Question

i would like to C++-read a file line by line, and split each line w.r.t to "\t" character so as to fill in a matrix. my code would go like this

    ifstream data_x;
        double** test_data = new double*[100];
        for(int j = 0 ; j <  ; j++)
             test_data[j] = new double[4]; 

    data_x.open("X.txt");

    int i = 0;
    if (data_x.is_open()) 
    {
        while (!data_x.eof()) 
        {
            char** split = data_x.split("\t") 
            for(int k = 1 ; k < 4 ; k++)
                 test_data[i][k];
            i++;
        }
    }

    data_x.close();

    ifstream data_y;
    data_y.open("Y.txt");

    i = 0;
    if (data_y.is_open()) 
    {
        while (!data_y.eof()) 
        {
            data_y >> test_data[i][0];
            i++;
        }
    }

    data_y.close(); 

where syntax

 char** split = data_x.split("\t") 
 for(int k = 1 ; k < 4 ; k++)
      test_data[i][k];

is approximative. how to make this properly with C++ ?

thanks

Answer 1

Assuming that your file only contains numbers, here’s the standard C++ idiom:

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

std::ifstream infile("data.txt");

std::vector<std::vector<double>> matrix;

for (std::string line; std::getline(infile, line); )
{
     std::istringstream iss(line);
     std::vector<double> row;

     for (double d; iss >> d; )
     {
         row.push_back(d);
     }

     matrix.push_back(row);
}

If you know the size of the matrix, you can add the relevant reserve calls to avoid vector reallocations. You can also add tests for whether there was any unrecognized data on a line, but for now this should get you started.