Java arrays, sorting and searching

Introduction to Arrays

Arrays can store multiple data of the same type. An array is also a data type and is a reference type.

Use of arrays

Usage 1-definition of dynamically initialized array

Data type array name [] = new data type [size]

int a[] = new int[5];//创建了一个数组，名字a,存放5个int

Explanation: This is one way to define an array.

import java.util.Scanner;
public class Array02 {
    
     

    //编写一个main方法
    public static void main(String[] args) {
    
    
        //演示 数据类型 数组名[]=new 数据类型[大小]
        //循环输入5个成绩，保存到double数组,并输出

        //步骤
        //1. 创建一个 double 数组，大小 5

        //(1) 第一种动态分配方式
        //double scores[] = new double[5];
        //(2) 第2种动态分配方式， 先声明数组，再 new 分配空间
        double scores[] ; //声明数组， 这时 scores 是 null
        scores = new double[5]; // 分配内存空间，可以存放数据


        //2. 循环输入
        //   scores.length 表示数组的大小/长度
        //   
        Scanner myScanner = new Scanner(System.in);
        for( int i = 0; i < scores.length; i++) {
    
    
            System.out.println("请输入第"+ (i+1) +"个元素的值");
            scores[i] = myScanner.nextDouble();
        }

        //输出，遍历数组
        System.out.println("==数组的元素/值的情况如下:===");
        for( int i = 0; i < scores.length; i++) {
    
    
            System.out.println("第"+ (i+1) +"个元素的值=" + scores[i]);
        }
    }
}

Usage 2 - dynamic initialization

1. Declare the array first

Syntax: data type array name []; also data type [] array name;

int a[]; or int[] a;

2. Create an array

Syntax: array name = new data type [size];

a=new int[10];

Usage 3 - static initialization

Initialize the array

grammar:

Data type array name[] = {element value, element value...}

int a[]={2,5,6,7,8,89,90,34,56},

Array Usage Notes and Details

1. The elements in the array can be of any data type, including basic types and reference types, but they cannot be mixed.

2. After the array is created, if there is no assignment, the default value is
   int 0

short 0

byte 0

long 0

float 0.0

double 0.0

char \u0000

boolean false

String null

3. Arrays are reference types, and array data is objects

Array application case

Create an array of 26 elements of type char, place 'A'-'Z' respectively. Use a for loop to iterate over all elements and print them. Tip: char type data operation 'A'+2 -> 'C'

public class ArrayExercise01 {
    
     

    //编写一个main方法
    public static void main(String[] args) {
    
    

        /*
        创建一个char类型的26个元素的数组，分别 放置'A'-'Z'。
        使用for循环访问所有元素并打印出来。
        提示：char类型数据运算 'A'+1 -> 'B'  

        思路分析
        1. 定义一个 数组  char[] chars = new char[26]
        2. 因为 'A' + 1 = 'B' 类推，所以老师使用for来赋值
        3. 使用for循环访问所有元素
         */
        char[] chars = new char[26];
        for( int i = 0; i < chars.length; i++) {
    
    //循环26次
            //chars 是 char[] 
            //chars[i] 是 char
            chars[i] = (char)('A' + i); //'A' + i 是int , 需要强制转换
        }

        //循环输出
        System.out.println("===chars数组===");
        for( int i = 0; i < chars.length; i++) {
    
    //循环26次
            System.out.print(chars[i] + " ");
        }

    }
}

Array assignment mechanism

Arrays are passed by reference by default, and the assigned value is an address. (compared to: variables are often passed by value)

int[] arr1 = {
    
    1,2,3};
int[] arr2 = arr1;

array copy

Write code to implement array copy (content copy)

int[] arr1 = {
    
    10,20,30};
int[] arr2 = new int[arr1.length];
for(int i = 0; i < arr1.length; i++) {
    
    
    arr2[i] = arr1[i];
}

Array addition/expansion

Requirements: Realize the effect of dynamically adding elements to the array, and realize the expansion of the array.

