Matrix

Matrix is also called 2-dimensional array.

class NaiveMatrix {

    private Integer[][] values;

    public NaiveMatrix(int n, int m) {
        this.values = new Integer[n][m];
        for (int row = 0; row < values.length; row++) {
            for (int column = 0; column < values[row].length; column++) {
                values[row][column] = 0;
            }
        }
    }

    public void set(int row, int column, Integer value) {
        values[row][column] = value;
    }

    public Integer get(int i, int j) {
        return values[i][j];
    }

    public String toString() {
        StringBuilder result = new StringBuilder();
        for (Integer[] row : values) {
            for (Integer value : row) {
                result.append(value);
                result.append(" ");
            }
            result.append("\n");
        }
        return result.toString();
    }
}

Lets try to use this matrix implementation.

public class MatrixDemo {

    public static void main(String... args) {
        NaiveMatrix matrix = new NaiveMatrix(2, 3);
        matrix.set(0, 0, 5);
        matrix.set(0, 1, 5);
        matrix.set(0, 2, 5);
        System.out.println(matrix);
    }
}

The code above prints out the following.

5 5 5 
0 0 0

Matrix operations

There are many operations that are described in linear algebra. We are going to implement couple of these. We are going to enhance NaiveMatrix class with more operations.

Addition

Adds to matrices together, summing each element on that position.

    public NaiveMatrix add(NaiveMatrix matrix) {
        NaiveMatrix sum = new NaiveMatrix(values.length, values[0].length);
        for (int row = 0; row < values.length; row++) {
            for (int column = 0; column < values[row].length; column++) {
                sum.values[row][column] = values[row][column] + matrix.values[row][column];
            }
        }
        return sum;
    }

Then we can create two matrices and sum them together.

public class MatrixDemo {

    public static void main(String... args) {
        NaiveMatrix matrix = new NaiveMatrix(2, 3);
        matrix.set(0, 0, 5);
        matrix.set(0, 1, 5);
        matrix.set(0, 2, 5);
        System.out.println(matrix);

        NaiveMatrix matrix2 = new NaiveMatrix(2, 3);
        matrix2.set(0, 0, 1);
        NaiveMatrix sum = matrix.add(matrix2);
        System.out.println(sum);
    }
}

Here is how the output would look like.

5 5 5 
0 0 0 

6 5 5 
0 0 0