How to read an "uneven" matrix from a file, and store into a 2D array?

Question

I'm working on an experiment that requires me to switch over to C++, which I'm still learning. I need to read data from a file into a 2D array, where the data in the file is composed of floating point numbers, laid out in a matrix format. However, each row of the matrix in the data file has a different number of columns, for example:

1.24 3.55 6.00 123.5
65.8 45.2 1.0
1.1 389.66 101.2 34.5 899.12 23.7 12.1

The good news is that I know the maximum number of possible rows/columns the file might have, and at least right now, I'm not particularly worried about optimization for memory. What I would like is to have a 2D array where the corresponding rows/columns match those of the file, and all the other elements are some known "dummy" value.

The idea I had was to loop through each element of the file (row by row), recognize the end of a line, and then begin reading the next line. Unfortunately I'm having trouble executing this. For example:

#include <iostream>
#include <fstream>

int main() {

const int max_rows = 100;
const int max_cols = 12;
//initialize the 2D array with a known dummy
float data[max_rows][max_cols] = {{-361}};

//prepare the file for reading
ifstream my_input_file;
my_input_file.open("file_name.dat");

int k1 = 0, k2 = 0; //the counters
while (!in.eof()) { //keep looping through until we reach the end of the file
    float data_point = in.get(); //get the current element from the file
    //somehow, recognize that we haven't reached the end of the line...?
        data[k1][k2] = next;
    //recognize that we have reached the end of the line
        //in this case, reset the counters
        k1 = 0;
        k2=k2+1;
}
}

And so I haven't been able to figure out the indexing. Part of the problem is that although I'm aware that the character "\n" marks the end of a line, it is of a different type compared to the floating point numbers in the file, and so I'm at a loss. Am I thinking about this the wrong way?

Answer 1

You don't need to know the limits in advance if you stick to std::vector. Heres some example code that will read the file (assuming no non-floats in the file).

using Row = std::vector<float>;
using Array2D = std::vector<Row>;

int main() {
    Array2D data;
    std::ifstream in("file_name.dat");
    std::string line;
    Row::size_type max_cols = 0U;
    while (std::getline(in, line)) { // will stop at EOF
        Row newRow;
        std::istringstream iss(line);
        Row::value_type nr;
        while (iss >> nr) // will stop at end-of-line
            newRow.push_back(nr);
        max_cols = std::max(max_cols, newRow.size());
        data.push_back(std::move(newRow)); // using move to avoid copy
    }
    // make all columns same length, shorter filled with dummy value -361.0f
    for(auto & row : data)
        row.resize(max_cols, -361.0f);
}

Answer 2

Here is starting point, I will be working on code that you can use. Use two dimensional vector vector<vector<double>> and you can use getline() to get the line as a string than use string stream to get decimals from string.

And here is the code

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

int main (void) {
    std::vector<std::vector<double>> matrix;

    std::ifstream inputFile;
    inputFile.open("test.txt");
    char line[99];
    for (int i = 0; inputFile.getline(line, sizeof(line)); ++i) {    
        std::stringstream strStream (line);
        double val = 0.0;
        matrix.push_back(std::vector<double>());
        while (strStream >> val)
            matrix[i].push_back(val);
    }

    return 0;
}