How to write vector values to a file

Question

I have a large vector.

The ways that I use multiply the run-time of the program hugely. The first is write all values to a string as they are calculated using

Answer 1

You can use std::copy and std::ostream_iterator.

Answer 2

Assuming you have C++11:

#include <fstream>
#include <vector>
#include <string>

int main()
{
    std::vector<std::string> v{ "one", "two", "three" };
    std::ofstream outFile("my_file.txt");
    // the important part
    for (const auto &e : v) outFile << e << "\n";
}

Answer 3

Using std::ofstream, std::ostream_iterator and std::copy() is the usual way to do this. Here is an example with std::strings using C++98 syntax (the question was asked pre-C++11):

#include <fstream>
#include <iterator>
#include <string>
#include <vector>

int main()
{
    std::vector<std::string> example;
    example.push_back("this");
    example.push_back("is");
    example.push_back("a");
    example.push_back("test");

    std::ofstream output_file("./example.txt");
    std::ostream_iterator<std::string> output_iterator(output_file, "\n");
    std::copy(example.begin(), example.end(), output_iterator);
}

Answer 4

Maybe I have missed something, but what is wrong with:

std::ofstream f("somefile.txt");
for(vector<X>::const_iterator i = v.begin(); i != v.end(); ++i) {
    f << *i << '\n';
}

That avoids having to do potentially quadratic string concatenation, which I assume is what's killing your run-time.