如何将两个不同大小的矩阵相乘

Question

I'm working on a simple encryption algorithm, so I need to multiply two matrices:

This is the first:

86  65  76  76
69  45  71  82
65  78  68  69

This is the second:

13  9   3   5
2   1   4   6
4   6   2   7
8   5   4   1

According to this page with which I work, the result should be:

2160    1675    974 1428
1927    1502    857 1194
1825    1416    919 1338

Here I leave the code that does not work:

public class encriptar {

public static void main(String[] args) {
    double[][] encriptador = new double[4][4];
    double[][] mensaje = new double[4][3];
    double[][] resultado = new double[4][3];

    encriptador[0][0] = 13;
    encriptador[0][1] = 9;
    encriptador[0][2] = 3;
    encriptador[0][3] = 5;
    encriptador[1][0] = 2;
    encriptador[1][1] = 1;
    encriptador[1][2] = 4;
    encriptador[1][3] = 6;
    encriptador[2][0] = 4;
    encriptador[2][1] = 6;
    encriptador[2][2] = 2;
    encriptador[2][3] = 7;
    encriptador[3][0] = 8;
    encriptador[3][1] = 5;
    encriptador[3][2] = 4;
    encriptador[3][3] = 1;

    mensaje[0][0] = 86;
    mensaje[1][0] = 65;
    mensaje[2][0] = 76;
    mensaje[3][0] = 76;
    mensaje[0][1] = 69;
    mensaje[1][1] = 45;
    mensaje[2][1] = 71;
    mensaje[3][1] = 82;
    mensaje[0][2] = 65;
    mensaje[1][2] = 78;
    mensaje[2][2] = 68;
    mensaje[3][2] = 69;

    resultado = multiplicarMatrizes(encriptador, mensaje);

    imprimirMatriz(resultado);

}

public static double[][] multiplicarMatrizes(double[][] llave, double[][] mensaje) {
    double[][] resultado = new double[llave.length][mensaje[0].length];
    if (llave.length == llave[0].length && mensaje.length == llave.length) {
        for (int k = 0; k < llave.length; k++) {
            for (int mc = 0; mc < mensaje.length; mc++) {
                for (int lf = 0; lf < mensaje.length; lf++) {
                    resultado[lf][k] += mensaje[k][mc] * llave[mc][lf];
                }
            }
        }
    }
    return resultado;
}

public static void imprimirMatriz(double[][] matriz) {
    for (int i = 0; i < matriz[0].length; i++) {
        for (int l = 0; l < matriz.length; l++) {
            System.out.print(matriz[l][i] + " ");
        }
        System.out.println("");
    }
}
}

Apparently in the count of the variables, at some point, a variable does not match the size of a matrix but I have not been able to solve it.

Answer 1

Here is a solution similar to what you are trying to do, you can use it if you wish. Your problem stems from the fact that you are confused with your dimensions.

public class Main {

public static void main(String[] args) {
    int r1 = 3, c1 = 4;
    int r2 = 4, c2 = 4;
    int[][] firstMatrix = { {x, x, x, x}, {x, x, x, x}, {x, x, x, x} };
    int[][] secondMatrix = { {x, x, x, x}, {x, x, x, x}, {x, x, x, x}, {x, x, x, x} };

    // Mutliplying Two matrices
    int[][] product = multiplyMatrices(firstMatrix, secondMatrix, r1, c1, c2);

    // Displaying the result
    displayProduct(product);
}

public static int[][] multiplyMatrices(int[][] firstMatrix, int[][] secondMatrix, int r1, int c1, int c2) {
    int[][] product = new int[r1][c2];
    for(int i = 0; i < r1; i++) {
        for (int j = 0; j < c2; j++) {
            for (int k = 0; k < c1; k++) {
                product[i][j] += firstMatrix[i][k] * secondMatrix[k][j];
            }
        }
    }

    return product;
}

public static void displayProduct(int[][] product) {
    System.out.println("Product of two matrices is: ");
    for(int[] row : product) {
        for (int column : row) {
            System.out.print(column + "    ");
        }
        System.out.println();
    }
}

}

In a double array, its [rows][columns] - your output will have 3 rows and 4 columns (as you can see from your website), so your dimensions for result should be resultado[3][4]. Your mensaje matrix should be [3][4] and not [4][3]. I think you can figure out the rest.