JAVA数组学习笔记

:::info JAVA课程中讲到数组这类数据结构的笔记，做此记录 :::

一维数组

创建

double [] array; // 声明数组方式一
double array []; // 生命数组方式二

创建好编译器会有初始值

初始化

double [] array = new double[10]; // 生命数组并且初始化数组大小
double [] array = {1.9, 2.9, 3.4, 3.5}; // 用值初始化数组

相关属性

是一个对象，有自己的属性自己的方法

String里面长度length有()，也就是String里面是方法，而数组没有括号()（写成array.length），理解为成员变量/属性，而不是方法（封装好了许多功能，体现出语言优越性）

相关操作

• [x] Initializing arrays with input values

  java.util.Scanner input = new java.util.Scanner(System.in);
  for (int i=0;i<mylist.length;i++)
    mylist[i] = input.nextDouble();

• [x] Initializing arrays with random values

  for (int i = 0; i < myList.length; i++) {
  myList[i] = Math.random() * 100;
  }

• [x] Printing arrays

  for (int i = 0; i < myList.length; i++) {
  System.out.print(myList[i] + " ");
  }

• [x] Summing all elements

  double total = 0;
  for (int i = 0; i < myList.length; i++) {
  total += myList[i];
  }

• [x] Finding the largest element

  double max = myList[0];
  for (int i = 1; i < myList.length; i++) {
  if (myList[i] > max) max = myList[i];
  }

• [ ] Finding the smallest index of the largest element

• Random shuffling

• Shifting elements

数组的值传递

java核心知识点之一

数组名（数组地址）作为函数参数

java当中是值传递的，没有引用没有指针的概念，所以在做函数参数当中，一直都是值传递，python也是。

Array类

import java.util.Arrays;

• Array类中常用的函数

  sort()、parallelSort()、fill()、toString()、binarySearch()（Binary Search二分查找（事先得排好序））、equals()

多维数组

二维数组的创建、初始化

dataType[][] refVar = new dataType[10][10]; 
int[][] array = {
  {1, 2, 3},
  {4, 5, 6},
  {7, 8, 9},
  {10, 11, 12}
};

数组长度的计算

array.length = 4
array[0].length = 3
array[4].length ==> ArrayIndexOutOfBoundsException

二维数组的相关操作

• [x] Initializing arrays with input values

  java.util.Scanner input = new Scanner(System.in);
  System.out.println("Enter " + matrix.length + " rows and " +
  matrix[0].length + " columns: ");
  for (int row = 0; row < matrix.length; row++) {
  for (int column = 0; column < matrix[row].length; column++) {
    matrix[row][column] = input.nextInt(); 
  }
  }

• [x] Initializing arrays with random values

  for (int row = 0; row < matrix.length; row++) {
  for (int column = 0; column < matrix[row].length; column++) {
    matrix[row][column] = (int)(Math.random() * 100);
  }
  }

• [x] Printing arrays

  for (int row = 0; row < matrix.length; row++) {
  for (int column = 0; column < matrix[row].length; column++) {
    System.out.print(matrix[row][column] + " ");
  }
  System.out.println();
  }

• [x] Summing all elements

  int total = 0;
  for (int row = 0; row < matrix.length; row++) {
  for (int column = 0; column < matrix[row].length; column++) {
    total += matrix[row][column];
  }
  }

• [x] Summing all elements by column

  for (int column = 0; column < matrix[0].length; column++) {
  int total = 0;
  for (int row = 0; row < matrix.length; row++)
    total += matrix[row][column];
  System.out.println("Sum for column " + column + " is " 
    + total);
  }

• [x] Which row has the largest sum

• Finding the smallest index of the largest element
• Random shuffling

  for (int i = 0; i < matrix.length; i++) {
  for (int j = 0; j < matrix[i].length; j++) {
    int i1 = (int)(Math.random() * matrix.length);
    int j1 = (int)(Math.random() * matrix[i].length);
    // Swap matrix[i][j] with matrix[i1][j1]
    int temp = matrix[i][j];
    matrix[i][j] = matrix[i1][j1]; 
    matrix[i1][j1] = temp;
  }
  }

多维数组

double[][][] scores = {
  {{7.5, 20.5}, {9.0, 22.5}, {15, 33.5}, {13, 21.5}, {15, 2.5}},
  {{4.5, 21.5}, {9.0, 22.5}, {15, 34.5}, {12, 20.5}, {14, 9.5}},
  {{6.5, 30.5}, {9.4, 10.5}, {11, 33.5}, {11, 23.5}, {10, 2.5}},
  {{6.5, 23.5}, {9.4, 32.5}, {13, 34.5}, {11, 20.5}, {16, 7.5}},
  {{8.5, 26.5}, {9.4, 52.5}, {13, 36.5}, {13, 24.5}, {16, 2.5}},
  {{9.5, 20.5}, {9.4, 42.5}, {13, 31.5}, {12, 20.5}, {16, 6.5}}
};