JAVA's Random class, array learning

Random tool class

Function: Generate a random number

Usage steps:
1. Import the corresponding package
import java.util.Random;
2. Create a reference type variable
Data type Variable name = new Data type();
Random rd = new Random();
3. Call function
Variable name. Function
Generate a random integer in the range [0,n), generate a random integer that contains 0, but does not contain n
nextInt(n);
rd.nextInt(n);

1 获取 1-100 之间的随机数    [1,100]  ---> [0,99] + 12 public static void main(String[] args) {3         //创建引用类型变量 4         Random rd = new Random();5         //调用功能 6         int num = rd.nextInt(100) + 1;7         System.out.println("num = "+num);    
8     }

Array: Store and operate batches of data
Array storage The value of the data can be changed
The data type of the stored batch data must be the same

Definition format of the array: Format 1: Data type [] array name; (recommended Format)
Format 2: Data type array name[]; (recognizable)
Allocate memory space to the array and assign default values ​​to the elements

Two forms: Chapter One: Dynamic initialization, the length of the array is given while defining the array (the maximum number of elements that the array can store), the default value of the array element is specified by the system
The second type: Static initialization, while defining the array Give the data to be stored, and the length of the array is specified by the system

Dynamic initialization format: Data type[] Array name = new data type[M];
Data type: The data type of the element, the data types on both sides must be consistent.
[]: The flag of the array, one [] represents a one-dimensional array and two [] two-dimensional arrays
Array name: The identifier can be obtained through the array name Elements of the array
M: The length of the array, that is, the maximum number of elements that can be stored

Access array element format
Array name [index value] - ---> Array name [subscript] Array name [subscript]
The index value starts from 0
The index value of the last element is n-1


Static initialization Define the array and give the specified value at the same time. The length of the array is calculated by the system
Syntax format:
Data type [] Array name = new data type [] {Element value 1, Element value 2 ,...};
Simplified format:
Data type [] array name = {element value 1, element value 2,...};

Array out-of-bounds exception: java.lang.ArrayIndexOutOfBoundsException: 4Null pointer exception: java.lang.NullPointerException

Obtain one by one Elements of the array Automatically obtain the length of the array
Array name.length


Traverse the array code demonstration:
* Find the number sequence 1,3,7,4 , the largest number among 5,6 * Define an array to store the above number sequence
* int[] arr = {1,3,7,4,5,6};
* Idea:
* Define a temporary variable to first store the value of the element at position 0, and compare the value in the temporary variable with the value of the element in sequence. Once it is found that the value of an element is greater than the value in the temporary variable, the value in the element is Value
* Assign a value to a temporary variable. From beginning to end, the temporary variable stores the maximum value

 1 public static void main(String[] args) { 2         // 3         int[] arr = {1,3,7,40,5,6}; 4          5         int tmp = arr[0]; 
 6          7         for(int i = 1;i<arr.length;i++){ 8             //比较  9             if(tmp < arr[i]){10                 //一旦满足条件 就赋值 11                 tmp = arr[i];// 把元素值 赋值给临时的tmp12             }13         }14         15         System.out.println("最大值= "+tmp);16     }

:Array elements are one-dimensional arrays

Data type[][] array name; Recommended format
Data type[] Array name[];
Data type Array name[ ][];

Dynamic initialization
Data type[][] Array name = new data type[M][N]; Data type: Data type of array elements, before and after Consistent
[]: The flag of the array, two [] represent the two-dimensional array
Array name: Identifier You can access the array elements through the array name
M: The length of the two-dimensional array, which constitutes this two-dimensional array The number of one-dimensional arrays in the array
N: The number of elements in each one-dimensional array in the two-dimensional array

Static initialization
Data type[][] Array name = new data type [][]{{element value 1,element value 2,...},{element value 1,element value 2,...},{element value 1,element value 2,...} ,...}; Abbreviated format
Data type [][] Array name = {{Element value 1, Element value 2,...},{Element value 1, Element value 2,... },{element value 1,element value 2,...},...};
1 2 3
4 5 6
7 8 9
int[][] arr = { {1,2,3},{4,5,6},{7,8,9}}
Access the elements of the two-dimensional array
Array name[index value 1][index value 2]
The length of the two-dimensional array
Array name.length
Get the length of an array in the two-dimensional array
Array name [index value].length

Two-dimensional array outer loop Control row, inner loop control column

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