c ++ vs MATLAB优化以提高速度。 例如我的matlab代码比c ++运行得快？

Question

Ive written a program that reads in a ascii file with cell vertice coordinates and then outputs a file with cell centres. Is there any way I can speed up the c++ code, to at least match my matlab performance. Im using push_back vectors in my c++ code but I don't know how I would achieve similar with arrays??

Im trying to use sleepy for profiling but how do you get sleepy to start profiling before you set of the program it can only program runs that are already running, also the outputs are very confusing and dont go to my lines of code but lines like std::_Vector_cal >.... etc ect. Whats the difference between exclusive and inclusive, and the highest % exclusive name is operator delete and second is operator new????

My c++ source:

#include <algorithm>
#include <fstream>
#include <iostream>
#include <iterator>
#include <sstream>
#include <string>
#include <vector>
#include <cstdlib>

std::vector<double> GetValues_n(const std::vector<std::string>& src, int start, int end)
{
    std::vector<double> ret;
    for(int i = start; i <= end; ++i)
    {
        ret.push_back(std::strtod(src[i].c_str(), nullptr));
    }
    return ret;
}

std::vector<int> GetValues_c(const std::vector<std::string>& src, int start, int end)
{
    std::vector<int> ret;
    for(int i = start; i <= end; ++i)
    {
        ret.push_back(std::atoi(src[i].c_str()));
    }
    return ret;
}

std::vector<double> polycentre(const std::vector<double>&  x,const std::vector<double>&  y,size_t ID)
{
    std::vector<double> C(3, 0);
    std::vector<double> x1(x.size(),0);
    std::vector<double> y1(y.size(),0);
    int sizx = x.size();
    int sizy = y.size();
    if(sizy != sizx)
    {
        std::cerr << "polycentre inputs not equal length";
    }
    double x0 = x[0];
    double y0 = y[0];
    for(int aa = 1; aa < sizx; ++aa)
    {
        if(x[aa] < x0)
        {
            x0 = x[aa];
        }
        if(y[aa] < y0)
        {
            y0 = y[aa];
        }
    }
    double A = 0.0;
    double B1 = 0.0;
    double B2 = 0.0;
    for(int aa = 0; aa < sizx; ++aa)
    {
        x1[aa] = x[aa] - x0;
        y1[aa] = y[aa] - y0;
    }
    for(int aa = 0; aa < sizx; ++aa)
    {
        if(aa != sizx-1)
        {
            A = A + (x1[aa]*y1[aa+1] - x1[aa+1]*y1[aa]);
            B1 = B1 + ((x1[aa]+x1[aa+1])*(x1[aa]*y1[aa+1]-x1[aa+1]*y1[aa]));
            B2 = B2 + ((y1[aa]+y1[aa+1])*(x1[aa]*y1[aa+1]-x1[aa+1]*y1[aa]));
        }
        else if(aa == sizx-1)
        {
            A = A + (x1[aa]*y1[0] - x1[0]*y1[aa]);
            B1 = B1 + ((x1[aa]+x1[0])*(x1[aa]*y1[0]-x1[0]*y1[aa]));
            B2 = B2 + ((y1[aa]+y1[0])*(x1[aa]*y1[0]-x1[0]*y1[aa]));
        }
    }
    A = A*0.5;
    C[0] = ID;
    C[1] = (((1/6.0)/A)*B1) + x0;
    C[2] = (((1/6.0)/A)*B2) + y0;
    return C;
}

template <typename T>

void PrintValues(const std::string& title, std::vector<std::vector<T>>& v, std::ofstream& outfil)
{
    if(outfil.is_open())
    {
        outfil << "ID,X,Y,Z \n";
        std::cout << title << std::endl;
        for(size_t line = 0; line < v.size(); ++line)
        {
            for(size_t val = 0; val < v[line].size(); ++val)
            {
                //std::cout << v[line][val] << " ";
                outfil.precision(10); 
                outfil << v[line][val] << ",";
            }
            outfil << "\n";
            //std::cout << std::endl;
        }
        //std::cout << std::endl;
    }
}

int main(int argc, char* argv[])
{

    std::ofstream outfil;

    if (argc < 2)
    {
        std::cerr << argv[0] << " needs to get input file (2dm)" << std::endl;
    }

    else if (argc == 3)
    {
        outfil.open(argv[2]);
    }

    else
    {
        outfil.open(std::string(argv[1]) + ".csv");
    }

    std::vector<std::vector<std::string>> values;
    std::ifstream fin(argv[1]);

    for (std::string line; std::getline(fin, line); )
    {
        std::istringstream in(line);
        values.push_back(
                         std::vector<std::string>(std::istream_iterator<std::string>(in),
                                                  std::istream_iterator<std::string>()));
    }

    std::vector<std::vector<int>> cells;
    std::vector<std::vector<double>> nodes;

    for (size_t i = 0; i < values.size(); ++i)
    {
        if(values[i][0] == "E3T")
        {
            cells.push_back(GetValues_c(values[i], 1, 5));
        }
        else if(values[i][0] == "E4Q")
        {
            cells.push_back(GetValues_c(values[i], 1, 6));
        }
        else if(values[i][0] == "ND")
        {
            nodes.push_back(GetValues_n(values[i], 1, 4));
        }
    }

    std::vector<std::vector<double>> cell_centres;

    for (size_t aa = 0; aa < cells.size(); ++aa)
    {
        if(cells[aa].size() == 5)
        {
            std::vector<double> xs;
            xs.push_back(nodes[cells[aa][1] - 1][1]);
            xs.push_back(nodes[cells[aa][2] - 1][1]);
            xs.push_back(nodes[cells[aa][3] - 1][1]);
            std::vector<double> ys;
            ys.push_back(nodes[cells[aa][1] - 1][2]);
            ys.push_back(nodes[cells[aa][2] - 1][2]);
            ys.push_back(nodes[cells[aa][3] - 1][2]);
            cell_centres.push_back(polycentre(xs,ys,aa+1));
        }
        else if(cells[aa].size() == 6)
        {
            std::vector<double> xs;
            xs.push_back(nodes[cells[aa][1] - 1][1]);
            xs.push_back(nodes[cells[aa][2] - 1][1]);
            xs.push_back(nodes[cells[aa][3] - 1][1]);
            xs.push_back(nodes[cells[aa][4] - 1][1]);
            std::vector<double> ys;
            ys.push_back(nodes[cells[aa][1] - 1][2]);
            ys.push_back(nodes[cells[aa][2] - 1][2]);
            ys.push_back(nodes[cells[aa][3] - 1][2]);
            ys.push_back(nodes[cells[aa][4] - 1][2]);
            cell_centres.push_back(polycentre(xs,ys,aa+1));
        }
    }

    PrintValues("Cell Centres", cell_centres, outfil);
    return 0;
}

Answer 1

The most obvious thing here is to reserve the storage before push_back calls. That way the vector is only resized once:

std::vector<double> GetValues_n(const std::vector<std::string>& src, int start, int end)
{
    std::vector<double> ret;
    ret.reserve( end - start + 1 );

    for(int i = start; i <= end; ++i)
    {
        ret.push_back(std::strtod(src[i].c_str(), nullptr));
    }
    return ret;
}

Answer 2

In addition to what @Paddy said, Keep this in mind too : Copy cTor's are involved when you are doing a vector.push_back. You are using "c_str()". Do investigate if it is efficient enough.

One more thing: GetValues_n and GetValues_c are returning vectors by value. You have taken the vector argument by reference. Which is good thing efficiency wise. Can you also try to return the vectors by reference?