Last Updated : 23 Jul, 2025
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);
If you are serious for competitive programming then learning only fast I/O is not the solution there are many other different concepts and short tricks which are very important to learn in order to become good competitive programmer. For you guys we launched a special competitive programming course in which we have covered all these concepts
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.
C++
// A normal IO example code
#include <bits/stdc++.h>
using namespace std;
int main()
{
int n, k, t;
int cnt = 0;
cin >> n >> k;
for (int i=0; i<n; i++)
{
cin >> t;
if (t % k == 0)
cnt++;
}
cout << cnt << "\n";
return 0;
}
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.
C++
// A fast IO program
#include <bits/stdc++.h>
using namespace std;
int main()
{
// added the two lines below
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n, k, t;
int cnt = 0;
cin >> n >> k;
for (int i=0; i<n; i++)
{
cin >> t;
if (t % k == 0)
cnt++;
}
cout << cnt << "\n";
return 0;
}
Now, talking about competitive contests like ACM ICPC, Google CodeJam, TopCoder Open, here is an exclusive code to read integers in the fastest way.
C++
void fastscan(int &number)
{
//variable to indicate sign of input number
bool negative = false;
register int c;
number = 0;
// extract current character from buffer
c = getchar();
if (c=='-')
{
// number is negative
negative = true;
// extract the next character from the buffer
c = getchar();
}
// Keep on extracting characters if they are integers
// i.e ASCII Value lies from '0'(48) to '9' (57)
for (; (c>47 && c<58); c=getchar())
number = number *10 + c - 48;
// if scanned input has a negative sign, negate the
// value of the input number
if (negative)
number *= -1;
}
// Function Call
int main()
{
int number;
fastscan(number);
cout << number << "\n";
return 0;
}
getchar_unlocked() for faster input in C for competitive programming
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