Shows how to generate a file with 128 rows of 3300 random floating point values, then load it back in

random_data_save_load.cpp

#include <iostream>
#include <vector>
#include <sstream>
#include <random>
#include <fstream>

void save_random_data();
void read_random_data();

int main()
{
    {
        auto start = std::chrono::high_resolution_clock::now();
        save_random_data();
        auto end = std::chrono::high_resolution_clock::now();

        std::cout << "Saving data: "
                  << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count() / 1000.0 << " seconds\n";
    }


    {
        auto start = std::chrono::high_resolution_clock::now();
        read_random_data();
        auto end = std::chrono::high_resolution_clock::now();

        std::cout << "Reading data: "
                  << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count() / 1000.0 << " seconds\n";
    }
}

void save_random_data()
{
    constexpr size_t no_rows = 128;
    constexpr size_t no_cols = 3300;

    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_real_distribution<> dis(-1, 1);

    std::ofstream output("file.txt");

    for (size_t i = 0; i < no_rows; ++i)
    {
        for (size_t j = 0; j < no_cols; ++j)
        {
            output << ' ' << dis(gen);
        }
        output << '\n';
    }
}

void read_random_data()
{
    std::vector<std::vector<double>> data(128, std::vector<double>(3300));

    std::ifstream input("file.txt");

    std::stringstream input_buffer;
    input_buffer << input.rdbuf();

    std::string line;
    for (size_t i = 0; std::getline(input_buffer, line); ++i)
    {
        std::istringstream iss(line);
        size_t j = 0;

        double v;
        while (iss >> v)
            data[i][j++] = v;
    }
}

Writing seems consistent to hover a little over 0.2 seconds.
Reading have in my simple testing been between 0.3 and 0.5 seconds.