Question

I was trying to solve a problem on InterviewStreet. After some time I determine that I was actually spending the bulk of my time reading the input. This particular question had a lot of input, so that makes some amount of sense. What doesn’t make sense is why the varying methods of input had such different performances:

Initially I had:

std::string command;
std::cin >> command;

Replacing it made it noticeably faster:

char command[5];
cin.ignore();
cin.read(command, 5);

Rewriting everything to use scanf made it even faster

char command;
scanf("get_%c", &command);

All told I cut the time reading the input down by about a 1/3.

I’m wondering there is such a variation in performance between these different methods. Additionally, I’m wondering why using gprof didn’t highlight the time I was spending in I/O, rather seeming to point the blame to my algorithm.

Answer 1

There is a big variation in these routines because console input speed almost never matters.

And where it does (Unix shell) the code is written in C, reads directly from the stdin device and is efficient.