Fourth day of Java learning

1. Array (static initialization-FAQ)

public class test1{
public static void Main (string[] args) {
Int[] Arr=new int[3];

Arr=null;
System.out.print (arr[1]);
}

At this point the program will error: NullPointerException: null pointer exception, the ARR pointer has not only pointed to the array.

ArrayIndexOutOfBoundsException the array, it accesses a non-existent corner mark.

NullPointerException NULL pointer exception: When the reference does not have any pointer to a value of NULL, the reference also exists in the action entity.

2. Traversal of the array (gets the elements in the array):

public class test1{
public static void Main (string[] args) {
Int[] arr={3,1,2,5,4,6};
for (int i=0;i<arr.length;i++) {
System.out.println ("arr[" +i+ "]=" +arr[i]);
}
}

}

The result of the output is:

Arr[0]=3
Arr[1]=1
arr[2]=2
Arr[3]=5
Arr[4]=4
Arr[5]=6

========================================================================================

3. Define a function output array element and separate it with commas:

public class test1{
public static void Main (string[] args) {
Int[] arr={1,2,3,4,5,6,7,8,9};
PrintArray (arr);
}
public static void PrintArray (int[] arr) {
for (int x=0;x<arr.length;x++) {
if (x!=arr.length-1)
System.out.print (arr[x]+ ",");
Else
System.out.print (Arr[x]);
}
}
}

Output Result: 1,2,3,4,5,6,7,8,9

===========================================================================================

4. Get the maximum value in the array: The idea is as follows

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The code is as follows:

public class test1{
public static int Getmax (int[] arr) {
int max=arr[0];
for (int x=1;x<arr.length;x++) {
if (Arr[x]>max)
MAX=ARR[X];

}
return Max;
}

public static void Main (string[] arg) {
Int[] arr={12,2,3,6,5,8,9,88,99,100};
int Max=getmax (arr);
System.out.print ("max=" +max);
}
}

The output result is 100:

Method Two (max initialized to 0) code is as follows--exists as an array of corner marks:

public class test1{
public static int Getmax (int[] arr) {
int max=0;
for (int x=1;x<arr.length;x++) {
if (Arr[x]>arr[max])
Max=x;

}
return Arr[max];
}

public static void Main (string[] arg) {
Int[] arr={12,2,3,6,5,8,9,88,99,100};
int Max=getmax (arr);
System.out.print ("max=" +max);
}
}

===========================================================================================

5. Sort the array (select sort and bubble sort):

The code is as follows:

public class test1{
public static void Selectsort (int[] arr) {
for (int x=0;x<arr.length-1;x++) {///Last number do not compare
for (int y=x+1;y<arr.length;y++) {
if (Arr[x]>arr[y]) {
int temp=arr[x];
Arr[x]=arr[y];
Arr[y]=temp;
}
}
}

}
public static void Main (string[] args) {
Int[] arr={5,6,4,3,2,1,9,8,7};
PrintArray (arr);
Selectsort (arr);
PrintArray (arr);
}
public static void PrintArray (int[] arr) {
System.out.print ("[");
for (int x=0;x<arr.length;x++) {
if (x!=arr.length-1)
System.out.print (arr[x]+ ",");
Else
System.out.print (arr[x]+ "]");
}
}
}

The output results are as follows:

[5, 6, 4, 3, 2, 1, 9, 8, 7] [1, 2, 3, 4, 5, 6, 7, 8, 9]

Bubble sort (two adjacent elements are compared if they match):

The code is as follows:

public  static void Maopao (int[] arr) {
        for (int x=0;x<arr.length-1 ; x + +) {
            for (int y=0;y<arr.length-x-1;y++)
             {
                 if (arr[y]<arr[y+1]) {
                     int Temp=arr[y];
                    arr[y]=arr[y+1];
                    arr[y+1]=temp;
                }
                }
    }

=============================================================================

A common feature of combining the above selection sort and bubbling sort is to replace the values of the two array locations that meet the criteria:

public static void Swap (int[] Arr,int A,int b) {

int Temp=arr[a];

ARR[A]=ARR[B];

Arr[b]=temp;

}

Just call this function and it's OK:

# # #Arrays. Sort (arr); A sort order that has been defined in Java requires the use of this code in the development of an array sort. # # # # # #

===========================================================================================

6, array lookup (binary lookup):

Find the ordinal of the sequence in which the elements in an array are located, with the following code:

public class test1{
public static int GetIndex (int[] arr,int key) {
for (int x=0;x<arr.length;x++) {
if (Arr[x]==key)
return x;
}
return-1;
}
public static void Main (string[] args) {
Int[] arr={2,5,6,9,8,7,9};
int Index=getindex (arr,5);
System.out.print ("index=" +index);
}
}
The above is in order to improve the efficiency of the direct use of binary to find, the code is as follows:

public class test1{
public static int GetIndex (int[] arr,int key) {
for (int x=0;x<arr.length;x++) {
if (Arr[x]==key)
return x;
}
return-1;
}
public static int Halfsearch (int[] arr,int key) {
int Min,mid,max;
min=0;
Max=arr.length-1;
Mid= (max+min)/2;
while (Arr[mid]!=key) {
if (Key>arr[mid])
min=mid+1;
else if (Key<arr[mid])
Max=mid-1;
if (Min>max)
return-1;
Mid= (max+min)/2;
}
return mid;
}
public static void Main (string[] args) {
Int[] arr={2,5,6,7,8,9};
int Index=halfsearch (arr,7);
System.out.print ("index=" +index);
}
}

Fourth day of Java learning