Store Matrix with integer from a text file into an array in c++

Question

I have a file named matrices.txt that has two matrices that are 3 by 3. They are:

1 2 3
4 5 6
7 8 9
1 2 3 
4 5 6 
7 8 9

I'm trying the read from this file and store it into an array proto_matrix to later split that into two matrices.

The problem I'm having is that I can't store the numbers in the array.

My code is

#include <iostream>
#include <fstream>

using namespace std;

void main()
{
    int i = 0, j, k = 0, n, dimension;
    cout << "Enter the dimension of square matrices (3 by 3 would be 3) \n";
    cin >> n;
    dimension = n * n;
    int proto_matrix[2 * dimension];

    // make array of two matrices combined, this will be split into two matrices
    ifstream matrix_file("matrices.txt");

    while(matrix_file)
    {
        matrix_file >> proto_matrix[i];
    }
    matrix_file.close();
}

I tried debugging the code and it seems like the no integer is stored in the array, just random numbers.

Answer 1

This:

int proto_matrix[2 * dimension];

is a Variable Length Array (VLA), which is not Standard C++, but is supported by some compiler extensions. If compile with g++ -pedantic main.cpp, you will surely get an error.

That means that you need to dynamically allocate your memory, if you had to use an array.

You really don't though, C++ offers std::vector, which can achieve what you want relatively easily. I say relatively, because you have two matrices in one file, which makes parsing the file a bit tricky. Usually we put the one matrix in one file, and the other matrix to another file.

• A 2D array can be represented as a 2D vector.
• You want two matrices, thus two 2D vectors.

For that reason, I will use an array of size 2 (fixed size!), where every element is going to be a 2D vector.

Moreover, I will keep track of which matrix I am reading from the file right now (the first or the second), and how many rows I have read currently, in order to populate the correct cell of the matrix.

index is going to be equal to 0 or 1, to index the array.

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

using namespace std;

int main()
{
    int i = 0, j, k = 0, n, dimension;
    cout << "Enter the dimension of square matrices (3 by 3 would be 3) \n";
    cin >> n;
    dimension = n * n;
    vector< vector<int> > v[2];
    v[0].resize(n);
    v[1].resize(n);
    // make array of two matrices combined, this will be split into two matrices
    ifstream matrix_file("matrices.txt");
    string line;
    int rows_read = 0, cols_read = 0, index = 0;
    while (std::getline(matrix_file, line))
    {
        std::istringstream iss(line);
        int a, b, c;
        if (!(iss >> a >> b >> c)) { break; } // error
        // process tuple (a, b, c)
        if(index == 0 && rows_read >= n)
        {
            rows_read = 0;
            index = 1;
        }
        //cout<<"rows = " << rows_read << " " << index<<endl;

        v[index][rows_read].push_back(a);
        v[index][rows_read].push_back(b);
        v[index][rows_read++].push_back(c);
    }
    matrix_file.close();

    for(int i = 0; i < 2; ++i)
    {
        cout << "Printing matrix " << i << endl;
        for(auto& matrix: v[i])
        {
            for(auto& number: matrix)
                cout << number << " ";
            cout << endl;
        }
    }
    return 0;
}

Output:

Printing matrix 0
1 2 3 
4 5 6 
7 8 9 
Printing matrix 1
1 2 3 
4 5 6 
7 8 9 

---

PS: Unrelated to your problem, but What should main() return in C and C++? An int.

Answer 2

Based on my answer, I will post here a cleaner example, which achieves what your code tries to achieve: Read all the numbers in a 1D data structure (which will be split in two matrices somehow later).

So for that case, the code boils down to just this:

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

using namespace std;

int main()
{
    int i = 0, j, k = 0, n, dimension;
    cout << "Enter the dimension of square matrices (3 by 3 would be 3) \n";
    cin >> n;
    dimension = n * n;
    // make array of two matrices combined, this will be split into two matrices
    vector<int> proto_matrix;
    ifstream matrix_file("matrices.txt");
    string line;
    while (std::getline(matrix_file, line))
    {
        std::istringstream iss(line);
        int a, b, c;
        if (!(iss >> a >> b >> c)) { break; } // error
        proto_matrix.push_back(a);
        proto_matrix.push_back(b);
        proto_matrix.push_back(c);
    }
    matrix_file.close();

    for(auto& number: proto_matrix)
        cout << number << "\n";
    return 0;
}

Answer 3

You don't increment i, thus always writing to the first element. It should be:

matrix_file >> proto_matrix[i++];

Answer 4

You could just read all the values into a flat container (such as std::vector) and do the re-arrangement at a later point. The following code may give you an idea:

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

int main() {
  std::vector<int> values;

  std::ifstream fp("matrices.txt");

  for (int value; fp >> value; ) {
    values.emplace_back(value);
  }

  std::cout << "I read " << values.size() << " values:\n";

  for (auto && value : values) std::cout << value << " ";

  std::cout << "\n";
}

Result:

$ clang++ matrixread.cpp -std=c++17
$ ./a.out 
I read 18 values:
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9