Read matrix from text file using vectors in C++ - c++

Question

we have a matrix in text file like this with commas between numbers, but there is no comma at the end of each line.
1,2,3,4
7,8,2,1
3,4,5,6
7,2,1,3
I was trying to do this with a 2D array like this but it wasn't really working out because also size of matrix is unknown.
string array[4][4];
int id;
for (int i = 0; i < 4; i++) { // go through each line
for (int j = 0; j < 4; j++) {
getline(filein, numbers, ',');
array[i][j] = numbers;
cout << array[i][j] << endl;
}
}
I want to do that with using 2D vectors but I have no idea how to do that. Like after creating a vector with
vector<vector<string>> matrix;
Should I create one more vector inside the loops?

Answer 1

Use vector of vectors. Here is the commentary for each line:
std::vector<std::vector<int>> v; // declare vector of vectors
std::ifstream ifs("myfile.txt"); // open the file
std::string tempstr; // declare a temporary string
int tempint; // declare a temporary integer
char delimiter; // declare a temporary delimiter
while (std::getline(ifs, tempstr)) { // read line by line from a file into a string
std::istringstream iss(tempstr); // initialize the stringstream with that string
std::vector<int> tempv; // declare a temporary vector for the row
while (iss >> tempint) { // extract the numbers from a stringstream
tempv.push_back(tempint); // push it onto our temporary vector
iss >> delimiter; // read the , delimiter
}
v.push_back(tempv); // push the vector onto vector of vectors
}
The full source code is:
#include <iostream>
#include <fstream>
#include <vector>
#include <sstream>
#include <string>
int main() {
std::vector<std::vector<int>> v;
std::ifstream ifs("myfile.txt");
std::string tempstr;
int tempint;
char delimiter;
while (std::getline(ifs, tempstr)) {
std::istringstream iss(tempstr);
std::vector<int> tempv;
while (iss >> tempint) {
tempv.push_back(tempint);
iss >> delimiter;
}
v.push_back(tempv);
}
for (auto row : v) {
for (auto el : row) {
std::cout << el << ' ';
}
std::cout << "\n";
}
}

Answer 2

Use a double std::vector, read the file line by line, and parse the commas as well, and you are done. Every time you read a line, increase the vector's size by 1. You can do that with std::vector::resize().
Example, using your file as "matrix.txt":
#include <iostream>
#include <sstream>
#include <string>
#include <fstream>
#include <vector>
int main(void) {
std::vector<std::vector<int>> matrix;
std::ifstream infile("matrix.txt");
int a, b, c, d;
char comma;
while (infile >> a >> comma >> b >> comma >> c >> comma >> d)
{
//std::cout << a << " " << b << " " << c << " " << d << std::endl;
matrix.resize(matrix.size() + 1);
matrix[matrix.size() - 1].push_back(a);
matrix[matrix.size() - 1].push_back(b);
matrix[matrix.size() - 1].push_back(c);
matrix[matrix.size() - 1].push_back(d);
}
for(auto row: matrix) {
for(auto v: row) {
std::cout << v << " ";
}
std::cout << "\n";
}
return 0;
}
Output:
Georgioss-MacBook-Pro:~ gsamaras$ g++ -std=c++0x main.cpp
Georgioss-MacBook-Pro:~ gsamaras$ ./a.out
1 2 3 4
7 8 2 1
3 4 5 6
7 2 1 3