When you use C++ and the input is really big you can't just use cin and cout. You need to speed up it with
ios::sync_with_stdio(0);
cin.tie(0);
Someone argues that the second line is unnecessary but it's not true. if the input and output alternate then adding the second line makes I/O more than twice faster. But then... Someone argues that we also need to use cout.tie(0)
.
I personally never use this and I don't know any case where it can help. So my question for today is the following: "Is there any case where we actually need cout.tie
? Or is it completely useless?"
I have read several comments related to it by a user -is-this-fft- and I quote "cout.tie(NULL) does literally nothing because cout is already tied to NULL", "It sounds very strange when you consider what tie does. Every input or output stream in C++ is tied to an ostream or to null. Tyig cin to null means it's not tied to cout anymore which can speed things up. But cout is generally already tied to null."
Link to the comments — https://codeforces.me/blog/entry/87419?#comment-756655, https://codeforces.me/blog/entry/83672?#comment-709924
Yes, you are correct.
I think it would be good to reiterate what
tie
does because I see a lot of people copying these lines with only a vague understanding of what they do.Every stream in C++ is tied to an output stream, which can be null.
What does this mean? First of all, it's important to understand that when you write
std::cout << "asdf"
, it is not necessarily immediately printed on the screen. It turns out that it is much better (in terms of performance) to collect it into a buffer and then, at some point, flush the buffer — i.e. empty its contents to the screen (or file, or any other "device"), all at once.But now, consider the following. You are developing a console application and write something like:
What would happen if
std::cout
didn't get flushed before line 3? The program would expect a reply from the user, but the user hasn't even had a chance to read the question. In an application like this, it would be a good idea ifstd::cout
was somehow automatically flushed when we try to read fromstd::cin
.And this is exactly the problem
tie
solves. If you have two streamsfin
andfout
, andfin
is tied tofout
, thenfout
is automatically flushed when you try to read fromfin
. So what are the defaults? What is tied to what? Quoting the C++ reference:Now it becomes clear why people recommend using
cin.tie(0)
in competitive programming. Suppose you are solving a query problem like this.If you didn't use
cin.tie(0)
, we would flushcout
every time we hit line 4; this is essentially as bad as writingendl
on line 5. Concerns about the user not seeing the question are also irrelevant now because we are given the entire input at once.The reference doesn't explicitly say that
cout
isn't tied to anything, but on every C++ compiler I tried,cout << cout.tie() << endl;
outputs 0. Also, even if you are using some strange compiler that tiescout
to some other streamsout
, you would only notice the performance hit if you wrote a lot tosout
which doesn't really happen in competitive programming.So yes,
cout.tie(0)
doesn't do anything and in some sense it's a sign that the user doesn't understandcin.tie(0)
very well either.While you are probably right in the sense that what you are saying is correct, I think it is a wrong way to understand
cin.tie(0)
in competitive programming. And the last line from your comment for me proves that you are wrong. The last line reads like the fact that "the user doesn't understand it" is bad somehow. I don't agree.cin.tie(0)
is just a bullshit line to makecin
faster. I don't give a fuck what it does inside or how it works or why it even exists. The same withusing namespace std
— it's just something that stupid computer needs to understand you, that's it.Well, I don't think it is very bad to not know what
tie
does. Still, I prefer to know these things to prevent any unpleasant surprises.Sorry, you answered a question that Um_nik wasn't interested in knowing the answer to. Please apologize for wasting his time.
hi
for me Um_nik is a legend who doesn't gives a F about anything and when he does he F's that thing
No, it was an interesting read. Useless, but interesting.
Classic um_nik way lol : useless but interesting
Pretty much the same as CP. Right ?
That depends on how you define usefulness.
Thanks. Now I completely understand why using
tie
would recieveIdleness Limit Exceeded
for those interactive problems. :DUm_nik, a guy with zero haters
Why do we still need to flush the buffer for interactive problems if one has not added
cin.tie(0)
?You know, I'm starting to think that you don't. I just went and resubmitted some solved interactive problems, removing any
endl
(those flush the output) and making sure there is nocin.tie(0)
. All of them got Accepted. Then I addedcin.tie(0)
and got (I think for the first time ever) the Idleness Limit Exceeded verdict.Your answer is very good but I mistakenly voted against, I'm sorry...
Thank you!
This blog is just to gain contribution.
I've seen your comments a lot lately on a lot of blogs. Now I think you're the one seeking +ve contribution.
cout.tie(0)
is not necessary, because by defaultcin
ties withcout
, but cout ties with nullptr so it's basically does nothing.You can check it by code:
If you do not properly understand, what
cin.tie(smth)
andcout.tie(smth)
do, you might want to write them all together. It doesn't slow your code down (does it?) and it decreases the chance you mix them up and write onlycout.tie(nullptr)
instead ofcin.tie(nullptr)
.I have somehow memorized that
cout.tie(nullptr)
is useless and excluded it from my pre-written pattern. I also do not write it in competitions that doesn't allow participants to use pre-written code. I don't know a simple way to remember, which string is useless (is cin kind of tied to cout, or is it the opposite?). So, I have just realised, it is better to write both of them.You're absolutely right. This question isn't about practice usage. I'm just interested in this.
Should we use
cin.tie(0);
in Interactive Problems where next input depends on what we print usingcout
?I think there isn't actually any need for fast IO in interactive problems most of the time because we use flush and it's really slow. But at the same time, there is no reason not to use fast IO always.
"Once and forever".... What a joke)
There still exists people that don't know that comparator must return true if and only if lhs is strictly less than rhs.
cout.tie in competitive programming is useless AFAIK. But it is usefull when you need to guarantee that you write in tied output strictly before cout. But I don't know when there is need for it;)
Well, I also had a question related to this, that Is their any difference btw
ios_base::sync_with_stdio(false); cin.tie(NULL);
andcin.tie(0)->sync_with_stdio(0);
(i saw this in Benq's template)Both do same
But I'd say first is preferable since second way is confusing.
sync_with_stdio
is static method and it supposed to be called as static not as member ofcout
.I am not sure of the logic behind it, but on a dp question on cses, my code gave runtime error without cout.tie(NULL) and got accepted when i added that line. https://cses.fi/paste/2f6f0fdf60a71e537a397d/ try submitting this same code without the cout.tie(NULL) line. it should give you a good idea
I have tried to submit the linked code with and without
cout.tie(NULL)
and it was accepted both times. No ideas where to find a good idea :)my bad, my code gave runtime error cause i used long long instead of int. don't know why it is, but you're right cout.tie(0) isnt the problem.
Can you please provide the problem link?
https://cses.fi/problemset/task/1158/
With Problem 2005-E2 - Subtangle Game (Hard Version), Codeforces is now openly telling people to use
cout.tie(NULL);
. I urge its authors, BiNARyBeastt and Tsovak, to read this blog and avoid spreading misinformation.TLDR: Do not use
cout.tie(0)
since it does nothing.Quoting purplesyringa:
Thanks for the information. I've been using that in my template all the time lol. I'll change it on polygon soon.