有没有办法比getchar（）（C / C ++）更快地读取字符串？

Question

I am participating in some programming competitions, and on many problems there's the need to read strings from an input file. Obviously performance is a big issue on those competitions, and strings might be huge, so I am trying to understand the most efficient way to read those strings.

My guess is that reading the strings char by char, with getchar(), is the fastest you can go. That's because even if you use other functions, say fgets() or getline(), those functions will still need to read every char anyway.

Update : I know that I/O won't be a bottleneck on most algorithmic problems. That being said I would still very much like to know what's the fastest way you can use to read strings, should this become an issue on any future problem.

Answer 1

You can use std::istream::read() function to read a chunk of unformatted data. It is relatively faster precisely because the data is unformatted . All overloads of operator>> read formatted data which makes reading from stream slower compared to read() .

Similarly, you can use std::ostream::write() function to write a chunk of data to output stream at once.

Answer 2

The reverse is true, reading larger chunks of data into memory in one go is far faster than reading one character at a time. The OS and/or hard drive will likley cache the data in any case, but the function call overhead alone of repeatedly cycling through the standard-library, OS, file system and device driver for each character is significant for large data sets.

When handling strings there are some more important performance issues you might consider: Back to Basics by Joel Spolsky

Either way, the most convincing way to answer the question for yourself is to write test code that investigates the difference between different I/O methods.