Fast I/O for Competitive Programming

In competitive programming, it is important to read input as fast as possible so we save valuable time.

You must have seen various problem statements saying: “Warning: Large I/O data, be careful with certain languages (though most should be OK if the algorithm is well designed)”. The key for such problems is to use Faster I/O techniques. 

It is often recommended to use scanf/printf instead of cin/cout for fast input and output. However, you can still use cin/cout and achieve the same speed as scanf/printf by including the following two lines in your main() function:

    ios_base::sync_with_stdio(false);

It toggles on or off the synchronization of all the C++ standard streams with their corresponding standard C streams if it is called before the program performs its first input or output operation. Adding ios_base::sync_with_stdio (false); (which is true by default) before any I/O operation avoids this synchronization. It is a static member of the function of std::ios_base. 

    cin.tie(NULL);

tie() is a method that simply guarantees the flushing of std::cout before std::cin accepts an input. This is useful for interactive console programs which require the console to be updated constantly but also slows down the program for large I/O. The NULL part just returns a NULL pointer.

Moreover, you can include the standard template library (STL) with a single include:  

    #include <bits/stdc++.h>

So your template for competitive programming could look like this:  

#include <bits/stdc++.h>
using namespace std;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}

It is recommended to use cout << “\n”; instead of cout << endl;. endl is slower because it forces a flushing stream, which is usually unnecessary (See this for details). (You’d need to flush if you were writing, say, an interactive progress bar, but not when writing a million lines of data.) Write ‘\n instead of endl.

We can test our input and output methods on the problem INTEST – Enormous Input Teston SPOJ. Before further reading, I would suggest you solve the problem first.
Solution in C++ 4.9.2

Normal I/O: The code below uses cin and cout. The solution gets accepted with a runtime of 2.17 seconds. 

Fast I/O However, we can do better and reduce the runtime a lot by adding two lines. The program below gets accepted with a runtime of 0.41 seconds.

Now, talking about competitive contests like ACM ICPC, Google CodeJam, TopCoder Open, here is an exclusive code to read integers in the fastest way.

getchar_unlocked() for faster input in C for competitive programming