[Java笔记]数组&字符串

public class Main {
    public static void main(String[] args) {
        // 声明数组
        int[] arr_1;
        String arr_2[];   // 两种形式
        // 创建数组
        arr_1 = new int[5];
        arr_2 = new String[5];
        // Java数组大小可以在运行时指定, C/C++不允许, 因为声明的时候仅要求确定引用(数组名);

        // 数组初始化器
        double[] arr_3 = new double[]{1.0, 2.1, 3.2, 4.3};
        double[] arr_s = {1.0, 4.0};        // 简写
        // 声明数组的同时也可以对数组元素初始化, 也称静态初始化

        // 增强的for循环, 标识符获取每次循环的值
        for (double v : arr_3) {
            System.out.print(v + " ");
        }
        System.out.println();

        // 数组复制, 调用System.arraycopy()方法
        double[] arr_4 = new double[6];
        System.arraycopy(arr_3,0,arr_4, 0, 3);
        for (double v : arr_4) {
            System.out.print(v + " ");
        }
        System.out.println();
    }
}

public class Main {
    // 函数参数表的最后一个参数为 ... arr, 有点像 js 里面的 数组 解构符
    public static double getAVG(String sign, double ... params) {
        System.out.println(sign);
        double avg = 0;
        for (int i = 0; i < params.length; i++) {
            avg += params[i];
        }
        return avg / params.length;
    }
    public static void main(String[] args) {
        System.out.println(getAVG("slacr_", 3.0, 4.0, 5.0));
        System.out.println(getAVG("re", 2.0, 8.0));
    }
}

// Arrays.sort()
public class Main {
    public static void main(String[] args) {
        String[] arr_s = new String[]{"Bob", "Alice", "Haskell", "Ruby", "Scala"};
        for (String arr : arr_s) {
            System.out.print(arr + " ");
        }
        System.out.println();
        
        Arrays.sort(arr_s);

        for (String arr : arr_s) {
            System.out.print(arr + " ");
        }
        System.out.println();


    }
}

// Arrays.binarySearch()
public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 3, 2, 4, 5};
        Arrays.sort(arr);           // 先排序
        int res = Arrays.binarySearch(arr, 4);
        System.out.println(res);   // 找到返回下标
        res = Arrays.binarySearch(arr, 100);
        System.out.println(res);   // 找不到返回 - 应该插入的位置 - 1， -6
    }
}

public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3};
        int[] arr_c = Arrays.copyOf(arr, 6);
        for (int i = 0; i < arr_c.length; i++) {
            System.out.print(arr_c[i] + " ");
        }
        System.out.println();  // 1 2 3 0 0 0

        int[] arr_cor = Arrays.copyOfRange(arr_c, 1, 5);
        for (int i = 0; i < arr_cor.length; i++) {
            System.out.print(arr_cor[i] + " ");
        }
        System.out.println();  // 2 3 0 0 
    }
}

public class Main {
    // fill() equals()
    public static void main(String[] args) {
        int[] arr = new int[5];
        Arrays.fill(arr, 2, 5, 1);
        for (int i : arr) {
            System.out.print(i + " ");
        }
        System.out.println();  //  0 0 1 1 1
        
        int[] arr_2 = new int[5];
        for (int i = 0; i < arr_2.length; i++) {
            Arrays.fill(arr_2, i, i+1, arr[i]);
        }
        
        if(Arrays.equals(arr, arr_2)) System.out.println("equal");  // equal
    }
}

public class Main {
    public static void main(String[] args) {
        int[][] arr = new int[2][3];

        int[][] arr_1 = new int[2][];
        arr_1[0]= new int[10];
        arr_1[1] = new int[5];
        for (int i = 0; i < arr_1.length; i++) {
            for (int j = 0; j < arr_1[i].length; j++) {
                System.out.print(arr_1[i][j] + " ");
            }
            System.out.println();
        }
    }
}

public class Matrix {
    public int[][] m;
    public Matrix(int m[][]) {
        this.m = m;
    }
    public Matrix multiply(Matrix M){
        int res[][] = new int[m.length][M.m[0].length];
        for (int i = 0; i < res.length; i++) {
            for (int j = 0; j < res[0].length; j++) {
                for (int k = 0; k < m[0].length; k++) {
                    res[i][j] = res[i][j] + m[i][k] * M.m[k][j];
                }
            }
        }
        return new Matrix(res);
    }

    public void show(){
        for (int i = 0; i < m.length; i++) {
            if( i == 0 ) {
                for (int j = 0; j < m[0].length; j++) {
                    if(j == 0)    System.out.printf("┏     ");
                    else if(j == m[0].length - 1)   System.out.printf("     ┓");
                    else  System.out.printf("     ");
                }
                System.out.println();
            }
            for (int j = 0; j < m[i].length; j++) {
                if( j == 0)  System.out.printf("┃%-5d", m[i][j]);
                else if( j == m[i].length - 1) System.out.printf("%-5d┃", m[i][j]);
                else System.out.printf("%-5d", m[i][j]);
            }
            System.out.println();
            if( i == m.length - 1 ) {
                for (int j = 0; j < m[0].length; j++) {
                    if(j == 0)    System.out.printf("┗     ");
                    else if(j == m[0].length - 1)   System.out.printf("     ┛");
                    else  System.out.printf("     ");
                }
                System.out.println();
            }
        }
    }
}

// ========================================================

public class Main {
    public static void main(String[] args) {
        int[][] m_1 = {{1, 2, 1},
                {-2, 4, 1}};
        int[][] m_2 = {{4, 3, 0, -1},
                {2, 3, 5, 2},
                {1, 0, 6, 3}};
        Matrix M_1 = new Matrix(m_1);
        M_1.show();
        Matrix M_2 = new Matrix(m_2);
        M_2.show();
        Matrix M_3 = M_1.multiply(M_2);
        M_3.show();
    }
}

public class Main {
    public static void main(String[] args) throws UnsupportedEncodingException {
        // 字面量 创建字符串
        String str = "PHP is the best language all of the world!";
        String str_1 = new String("A cup of Java");
        byte[] b = str.getBytes("UTF-8");
        String str_2 = new String(b, "UTF-8");
        System.out.println(str_2);
    }
}