AcWing《Java语法基础课》第4章数组

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AcWing《Java语法基础课》第4章数组

作者：

姜萍Jumping , 2023-01-15 15:16:34 , 所有人可见 , 阅读 428

第4章数组

笔记

一维数组

数组的定义

int[] a = new int[10];
float[] f = new float[33];
double[] d = new double[123];
char[] c = new char[21];

数组的初始化

int[] a = {0, 1, 2};        // 含有3个元素的数组，元素分别是0, 1, 2
int[] b = new int[3];       // 含有3个元素的数组，元素的值均为0
char[] d = {'a', 'b', 'c'}; // 字符数组的初始化

访问数组元素

int[] a = {0, 1, 2};  // 数组下标从0开始
System.out.printf("%d %d %d\n", a[0], a[1], a[2]);      // 读取
a[0] = 5;           // 写入

多维数组

多维数组就是数组的数组。

// 二维数组
int[][] a = new int[3][4];  // 大小为3的数组，每个元素是含有4个整数的数组

// 三维数组
int[][][] b = new int[10][20][30];  
// 大小为10的数组，它的每个元素是含有20个数组的数组
// 这些数组的元素是含有30个整数的数组

初始化

int[][] a = {           // 三个元素，每个元素都是大小为4的数组
    {0, 1, 2, 3},       // 第1行的初始值
    {4, 5, 6, 7},       // 第2行的初始值
    {8, 9, 10, 11}      // 第3行的初始值
};

数组的范围遍历

for (int[] row: a) {  // 范围遍历
    for (int x: row)  // 范围遍历
        System.out.printf("%d ", x);
    System.out.println();
}

数组API

属性length：返回数组长度，注意不加小括号
Arrays.sort()：数组排序
Arrays.fill(int[] a, int val)：填充数组
Arrays.toString()：将数组转化为字符串
Arrays.deepToString()：将多维数组转化为字符串
使用Arrays需要import java.util.Arrays

注意数组创建后，长度不可变。

习题

AcWing 737. 数组替换

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int[] a = new int[10];

        Scanner scanner = new Scanner(System.in);
        for (int i = 0; i < 10; i++)
            a[i] = scanner.nextInt();
        scanner.close();

        for (int i = 0; i < 10; i++) {
            if (a[i] <= 0) a[i] = 1;
            System.out.printf("X[%d] = %d\n", i, a[i]);
        }
    }
}

AcWing 738. 数组填充

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int a = scanner.nextInt();
        scanner.close();

        for (int i = 0; i < 10; i++) {
            System.out.printf("N[%d] = %d\n", i, a);
            a *= 2;
        }
    }
}

AcWing 739. 数组选择

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        for (int i = 0; i < 100; i++) {
            double x = scanner.nextDouble();
            if (x <= 10)
                System.out.printf("A[%d] = %.1f\n", i, x);
        }
        scanner.close();
    }
}

AcWing 743. 数组中的行

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double[] sum = new double[12];
        Scanner scanner = new Scanner(System.in);
        int k = scanner.nextInt();
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++)
                sum[i] += scanner.nextDouble();
        scanner.close();

        if (op.equals("S")) System.out.printf("%.1f\n", sum[k]);
        else System.out.printf("%.1f\n", sum[k] / 12.0);
    }
}

AcWing 745. 数组的右上半部分

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double res = 0;
        Scanner scanner = new Scanner(System.in);
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++) {
                double x = scanner.nextDouble();
                if (j > i) res += x;
            }
        scanner.close();

        if (op.equals("M")) res /= 66.0;
        System.out.printf("%.1f\n", res);

    }
}

AcWing 747. 数组的左上半部分

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double res = 0;
        Scanner scanner = new Scanner(System.in);
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++) {
                double x = scanner.nextDouble();
                if (j < 11 - i) res += x;
            }
        scanner.close();

        if (op.equals("M")) res /= 66.0;
        System.out.printf("%.1f\n", res);

