Fast way to write data from a std::vector to a text file

c++ std ofstream
Diego Fernando Pava picture Diego Fernando Pava · Sep 28, 2016 · Viewed 10.8k times · Source

I currently write a set of doubles from a vector to a text file like this:

std::ofstream fout;
fout.open("vector.txt");

for (l = 0; l < vector.size(); l++)
    fout << std::setprecision(10) << vector.at(l) << std::endl;

fout.close();

But this is taking a lot of time to finish. Is there a faster or more efficient way to do this? I would love to see and learn it.

Answer

LogicStuff picture LogicStuff · Sep 28, 2016 
std::ofstream fout("vector.txt");
fout << std::setprecision(10);

for(auto const& x : vector)
    fout << x << '\n';

Everything I changed had theoretically worse performance in your version of the code, but the std::endl was the real killer. std::vector::at (with bounds checking, which you don't need) would be the second, then the fact that you did not use iterators.

Why default-construct a std::ofstream and then call open, when you can do it in one step? Why call close when RAII (the destructor) takes care of it for you? You can also call

fout << std::setprecision(10)

just once, before the loop.

As noted in the comment below, if your vector is of elements of fundamental type, you might get a better performance with for(auto x : vector). Measure the running time / inspect the assembly output.

Just to point out another thing that caught my eyes, this:

for(l = 0; l < vector.size(); l++)

What is this l? Why declare it outside the loop? It seems you don't need it in the outer scope, so don't. And also the post-increment.

The result:

for(size_t l = 0; l < vector.size(); ++l)

I'm sorry for making code review out of this post.

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