Matrix.java

package test;

public class Matrix {
    public static double[][] random(int m, int n) {
        double[][] C = new double[m][n];
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                C[i][j] = Math.random();
        return C;
    }

    public static double[][] identity(int n) {
        double[][] I = new double[n][n];
        for (int i = 0; i < n; i++)
            I[i][i] = 1;
        return I;
    }

    public static double[][] transpose(double[][] A) {
        int m = A.length;
        int n = A[0].length;
        double[][] C = new double[n][m];
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                C[j][i] = A[i][j];
        return C;
    }

    public static double[][] add(double[][] A, double[][] B) {
        int m = A.length;
        int n = A[0].length;
        double[][] C = new double[m][n];
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                C[i][j] = A[i][j] + B[i][j];
        return C;
    }

    public static double[][] subtract(double[][] A, double[][] B) {
        int m = A.length;
        int n = A[0].length;
        double[][] C = new double[m][n];
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                C[i][j] = A[i][j] - B[i][j];
        return C;
    }

    public static double[][] multiply(double[][] A, double[][] B) {
        int mA = A.length;
        int nA = A[0].length;
        int mB = B.length;
        int nB = B[0].length;
        if (nA != mB) throw new RuntimeException("Illegal matrix dimensions.");
        double[][] C = new double[mA][nB];
        for (int i = 0; i < mA; i++)
            for (int j = 0; j < nB; j++)
                for (int k = 0; k < nA; k++)
                    C[i][j] += A[i][k] * B[k][j];
        return C;
    }

    public static void print(double[][] x) {
    	int m = x.length;
        int n = x[0].length;
        for (int i = 0; i < m; i++) {
        	System.out.print("[  ");
            for (int j = 0; j < n; j++) {
            	 
            	System.out.print(String.format( "%.2f", x[i][j]) + "  ");
            }
            System.out.println("]");
        }
    }

    public static void main(String[] args) {
        double[][] d = { { 1, 2, 3 }, { 4, 5, 6 }, { 9, 1, 3} };
        double[][] c = Matrix.identity(5);
        double[][] a = Matrix.random(5, 5);
        double[][] b = Matrix.transpose(a);
        double[][] e = Matrix.add(a, b);
        
        print(a);
        System.out.println("---");
        print(b);
        System.out.println("--- Multiplication: ");
        double[][] f = Matrix.multiply(a, b);
        print(f);
    }
}