HitmanBBa's blog

Problem with cin.tie(0); / ios_base::sync_with_stdio(0);
By HitmanBBa, 10 years ago, In English

Hello everyone.

Today, while I'm trying to solve this easy problem 404A - Валера и X,

I write this code 11264735 using cin.tie(0); and ios_base::sync_with_stdio(0); and I got a wrong answer on test 2, but when I removed them from my code 11264741 I got Accepted, why??

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10 years ago, # |
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You can't use printf/scanf once you write ios_base::sync_with_stdio(0)

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    10 years ago, # ^ |
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    Can you explain, please :)

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      10 years ago, # ^ |
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      ios_base::sync_with_stdio(0) will de-synchronize cin from scanf and cout from printf. This is fine and will result in a speedup if you just use cin and cout, but if you mix that with stdio-style IO, then weird things happen. For example if the input is 5 6 and your code says

      int a, b;
scanf("%d", &a);
cin >> b;

      You might get a=6, b=5 or something like that. Similarly,

      printf("%s", "YES");
cout << "NO";

      might result in "NOYES" instead of "YESNO". At least that's my understanding of it.

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        10 years ago, # ^ |
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        Thanks a lot. :)

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        10 years ago, # ^ |
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        what is the use of cin.tie(0) ?

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          10 years ago, # ^ |
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          It seems that by default, cout is flushed before each input operation on cin:

          http://www.cplusplus.com/reference/ios/ios/tie/

          cin.tie(0) turns this off by instead "tying" cin to nothing.

          Presumably this is done for performance reasons. It would make a significant difference in programs that interweave numerous cin/cout calls, such as those that answer a large number of queries from the input in an online manner.

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