Parse .xyz file and load to std::vector

parse_xyz.cpp

#include <iostream>
#include <fstream>
#include <regex>
#include <string>


using XYZ = std::array<double, 3>;
using Coord3D = std::vector<XYZ>;


int main() {
    std::ifstream f;
    std::string line;
    std::string particle_type;
    std::regex head{"\\d+"};
    std::smatch matched;

    Coord3D coordinates {};
    XYZ xyz {0, 0, 0};

    double x = 0;
    int N = 0;
    int total_frame = 0;
    
    f.open("sample_bulk.xyz", std::ios::in);

    while (f) {
        getline(f, line);
        if (regex_match(line, matched, head)){
            coordinates.clear();
            N = stoi(line);
            total_frame += 1;
            std::cout << "processing frame " << total_frame;
            getline(f, line); // skip the comment line

            for (int i = 0; i < N; i++){
                getline(f, line); // read line with content [Type x y z]
                std::istringstream ss(line);
                ss >> particle_type;
                for (int j = 0; j < 3; j++){
                    ss >> x;
                    xyz[j] = x;
                }
                coordinates.push_back(xyz);
            }
            std::cout << " (N = " << coordinates.size() <<  ")" << std::endl;
        }
    }
    f.close();
}

The format of xyz file is

500
Type, x, y, z
A 6.83552 4.41063 6.6408
...