I have 10 large (34 million cell) 2D gridded vectors storing doubles. When written they are over 200 MB in size. I use an ofstream object to write them to a text file (.csv format), one element at a time, using two for loops (one for my rows, one for my columns). They take way to long to write. Is there a faster way to write from these vectors?

(I am using Visual Studio 2012.)

// Resizing of vectors
flowDirGrid.resize(rows, std::vector<double>(cols, NODATA));

// Do some processing

// Outputting processed data
ofstream outfile3("FlowDir.dat");

for(int i=0; i<rows; i++)        
        for (int j=0; j<cols; j++)
                outfile3 << flowDirGrid[i][j]<<" ";
                outfile3 << NODATA<<" ";
        outfile3 << std::endl;


The output-writing code should reside in a function that takes two parameters: the flowDirGrid and a filename.

Don't omit braces on multi-line conditionals. It's a filthy habit, and someday you will contribute to a coding accident like this.

A performance problem is outputting std::endl at the end of every line, which causes a flush. Output '\n' instead.


Looking at your conditional:

            outfile3 << flowDirGrid[i][j]<<" ";
            outfile3 << NODATA<<" ";

I rewrite as a ternary

 outfile3 << elevation[i][j] == NODATA ? NODATA : flowDirGrid[i][j] << " ";

The ternary poses importance on the fact that only the value changes. if should be used when also the action changes.

  • \$\begingroup\$ Will this really speed up the code? What's the typical performer improvement? \$\endgroup\$ – traggatmot Aug 15 '15 at 8:18
  • \$\begingroup\$ @traggatmot This may speed up the code a tiny bit but mainly enhances clarity. \$\endgroup\$ – Caridorc Aug 15 '15 at 8:22
  • \$\begingroup\$ That's funny. I find ternaries more confusing. \$\endgroup\$ – traggatmot Aug 15 '15 at 8:22
  • \$\begingroup\$ @traggatmot The result of a ternary can only be a value. You may find anything (an expression) in an if block. If the action (print) is the same, a ternary is more limited and therefore more easy to understand. \$\endgroup\$ – Caridorc Aug 15 '15 at 8:24

It might be worth considering using std::basic_streambuf as apposed to std::basic_ostream. You'd need to do the double formatting yourself, but this may still be quicker. Here's an example which shows how to do this for a single std::vector<double>, it should be fairly easy to generalise to a 2D version.

// streambuf.cpp

#include <fstream>
#include <iostream>
#include <iterator>
#include <vector>
#include <string>
#include <algorithm>
#include <cstddef>
#include <random>
#include <chrono>

void write_basic_ostream(const std::vector<double>& v, std::ofstream& of)
    std::copy(v.cbegin(), v.cend(), std::ostream_iterator<double> {of, " "});

void write_basic_streambuf(const std::vector<double>& v, std::ofstream& of)
    std::string str {};
    str.reserve(v.size() * (1 + sizeof(double)));

    for (const auto& e : v) {
        str += std::to_string(e);
        str += " ";

    std::copy(str.cbegin(), str.cend() - 1, std::ostreambuf_iterator<char>(of));

int main()
    std::size_t n {10000000};

    std::default_random_engine generator {};
    std::uniform_real_distribution<double> distribution {0.0, 1.0};

    std::vector<double> v(n);
    std::generate_n(v.begin(), n, [&generator, &distribution] { return distribution(generator); });

    std::ofstream of1 {"/Users/you/test_basic_ostream.txt"};
    std::ofstream of2 {"/Users/you/test_basic_streambuf.txt"};

    auto start = std::chrono::system_clock::now();

    write_basic_ostream(v, of1);

    auto end = std::chrono::system_clock::now();
    auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(end - start);

    std::cout << "basic_ostream: "  << duration.count() << std::endl;

    start = std::chrono::system_clock::now();

    write_basic_streambuf(v, of2);

    end = std::chrono::system_clock::now();
    duration = std::chrono::duration_cast<std::chrono::nanoseconds>(end - start);

    std::cout << "write_basic_streambuf: "   << duration.count() << std::endl;

    return 0;

Some quick benchmarks has write_basic_ostream about 20% slower than write_basic_streambuf on my system (g++ streambuf.cpp -std=c++14 -O3 -o test).


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