Exercise08_06.java

package ch_08;

import java.util.Scanner;


/**
 * 8.6 (Algebra: multiply two matrices)
 * Write a method to multiply two matrices. The header of the method is:
 * <p>
 * public static double[][] multiplyMatrix(double[][] a, double[][] b)
 * <p>
 * To multiply matrix a by matrix b, the number of columns in A must
 * be the same as the number of rows in B, and the two matrices must have
 * elements of the same or compatible types. Let c be the result of the
 * multiplication. Assume the column size of matrix a is n. Each element
 * cij is ai1 � b1j + ai2 � b2j + � + ain � bnj. For example, for two
 * 3 � 3 matrices a and b, c is..............
 * <p>
 * Write a test program that prompts the user
 * to enter two 3 � 3 matrices and displays
 * their product.
 */
public class Exercise08_06 {
    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);

        double[][] matrix1 = new double[3][3];

        System.out.println("Enter the values for the first 3 x 3 matrix: ");

        for (int i = 0; i < matrix1.length; i++) {
            for (int j = 0; j < matrix1[i].length; j++) {

                matrix1[i][j] = input.nextDouble();
            }
        }

        double[][] matrix2 = new double[3][3];

        System.out.println("Enter the values for the second 3 x 3 matrix: ");

        for (int i = 0; i < matrix2.length; i++) {
            for (int j = 0; j < matrix2[i].length; j++) {

                matrix2[i][j] = input.nextDouble();

            }
        }

        double[][] newMatrix = multiplyMatrix(matrix1, matrix2);

        System.out.println("The multiplication of the two matrices is as follows:");

        for (int i = 0; i < matrix1.length; i++) {


            for (int j = 0; j < matrix1[i].length; j++) {

                System.out.print(matrix1[i][j] + " ");
                if (i == 1 && j == 2) {
                    System.out.print(" *  ");
                } else if (i != 1 && j == 2) {
                    System.out.print("    ");
                }
            }

            for (int j = 0; j < matrix2[i].length; j++) {

                System.out.print(matrix2[i][j] + " ");
                if (i == 1 && j == 2) {
                    System.out.print(" =  ");
                } else if (i != 1 && j == 2) {
                    System.out.print("    ");
                }
            }

            for (int j = 0; j < newMatrix[i].length; j++) {

                System.out.print(newMatrix[i][j] + " ");
                if (i != 1 && j == 2) {
                    System.out.print("    ");
                }


            }
            System.out.println();
        }

    }

    public static double[][] multiplyMatrix(double[][] a, double[][] b) {

        double[][] result = new double[3][3];

        for (int i = 0; i < a.length; i++) {
            for (int j = 0; j < a[i].length; j++) {
                for (int z = 0; z < a.length; z++) {

                    result[i][j] += a[i][z] * b[z][j];
                }
            }
        }
        return result;
    }
}