1, Concept
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An array is a collection of multiple data types arranged in a certain order, named with a name, and managed uniformly by numbering.
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The array itself is a reference data type, and the elements in the array can be any data type, including basic data type and reference data type.
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Creating an array object will open up a continuous memory space in the content, and the first address of this continuous space is referenced in the array name.
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Once the length of the array is determined, it cannot be modified.
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data classification
- Divided by dimension: one-dimensional array, two-dimensional array
- By element type: array of basic data type elements and array of reference data type elements
2, One dimensional array
1. Declaration and initialization
public class ArrayTest {
public static void main(String[] args) {
// 1. Static initialization: array initialization and array element assignment are performed simultaneously
int[] ids; // statement
ids = new int[]{1001, 1002, 1003, 1004};
// 2. Dynamic initialization: array initialization and array element assignment are performed separately
String[] names;
names = new String[5];
// Declaration + initialization
String[] animals = new String[10];
char[] grades = new char[]{'A', 'B', 'C', 'D'};
// Other writing methods
int[] nums = {1, 2, 3, 4}; // Type inference
}
}
2. Array index
- The element at the specified position of the data is called by index.
- The index of the array starts at 0 and ends at array length - 1.
package com.example.www;
public class ArrayTest {
public static void main(String[] args) {
String[] names;
names = new String[5];
names[0] = "Guan Yu";
names[1] = "Fei Zhang";
names[2] = "Huang Zhong";
names[3] = "ma chao";
names[4] = "Zhao Yun";
}
}
3. Array length
public class ArrayTest {
public static void main(String[] args) {
String[] names;
names = new String[5];
System.out.println(names.length); // 5
}
}
4. Array traversal
public class ArrayTest {
public static void main(String[] args) {
String[] names;
names = new String[5];
names[0] = "Guan Yu";
names[1] = "Fei Zhang";
names[2] = "Huang Zhong";
names[3] = "ma chao";
names[4] = "Zhao Yun";
for (int i = 0; i < names.length; i++) {
System.out.println(names[i]);
}
}
}
5. Default initialization value
public class ArrayTest {
public static void main(String[] args) {
String[] names;
names = new String[5];
for (int i = 0; i < names.length; i++) {
System.out.println(names[i]); // null
}
}
}
Array element is an integer: 0
Array elements are floating point: 0.0
Array elements are of type char: 0 (null character) in ASCII
Array elements are boolean: false
Array element is a reference data type: null
6. Array memory parsing
3, Two dimensional array
1. Declaration and initialization
public class ArrayTest {
public static void main(String[] args) {
// 1. Static initialization
int[][] arr1 = new int[][]{{1,2,3},{4,5},{6}};
// 2. Dynamic initialization
int[][] arr2 = new int[3][];
int[][] arr3 = new int[3][2];
// Other writing methods
int arr4[][] = new int[3][];
int[] arr5[] = new int[3][];
int[][] arr6 = {{1,2,3},{4,5},{6}};
}
}
2. Array index
public class ArrayTest {
public static void main(String[] args) {
int[][] arr1 = new int[][]{{1,2,3},{4,5},{6}};
System.out.println(arr1[0][1]); // 2
int[][] arr2 = new int[3][];
arr2[1] = new int[4];
System.out.println(arr2[1][0]); // 0
}
}
3. Array length
public class ArrayTest {
public static void main(String[] args) {
int[][] arr1 = new int[][]{{1,2,3},{4,5},{6}};
System.out.println(arr1.length); // 3
System.out.println(arr1[1].length); // 2
}
}
4. Array traversal
public class ArrayTest {
public static void main(String[] args) {
int[][] arr = new int[][]{{1,2,3},{4,5},{6}};
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
System.out.print(arr[i][j] + " ");
}
System.out.println();
}
}
}
//1 2 3
//4 5
//6
5. Default initialization value
public class ArrayTest {
public static void main(String[] args){
// Initialization method 1:
int[][] arr1 = new int[4][3];
System.out.println(arr1[0]); // [ I@1b6d3586 Address value
System.out.println(arr1[0][0]); // 0
System.out.println(arr1); // [[I@4554617c
// Initialization mode 2:
int[][] arr2 = new int[4][];
System.out.println(arr2[0]); // null
System.out.println(arr2[0][1]); // NullPointerException, cannot be called, and an error is reported
}
}
6. Array memory parsing
4, Arrays tool class
import java.util.Arrays;
public class ArrayTest {
public static void main(String[] args) {
int[] arr1 = new int[]{1, 2, 3, 4};
int[] arr2 = new int[]{2, 1, 4, 3};
boolean isEquals = Arrays.equals(arr1,arr2);
// Determine whether the arrays are equal
System.out.println(isEquals); // false
// Output array information
System.out.println(Arrays.toString(arr1)); // [1, 2, 3, 4]
// Fills the array with the specified value
Arrays.fill(arr1, 10);
System.out.println(Arrays.toString(arr1)); // [10, 10, 10, 10]
// sort
Arrays.sort(arr2);
System.out.println(Arrays.toString(arr2)); // [1, 2, 3, 4]
// Binary search
int[] arr3 = new int[]{2, 3, 5, 7, 11, 13, 17, 19, 21, 23};
int index = Arrays.binarySearch(arr3, 17);
System.out.println(index); // 6
}
}
5, Array exercise
1. Yanghui triangle
// Investigate element assignment
public class YangHuiTest {
public static void main(String[] args) {
// 1. Declare and initialize a two-dimensional array
int[][] yangHui = new int[10][];
// 2. Assign values to the elements of the array
for (int i = 0; i < yangHui.length; i++) {
yangHui[i] = new int[i + 1];
// 2.1 assign values to the first and last elements first
yangHui[i][0] = yangHui[i][i] = 1;
// 2.2 assign values to non first and last elements of each line
if (i > 1) {
for (int j = 1; j < yangHui[i].length - 1; j++) {
yangHui[i][j] = yangHui[i-1][j-1] + yangHui[i-1][j];
}
}
}
// 3. Traverse two-dimensional array
for (int i = 0; i < yangHui.length; i++) {
for (int j = 0; j < yangHui[i].length; j++) {
System.out.print(yangHui[i][j] + " ");
}
System.out.println();
}
}
}
2. Find the maximum, minimum, average and sum of elements in a numeric array
Define a one-dimensional array of int type, which contains 10 elements and is given two random integers respectively. Then calculate the maximum, minimum, average and sum of all elements and output them.