1. Raw arrays use static allocation int[] arr = {1,2,3}
2. The added element 4 is placed directly at the end of the array arr = {1,2,3,4}
3. The user can decide whether to continue to add through the following methods, if the addition is successful, whether to continue? y/n

1. 定义初始数组int[] arr = {
    
    1,2,3}//下标0-2
2. 定义一个新的数组int[] arrNew = new int[arr.length+1];
3. 遍历arr 数组，依次将arr 的元素拷贝到arrNew 数组
4. 将4 赋给arrNew[arrNew.length - 1] = 4;把4 赋给arrNew 最后一个元素
5. 让arr 指向arrNew ; arr = arrNew; 那么原来arr 数组就被销毁
6. 创建一个Scanner可以接受用户输入
7. 因为用户什么时候退出，不确定，使用do-while + break 来控制

Multidimensional array Two-dimensional array

dynamic initialization 1

1. Syntax: type[][] array name=new type[size][size]
2. For example: int a[][]=new int[2][3]
3. The existence form of two-dimensional array in memory (!! drawing)

Dynamic initialization 2

First declare: type array name[][];

Redefine (open up space) array name = new type [size] [size]

Assignment (there is a default value, such as int type is 0)

static initialization

define type array name[][] = { {value1,value2...},{value1,value2...},{value1,value2...}}
int[][] arr = { {1,1, 1}, {8,8,9}, {100}};

Application case of two-dimensional array

Print a 10-line Yang Hui triangle using a two-dimensional array

1

1 1

1 2 1

1 3 3 1

1 4 6 4 1

1 5 10 10 5 1

1. The first row has 1 element, and the nth row has n elements

2. The first element and the last element of each row are both 1

3. Starting from the third row, for the value of the element that is not the first element and the last element.

arr[i][j] = arr[i-1][j] + arr[i-1][j-1];

public class YangHui {
    
     

    //编写一个main方法
    public static void main(String[] args) {
    
    
        /*
        使用二维数组打印一个 10 行杨辉三角
        1
        1 1
        1 2 1
        1 3 3  1
        1 4 6  4  1
        1 5 10 10 5 1

        规律
         1.第一行有 1 个元素, 第 n 行有 n 个元素
         2. 每一行的第一个元素和最后一个元素都是 1
         3. 从第三行开始, 对于非第一个元素和最后一个元素的元素的值. arr[i][j] 
          arr[i][j]  =  arr[i-1][j] + arr[i-1][j-1]; //必须找到这个规律

         */
        int[][] yangHui = new int[12][];
        for(int i = 0; i < yangHui.length; i++) {
    
    //遍历yangHui的每个元素

            //给每个一维数组(行) 开空间
            yangHui[i] = new int[i+1];
            //给每个一维数组(行) 赋值
            for(int j = 0; j < yangHui[i].length; j++){
    
    
                //每一行的第一个元素和最后一个元素都是1
                if(j == 0 || j == yangHui[i].length - 1) {
    
    
                    yangHui[i][j] = 1;
                } else {
    
    //中间的元素
                    yangHui[i][j]  =  yangHui[i-1][j] + yangHui[i-1][j-1];
                }
            }
        }
        //输出杨辉三角
        for(int i = 0; i < yangHui.length; i++) {
    
    
            for(int j = 0; j < yangHui[i].length; j++) {
    
    //遍历输出该行
                System.out.print(yangHui[i][j] + "\t");
            }
            System.out.println();//换行.
        }
    }
}

Two-dimensional array usage details and precautions

1. One-dimensional arrays are declared in the following ways:

int[] x 或者int x[]

2. Two-dimensional arrays are declared in the following ways:

int[][] y 或者int[] y[] 或者int y[][]

3. A two-dimensional array is actually composed of multiple one-dimensional arrays, and the lengths of each one-dimensional array can be the same or different. For example: map[][] is a two-dimensional array

int map [][] = {
    
    {
    
    1,2},{
    
    3,4,5}}

Map[0] is a one-dimensional array containing two elements, and map[1] is a one-dimensional array containing three elements. We also call it a two-dimensional array with different numbers of columns.

---

Articles and codes have been archived in [Github warehouse: https://github.com/timerring/java-tutorial] or the public account [AIShareLab] can also be obtained by replying to java .