    }
}

AcWing 749. 数组的上方区域

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double res = 0;
        Scanner scanner = new Scanner(System.in);
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++) {
                double x = scanner.nextDouble();
                if (i <= 4 && j > i && j < 11 - i) res += x;
            }
        scanner.close();

        if (op.equals("M")) res /= 30.0;
        System.out.printf("%.1f\n", res);

    }
}

AcWing 751. 数组的左方区域

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double res = 0;
        Scanner scanner = new Scanner(System.in);
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++) {
                double x = scanner.nextDouble();
                if (j >= 0 && j <= 4 && i > j && i < 11 - j) res += x;
            }
        scanner.close();

        if (op.equals("M")) res /= 30.0;
        System.out.printf("%.1f\n", res);

    }
}

笔记

类似“数组的上方区域”这一题，把描述上方区域条件的i和j交换后，就能得到描述左方区域的条件。此外，在描述这类区域时，可模仿二重积分积分域的描述方法。

AcWing 753. 平方矩阵 I

import java.util.Scanner;

public class Main {

    public static void print(int n) {
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                int di = Math.min(i + 1, n - i);
                int dj = Math.min(j + 1, n - j);
                System.out.printf("%d ", Math.min(di, dj));
            }
            System.out.println("");
        }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        while(true) {
            int n = scanner.nextInt();
            if (n == 0) break;
            print(n);
            System.out.println("");
        }
        scanner.close();
    }
}

笔记

每个元素的值为当前位置距离四个边界的最小值

AcWing 740. 数组变换

import java.util.Scanner;

public class Main {

    public static int n = 20;
    public static int[] a = new int[n];

    public static void reverse() {
        for (int i = 0, j = n - 1; i < j; i++, j--) {
            int tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
        }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        for (int i = 0; i < n; i++)
            a[i] = scanner.nextInt();
        scanner.close();

        reverse();

        for (int i = 0; i < n; i++)
            System.out.printf("N[%d] = %d\n", i, a[i]);
    }
}

AcWing 741. 斐波那契数列

import java.util.Scanner;

public class Main {

    public static int n = 61;
    public static long[] a = new long[n];

    public static void init() {
        a[0] = 0;
        a[1] = 1;
        for (int i = 2; i < n; i++)
            a[i] = a[i - 2] + a[i - 1];
    }

    public static void main(String[] args) {
        init();

        Scanner scanner = new Scanner(System.in);
        int m = scanner.nextInt();
        for (int i = 0; i < m; i++) {
            int k = scanner.nextInt();
            System.out.printf("Fib(%d) = %d\n", k, a[k]);
        }
        scanner.close();
    }
}

笔记

int类型无法保存第60个数，需要用long类型保存。

AcWing 742. 最小数和它的位置

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int min = 2000, pos = -1;
        for (int i = 0; i < n; i++) {
            int x = scanner.nextInt();
            if (x < min) {
                min = x;
                pos = i;
            }
        }
        scanner.close();

        System.out.println("Minimum value: " + min);
        System.out.println("Position: " + pos);
    }
}

AcWing 744. 数组中的列

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double[] sum = new double[12];
        Scanner scanner = new Scanner(System.in);
        int k = scanner.nextInt();
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++)
                sum[j] += scanner.nextDouble();
        scanner.close();

        if (op.equals("S")) System.out.printf("%.1f\n", sum[k]);
        else System.out.printf("%.1f\n", sum[k] / 12.0);
    }
}

AcWing 748. 数组的右下半部分

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double res = 0;
        Scanner scanner = new Scanner(System.in);
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++) {
                double x = scanner.nextDouble();
                if (j > 11 - i) res += x;
            }
        scanner.close();

        if (op.equals("M")) res /= 66.0;
        System.out.printf("%.1f\n", res);

    }
}

AcWing 746. 数组的左下半部分

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double res = 0;
        Scanner scanner = new Scanner(System.in);
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++) {
                double x = scanner.nextDouble();
                if (j < i) res += x;
            }
        scanner.close();

        if (op.equals("M")) res /= 66.0;
        System.out.printf("%.1f\n", res);