//Obtain the random number of [a,b] range: (int)(Math.random()*(b-a+1)+a)
public class ArrayTest {
public static void main(String[] args) {
int[] arr = new int[10];
for (int i = 0; i < arr.length; i++) {
arr[i] = (int) (Math.random() * (99 - 10 + 1) + 10);
}
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " ");
}
System.out.println();
//Maximum
int maxValue = arr[0];
for (int i = 1; i < arr.length; i++) {
if (maxValue < arr[i]) {
maxValue = arr[i];
}
}
System.out.println("The maximum value is:" + maxValue);
//minimum value
int minValue = arr[0];
for (int i = 1; i < arr.length; i++) {
if (minValue > arr[i]) {
minValue = arr[i];
}
}
System.out.println("The minimum value is:" + minValue);
//Sum
int sum = 0;
for (int i = 0; i < arr.length; i++) {
sum += arr[i];
}
System.out.println("And:" + sum);
//average
int avgValue = sum / arr.length;
System.out.println("The average is:" + avgValue);
}
}
3. Array copy
public class ArrayTest {
public static void main(String[] args) {
int[] arr1,arr2;
arr1 = new int[]{2, 3, 5, 7, 11, 13, 17, 19};
// Display the contents of arr1
for (int i = 0; i < arr1.length; i++) {
System.out.print(arr1[i] + " "); // 2 3 5 7 11 13 17 19
}
System.out.println();
// 1. Assign arr2 variable equal to arr1
// It cannot be called array copy. The address values of arr1 and arr2 are the same
// arr2 = arr1;
// 2. Traversal assignment
arr2 = new int[arr1.length];
for (int i = 0; i < arr1.length; i++) {
arr2[i] = arr1[i];
}
// Modify elements in arr2
for (int i = 0; i < arr2.length; i++) {
if (i % 2 == 0){
arr2[i] = i;
}
}
// Display the contents of arr1
for (int i = 0; i < arr1.length; i++) {
System.out.print(arr1[i] + " "); // 2 3 5 7 11 13 17 19
}
System.out.println();
// Displays the contents of arr2
for (int i = 0; i < arr2.length; i++) {
System.out.print(arr2[i] + " "); // 0 3 2 7 4 13 6 19
}
System.out.println();
}
}
4. Array inversion
public class ArrayTest {
public static void main(String[] args) {
int[] arr = new int[]{2, 3, 5, 7, 11, 13, 17, 19};
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " "); // 2 3 5 7 11 13 17 19
}
System.out.println();
// Array inversion
// Method 1
// for (int i = 0; i < arr.length / 2; i++) {
// int tmp = arr[i];
// arr[i] = arr[arr.length - i - 1];
// arr[arr.length - i - 1] = tmp;
// }
// Method 2
for (int i = 0, j = arr.length - 1; i<j;i++,j--) {
int tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " "); // 19 17 13 11 7 5 3 2
}
}
}
5. Find
5.1 linear search
public class SearchTest {
public static void main(String[] args) {
int[] arr = new int[]{2, 3, 5, 7, 11, 13, 17, 19};
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " "); // 2 3 5 7 11 13 17 19
}
System.out.println();
int dest = 17;
// Linear search
for (int i = 0; i < arr.length; i++) {
if (dest == arr[i]){
System.out.println("Index:"+i);
break;
}
}
}
}
5.2 dichotomy
Premise: the array to be found must be ordered
public class SearchTest {
public static void main(String[] args) {
int[] arr = new int[]{2, 3, 5, 7, 11, 13, 17, 19, 21, 23};
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " "); // 2 3 5 7 11 13 17 19
}
System.out.println();
int dest = 17;
// binary search
int head = 0; // Initial first index
int end = arr.length - 1; // Initial last index
boolean isFlag = true;
while (head <= end){
int middle = (head + end) / 2;
if (dest == arr[middle]){
System.out.println("Index:" + middle);
isFlag = false;
break;
}else if (arr[middle] > dest) {
end = middle -1 ;
}else {
head = middle + 1;
}
}
if(isFlag){
System.out.println("Not found");
}
}
}
6. Sorting
6.1 bubble sorting
public class BubbleSortTest {
public static void main(String[] args) {
int[] arr = new int[]{11, 17, 2, 13, 5, 23, 7, 3};
// Bubble sorting
for (int i = 0; i < arr.length - 1; i++) {
for (int j = 0; j < arr.length - 1 - i; j++) {
if (arr[j] > arr[j+1]) {
int tmp = arr[j];
arr[j] = arr[j+1];
arr[j+1] = tmp;
}
}
}
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " "); // 2 3 5 7 11 13 17 23
}
}
}