Fastest way to write vector <pair <double,double> to file

Question

I'm trying to write a lot of x/y double points to a file.

I came up with the following function to be the fastest solution, yet.

Are there any other ways speed up the process?

Writing to the stringstream first and then opening the file gave a nice speed boost.

bool printPoints(const vector <pair <double,double> > &points, const string &file)
{
    if(points.empty())
        return false;

    vector <pair <double,double> > const_iterator i;

    if(file != "")
    {
        stringstream ss;
        for(i=points.begin(); i != points.end();++i )
        {
           ss << i->first << " " << i->second << "\n";
        }

        ofstream out(file.c_str());
        if(out.fail())
        {
            out.close();
            return false;
        }
        out << ss.str();
        out.close();
    }
    return true;
}

Answer 1

I tested this. Writing to a stringstream buys you next to nothing. Using FILE * instead of fstream does give a reasonable improvement.

Here's my test-code:

#include <vector>
#include <utility>
#include <fstream>
#include <iostream>
#include <sstream>
#include <cstdio>

using namespace std;

bool printPoints(const vector <pair <double,double> > &points, const string &file)
{
    if(points.empty())
        return false;

    vector <pair <double,double> >::const_iterator i;

    if(file != "")
    {
        stringstream ss;
        for(i=points.begin(); i != points.end();++i )
        {
           ss << i->first << " " << i->second << "\n";
        }

        ofstream out(file.c_str());
        if(out.fail())
        {
            out.close();
            return false;
        }
        out << ss.str();
        out.close();
    }
    return true;
}

bool printPoints2(const vector <pair <double,double> > &points, const string &file)
{
    if(points.empty())
        return false;

    vector <pair <double,double> >:: const_iterator i;

    if(file != "")
    {
        ofstream out(file.c_str());
        if(out.fail())
        {
            out.close();
            return false;
        }
        for(i=points.begin(); i != points.end();++i )
        {
           out << i->first << " " << i->second << "\n";
        }

        out.close();
    }
    return true;
}


bool printPoints3(const vector <pair <double,double> > &points, const string &file)
{
    if(points.empty())
        return false;

    vector <pair <double,double> >:: const_iterator i;

    if(file != "")
    {
    FILE *out = fopen(file.c_str(), "w");
        if(!out)
        {
            return false;
        }
        for(i=points.begin(); i != points.end();++i )
        {
        fprintf(out, "%f %f", i->first, i->second);
        }

        fclose(out);
    }
    return true;
}

static __inline__ unsigned long long rdtsc(void)
{
    unsigned hi, lo;
    __asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi));
    return ( (unsigned long long)lo)|( ((unsigned long long)hi)<<32 );
}

int main()
{
    vector <pair <double,double> >  v;
    unsigned long long t1, t2;

    for(int i = 1; i <= 10000000; i++)
    {
    v.push_back(make_pair<double, double>((double)i, 1.0/i)); 
    }
    t1 = rdtsc();
    printPoints(v, "points.txt");
    t2 = rdtsc();
    cout << "time = " << t2 - t1 << endl;
    t1 = rdtsc();
    printPoints2(v, "points2.txt");
    t2 = rdtsc();
    cout << "time = " << t2 - t1 << endl;
    t1 = rdtsc();
    printPoints3(v, "points3.txt");
    t2 = rdtsc();
    cout << "time = " << t2 - t1 << endl;
}   

---

Results:
time = 55363637480
time = 54413392112
time = 33069402767

Obviously, the results may vary depending on the processor type, memory type, hard disk system (or network drive storage), etc, etc. But I've tested this in the past, and found similar results.