    }
}

AcWing 750. 数组的下方区域

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double res = 0;
        Scanner scanner = new Scanner(System.in);
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++) {
                double x = scanner.nextDouble();

                if (i >= 7) {
                    int k = i - 7;
                    if (j >= 5 - k && j <= 6 + k)
                        res += x;
                }  
            }
        scanner.close();

        if (op.equals("M")) res /= 30.0;
        System.out.printf("%.1f\n", res);

    }
}

AcWing 752. 数组的右方区域

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        double res = 0;
        Scanner scanner = new Scanner(System.in);
        String op = scanner.next();
        for (int i = 0; i < 12; i++)
            for (int j = 0; j < 12; j++) {
                double x = scanner.nextDouble();
                if (j >= 7) {
                    if (j > i && i + j > 11)
                        res += x;
                }
            }
        scanner.close();

        if (op.equals("M")) res /= 30.0;
        System.out.printf("%.1f\n", res);

    }
}

AcWing 754. 平方矩阵 II

import java.util.Scanner;

public class Main {

    public static void print(int n) {
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++)
                System.out.printf("%d ", Math.abs(i - j) + 1);
            System.out.println("");
        }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        while(true) {
            int n = scanner.nextInt();
            if (n == 0) break;
            print(n);
            System.out.println("");
        }
        scanner.close();
    }
}

AcWing 755. 平方矩阵 III

import java.util.Scanner;

public class Main {

    public static int n = 16;
    public static int[][] a;

    public static void init() {
        a = new int[n][n];
        for (int i = 0; i < n; i++)
            for (int j = 0; j < n; j++)
                a[i][j] = (int)Math.pow(2, i + j);
    }

    public static void print(int k) {
        for (int i = 0; i < k; i++) {
            for (int j = 0; j < k; j++)
                System.out.printf(a[i][j] + " ");
            System.out.println("");
        }
    }

    public static void main(String[] args) {
        init();

        Scanner scanner = new Scanner(System.in);
        while(true) {
            int k = scanner.nextInt();
            if (k == 0) break;
            print(k);
            System.out.println("");
        }
        scanner.close();
    }
}

AcWing 756. 蛇形矩阵

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int m = scanner.nextInt();
        int n = scanner.nextInt();
        scanner.close();

        int[][] a = new int[m][n];
        int[] dx = {0, 1, 0, -1}, dy = {1, 0, -1, 0};
        int cnt = 1, direction = 0, x = 0, y = 0;
        do{
            a[x][y] = cnt;
            cnt++;
            int new_x = x + dx[direction], new_y = y + dy[direction];
            if (new_x < 0 || new_x >= m || new_y < 0 || new_y >= n || a[new_x][new_y] > 0) {
                direction = (direction + 1) % 4;
                new_x = x + dx[direction];
                new_y = y + dy[direction];
            }
            x = new_x;
            y = new_y;
        } while(cnt <= m * n);

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++)
                System.out.printf(a[i][j] + " ");
            System.out.println("");
        }

    }
}

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AcWing《Java语法基础课》第4章 数组

第4章 数组

笔记

习题

AcWing 737. 数组替换

AcWing 738. 数组填充

AcWing 739. 数组选择

AcWing 743. 数组中的行

AcWing 745. 数组的右上半部分

AcWing 747. 数组的左上半部分

AcWing 749. 数组的上方区域

AcWing 751. 数组的左方区域

AcWing 753. 平方矩阵 I

AcWing 740. 数组变换

AcWing 741. 斐波那契数列

AcWing 742. 最小数和它的位置

AcWing 744. 数组中的列

AcWing 748. 数组的右下半部分

AcWing 746. 数组的左下半部分

AcWing 750. 数组的下方区域

AcWing 752. 数组的右方区域

AcWing 754. 平方矩阵 II

AcWing 755. 平方矩阵 III

AcWing 756. 蛇形矩阵

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AcWing《Java语法基础课》第4章数组

第4章数组