算法-二维数组的各种运算

public class 数组_二维数组 {
    //找出数组中最大的元素
    private static Double max(int[] a) {
        double max = a[0];
        //如果after > before，则将大的这个值赋值给上一个数
        for (int i = 1; i < a.length; i++) if (a[i] > max) max = a[i];
        return max;
    }


    //计算数组元素的平均值
    private static Double mean(int[] a) {
        int N = a.length;
        double sum = 0.0;
        for (int anA : a) sum += anA;

        return sum / N;
    }

    //复制数组
    private static double[] copyArray(int[] a) {
        int N = a.length;
        //新建一个数组长度是，原数组的长度
        double[] b = new double[N];
        //将原数组，赋值给新数组，且下标是对应的关系
        for (int i = 0; i < N; i++) b[i] = a[i];
        return b;
    }

    //颠倒数组元素的顺序
    private static int[] reverse_Order(int[] a) {
        int N = a.length;
        for (int i = 0; i < N / 2; i++) {
            int temp = a[i];
            a[i] = a[N - 1 - i];
            a[N - i - 1] = temp;
        }
        return a;
    }

    /**
     * 矩阵相乘
     * a[][] * b[][] = c[][]
     */
    private static double[][] matrix_Multiplication() {
        int[][] a = {{3, 2}, {1, 3}};
        int[][] b = {{2, 3}, {1, 1}};
        /**
         *
         * 二维数组a可以看作是一个矩阵
         * 3,2
         * 1,3
         *
         * a[0][0]：代表的是第一行第一列:3
         * a[0][1]：代表的是第一行第二列:2
         * a[1][0]：代表的是第二行第一列:1
         * a[1][1]：代表的是第二行第二列:3
         * */

        int N = a.length;
        double[][] c = new double[N][N];
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < N; j++) {
                //计算行i和列j的点乘

                for (int k = 0; k < N; k++) {
                    c[i][j] += a[i][k] * b[k][j];
                }
            }
        }

        return c;
    }

    //二维数组
    private static int[][] two_dimensional_array() {
        //M*N
        int[][] ss = new int[2][3];

        //初始化二维数组,第一次循环循环的是M(行)
        for (int i = 0; i < ss.length; i++) {
            //的二次循环循环的是N(列)
            for (int j = 0; j < ss.length; j++) {
                ss[i][j] = 0;
            }
        }
        return ss;
    }

    public static void main(String[] args) {
        int[] a = {1, 3, 12, 3, 5};

        System.out.println(max(a));
        System.out.println(mean(a));
        System.out.println(Arrays.toString(copyArray(a)));

        //将数组元素进行颠倒
        System.out.println("元素颠倒:\t"+Arrays.toString(reverse_Order(a)));
//        System.out.println(Arrays.toString(a));

        //矩阵相乘
        System.out.println(Arrays.deepToString(matrix_Multiplication()));

        //二维数组
        System.out.println(Arrays.deepToString(two_dimensional_array()));

    }
}