Common knowledge of one-dimensional array and two-dimensional array

Just a quick tip ： Enter a number from the console
import java.util.Scanner // Import jar package
Scanner scanner =new Scanner(System.in)
System.out.println(“ Please enter a number ：”);
// Get the entered number
int input=scanner.nextlnt();

One ： One dimensional array
1. Definition ： Arrays are reference data types , Used to store multiple data . Reference data types also include classes and interfaces
2. data structure ： Data structure is computer storage 、 How to organize data . Data structure refers to the existence of one or more A collection of data elements with specific relationships . Usually , Well chosen data structure can bring higher operation or storage efficiency . Data structures are often related to efficient retrieval algorithms and indexing techniques

  Data manipulation  :  Additions and deletions 

3. Array properties
Continuous storage in memory , And subscript from 0 Start ( Memory address offset )

•   Once the array length is determined   The length cannot be changed , This means that arrays cannot be added or deleted 
  

•   Unless you create a new array , Copy the original data into the new array , In the process of copying, you can add and delete 
  

•   therefore   Array query and change   Efficiency is very high , however   Adding and deleting are inefficient 
  

•   Arrays have a built-in property  length   Saved the length of the array 
  

•   stay java in   There is one java.util.Arrays  class   Provides some array operations 
  

4. Array declaration

•  ①  Static declaration  :  When each element is known , Use static declarations 
  

•   data type    Variable name  =  value ;
  

•  		int i = 2;
  

•  		 data type []   Variable name  = { value , value , value .....};   Shorthand way 
  

•  		 data type []   Variable name  = new  data type []{ value , value , value ......};   For example, assign a value to an array twice , You need to write 
  

•  		int[] arr = {1,2,3,6,1,2,3};
  

•  		int[][] arr = {
  

•  					{1,2,3},{2,1,3},{4,6}
  

•  			};
  

•  ②  Dynamic statement  :  When you don't know every element in the array , Use dynamic declarations 
  

•  		 data type []  Variable name  = new  data type [ length ];
  

•  		int[] arr = new int[5];
  

•  		int[][] arr = new int[2][3];
  

•  		 Dynamic statement , The default value of the corresponding type is saved , For example, in the above program   Will save 5 individual 0
  

•  		 Integers   0  ,  decimal   0.0  ,  Boolean  false ,  character  \u0000 ,  Reference type  null
  
       int[] arr={1,2,3};  // Static declaration 
        If you assign a value to an array twice , It must be written like this , Don't write directly ------ arr=new int[] {2,4,3} 
       int [] arr2=new int[10]   Also can put the “[]” Write after the variable name    int arr2[] =new int[10]
  

5. get data
int[] arr ={10,11,12,13} // Statically initialize an array
System.out.println(arr[0]); // Note subscript from 0 Start Represents the first number
System.out.println(arr[1]);
System.out.println(arr[2]);
6. Change data
Array [ Little sign ]= value ;
arr[1]=1;
System.out.println(arr[1]);
7. Traverse
for(int i=0;i<arr.length;i++){
System.out.println(arr[i]);
}

// enhance for loop foreach
// Put every element in the array , All assigned to variables
//for( data type Variable name : Array ){}
for(int element:arr){
System.out.println(element);
}
8. Common abnormal
The subscript crossing the line

Null pointer

9. Array passing
example ：m2（new int[]{1,2,3});
10.Main Method dissemination

10. Value transfer refers to the transfer of basic types of data After basic type data transmission , There is no influence on each other , Because it points to different spaces
Byref / The reference It refers to passing the value of the reference data type After the reference type is passed , Influence each other , Because it points to the same heap Save object space
11. Array copy
public static int[] copy(int[] src, int srcPos, int[] dest, int destPos,
int length) {
}
src： Source array srcPos： Source array start position dest： Target array destPos： Starting position of target array length： Insert number
notes ： Because it's insert copy , So the length of the array will certainly be changed , Because the array length cannot be changed , So we can only create new arrays , And return the array through the return value

Two ： Two dimensional array
1. Declaration method


2. storage

3. get data
int[] arr0 = arr4[0]; // Put the array arr4 The first value in is given to arr0 This array
int arr00 = arr0[0]; // Put the array arr0 The first value of is given to arr00 This array
System.out.println(arr00);// Output arr00 This array
System.out.println(arr4[0][0]);
System.out.println(arr4[3][1]);
4. Change data
change Array [ Subscript ][ Subscript ]= value
arr4[3][1]=33;
5. Traverse

6. Dynamic declaration zigzag

7. Exchange the values of variables
int x = 10;
int y = 11;
// 1 Intermediate variable ( Development commonly used )
int temp = x;
x = y;
y = temp;
System.out.println(“x=” + x + “,y=” + y);

	// 2  Displacement calculation ( For interview )
	x = 2; // 0000 0010
	y = 3; // 0000 0011
	//  Convert to the corresponding binary , Every XOR , Take the same 0, Different take 1
	x = x ^ y; // 0000 0001
	y = x ^ y; // 0000 0010
	x = x ^ y; // 0000 0011
	System.out.println("x=" + x + ",y=" + y);

	// 3  Addition and subtraction 
	x = 10;
	y = 20;
	x = x + y;
	y = x - y;
	x = x - y;
	System.out.println("x=" + x + ",y=" + y);