How to read int numbers (matrix) from file.txt and add to array?

Question

I want to multiply two matrices. I have 2 files with 2 different integers (matrices).

file1.txt
4 3 4 6
-1 10 4 -1
4 7 2 -8

file2.txt
3 0 0
0 3 0
0 0 3
0 2 4

How can I read these files separately into a two-dimensional array, so that it is convenient to multiply. I've a code where the size of the matrix is indicated at the beginning, but what if there could be different size of matrices? Here is my code with given size:

public static void main(String[] args) throws IOException {
    try {
        Scanner input = new Scanner(new File(file1.txt));
        int m = 3; // I need the size for random matrix
        int n = 5; // I need the size for random matrix
        int[][] a = new int[m][n];
        while (input.hasNextLine()) {
            for (int i = 0; i < m; i++) {
                for (int j = 0; j < n; j++) {
                    try{
                        a[i][j] = input.nextInt();
                        System.out.println("number is "+ a[i][j]);
                    }
                    catch (java.util.NoSuchElementException e) {
                    }
                }
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }

}

Answer 1

It would be better to implement a separate method reading a file into matrix if the dimensions of the matrix are known:

public static int[][] readFileWithMatrix(String filename, int rows, int cols) throws Exception {
    int[][] arr = new int[rows][cols];
    try (Scanner input = new Scanner(new File(filename))) { // use try-with-resources to close Scanner
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                arr[i][j] = input.nextInt();
            }
        }
    }
    return arr;
}

Then it would be simpler to use this method:

int[][] arr3x4 = readFileWithMatrix("file1.txt", 3, 4);
int[][] arr4x3 = readFileWithMatrix("file2.txt", 4, 3);
//... do matrix multiplication 

Answer 2

Here try the following code which takes all the input from file assuming that it contains only the matrix elements and store it into a dynamic 2D array a eg, list of list. First I read the first line of the file and take out all the numbers from it which gives me the total number of columns of the matrix. Next I keep on reading the input lines until the end of file which expands the row of the matrix a

public static void main(String[] args) throws IOException {
    try {
        Scanner input = new Scanner(new File(file1.txt));
        List<ArrayList<Integer>> a = new ArrayList<ArrayList<Integer>>();
        int row=0;
        String cols[] = input.nextLine().split(" ");
        a.add(row, new ArrayList<Integer>());
        for (int j = 0; j < cols.length; j++) {
             try {
                 a.get(row).add(Integer.parseInt(cols[j]));        
              }  catch (Exception e) {
              }
        }
        row++;
        
        while (input.hasNextLine()) {
            a.add(row, new ArrayList<Integer>());
            for (int j = 0; j < cols.length; j++) {
              try {
                    a.get(row).add(input.nextInt());
              } catch (java.util.NoSuchElementException e) {
              }
            }
            row++;
        }
       
       System.out.println("Rows: "+row+"  Columns: "+cols.length);
       for (int i = 0; i < a.size(); i++) {
        for (int j = 0; j < a.get(i).size(); j++) {
            System.out.println("Number is "+ a.get(i).get(j));
        }
       }

    } catch (Exception e) {
        e.printStackTrace();
    }
}

Demo code in here: https://ideone.com/ZEcgPR#stdin