things up a bit.

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