Performance Difference Between C and C++ Style File IO
c, c++, file-io, stream
Solution
You're using `endl` to print a newline. That is the problem here, as it does more than just printing a newline — `endl` also flushes the buffer which is an expensive operation (if you do that in each iteration).
Use `\n` if you mean so:
file << i << '\n';
And also, must compile your code in release mode (i.e turn on the optimizations).
Problem
I've always heard that C++ file I/O operations are much much slower than C style I/O. But I didn't find any practical references on comparatively how slow they actually are, so I decided to test it in my machine (Ubuntu 12.04, GCC 4.6.3, ext4 partition format). First I wrote a ~900MB file in the disk. C++ (`ofstream`): 163s ``` ofstream file("test.txt"); for(register int i = 0; i < 100000000; i++) file << i << endl; ``` C (`fprintf`): 12s ``` FILE *fp = fopen("test.txt", "w"); for(register int i = 0; i < 100000000; i++) fprintf(fp, "%d\n", i); ``` I was expecting such output, it shows that writing to a file is much slower in C++ than in C. Then I read the same file using C and C++ I/O. What made me exclaimed that there is almost no difference in performance while reading from file. C++ (`ifstream`): 12s ``` int n; ifstream file("test.txt"); for(register int i = 0; i < 100000000; i++) file >> n; ``` C (`fscanf`): 12s ``` FILE *fp = fopen("test.txt", "r"); for(register int i = 0; i < 100000000; i++) fscanf(fp, "%d", &n); ``` So, why is taking so long to execute writing using stream? Or, why reading using stream is so fast compared to writing? Conclusion: The culprit is the `std::endl`, as the answers and the comments have pointed out. Changing the line `file << i << endl;` to `file << i << '\n';` has reduced running time to 16s from 163s.