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Fast input-output in C++ is a significant element to master when attempts are made to reduce the time of execution in competitive programming. It is a popular and common technique used especially in problems where the standard input-output functions like cin and cout fall short because of their slower execution time.
Let’s start with addressing the problem first. When dealing with competitive programming or large data sets, the conventional i/o methods in C++ (cin and cout) become inefficient. This is partly because they are synchronized with stdio, meaning they maintain an order between the C (stdio) and C++ streams (cin or cout). This synchronization is time-consuming when it comes to processing large data sets. Hence, the need for fast i/o.
Fast I/O Techniques
The most common methods for fast i/o in C++ include using scanf/printf instead of cin/cout, and using cin/cout with std::ios::sync_with_stdio(0); and cin.tie(NULL); These methods significantly reduce synchronization cost.
#include
using namespace std;
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
// rest of your code
}
In the code snippet above, ios_base::sync_with_stdio(false) disables 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. cin.tie(NULL) unties cin from cout, ensuring they do not wait for each other’s operations to complete.
Step by Step Explanation
The first step is to call ios_base::sync_with_stdio(false). It’s important to do this before performing any input or output operation. It disconnects the C++ standard streams from their corresponding C streams. This prevents any overhead that might occur due to synchronisation.
The next step is to call cin.tie(NULL). Normally, cin waits for cout to flush its buffer before executing, or in other words, they’re tied. Tying of streams guarantees a predictable order of reads and writes. But it might lead to slowing down of input/output operations and hence we untie them.
#include
int main(){
std::ios::sync_with_stdio(0);
std::cin.tie(0);
// Now, cin/cout will work faster
}
Drawbacks
While these methods improve speed, they have their drawbacks. These methods make mixed input and output, i.e., usage of both cin/cout and scanf/printf, in the same program behave inconsistently. Thus, it’s recommended to stick with one type of functions only.
Fast i/o can significantly reduce your program’s execution time in C++, providing a significant edge in competitive programming scenarios. It’s considered a best practice when dealing with massive datasets and can be the difference between an accepted solution and a timeout error.
It is a crucial tool in the kit of every serious competitive programmer, and understanding its workings, benefits, and tradeoffs is valuable for getting the most out of your code.
