Day8 矩阵乘法

1、矩阵相乘的条件就是A矩阵的列等于B矩阵的行，且最后求得的C矩阵的行和列分别为A矩阵的行、B矩阵的列；

2、二阶数组的话， a.length 表示行，a[0].length 表示列

 1 import java.util.Arrays;
 2 public class MatrixMultiplication {
 3     /**
 4      * The entrance of the program
 5      * @param args
 6      */
 7     public static void main(String args[]) {
 8         matrixMultiplicationTest();
 9     }//Of main
10 
11     /**
12      * Matrix multiplication.
13      * The columns of the first matrix should be equal to the rows of the second one.
14      * 
15      * @param paraFirstMatrix
16      * @param paraSecondMatrix
17      * @return
18      */
19     public static int[][] multiplication(int[][] paraFirstMatrix, int[][] paraSecondMatrix) {
20         int m = paraFirstMatrix.length;//Ai
21         int n = paraFirstMatrix[0].length;//Aj
22         int p = paraSecondMatrix[0].length;//Bj
23         
24         //Step1. Dimension check
25         if (paraSecondMatrix.length != n) {//Bi != Aj
26             System.out.println("The two matrices cannot be multiplied.");
27             return null;
28         }
29 
30         //Step2. The loop.
31         int[][] resultMatrix = new int[m][p];
32         for (int i = 0; i < m; i++) {
33             for (int j = 0; j < p; j++) {
34                 for (int k = 0; k < n; k++) {
35                     resultMatrix[i][j] += paraFirstMatrix[i][k] * paraSecondMatrix[k][j];
36                 }// Of for k 
37             }// Of for j
38         }// Of for i
39         return resultMatrix;
40     }// Of multiplication
41     
42     /**
43      * Unit test for respective method
44      */
45     public static void matrixMultiplicationTest() {
46         int[][]tempFirstMatrix = new int[2][3];
47         for (int i = 0; i < tempFirstMatrix.length; i++) {
48             for (int j = 0; j < tempFirstMatrix[0].length; j++) {
49                 tempFirstMatrix[i][j] = i + j;
50             }// Of for j
51         }// Of for i
52         System.out.println("The first matrix is: \r\n" + Arrays.deepToString(tempFirstMatrix));
53         
54         int[][]tempSecondMatrix = new int[3][2];
55         for (int i = 0; i < tempSecondMatrix.length; i++) {
56             for (int j = 0; j < tempSecondMatrix[0].length; j++) {
57                 tempSecondMatrix[i][j] = i*10 + j;
58             }// Of for j
59         }// Of for i
60         System.out.println("The second matrix is: \r\n" + Arrays.deepToString(tempSecondMatrix));
61         
62         int[][] tempThirdMatrix = multiplication(tempFirstMatrix, tempSecondMatrix);
63         System.out.println("The Third matrix is: \r\n" + Arrays.deepToString(tempThirdMatrix));
64 
65         System.out.println("Try to multiply the first matrix with itself.\r\n");
66         tempThirdMatrix = multiplication(tempFirstMatrix, tempFirstMatrix);
67         System.out.println("The result matrix is: \r\n" + Arrays.deepToString(tempThirdMatrix));
68 
69     }// Of matrixMultiplicationTest
70     
71 }// Of class MatrixMultiplication