数组的使用
数组概述
定义：多个相同类型的的数据按照一定顺序排列的***，使用一个名字命名并通过编号的方式对这些数据进行统一管理。概念：数组名、索引（下标）、元素、长度特点：有序排列；数组属于引用数据类型，数组的元素可以是基本数据类型或者引用数据类型；创建数组对象会在内存中开辟一整块连续的空间；长度固定，不能修改；分类：一维数组和多维数组；基本数据类型元素数组和引用数据类型元素数组；一维数组
概念声明和初始化：静态数组、动态数组默认值数组元素的调用数组的长度数组遍历数组的内存
栈stack堆heap：new出来的结构：对象、数组 区：常量池（String）、静态域（static）自动垃圾回收机制package www.wangwei.com; public class array1 { public static void main(String[] args) { System.out.println("数组的使用"); //静态数组声明和初始化 int[] arr1 = new int[]{1,2,3,4,5}; //动态数组声明和初始化 String[] arr2 = new String[5]; //调用指定位置的元素 arr2[0] = "元素"; arr2[1] = "元素"; arr2[2] = "元素"; arr2[3] = "元素"; arr2[4] = "元素"; //数组的长度 System.out.println(arr1.length); System.out.println(arr2.length); //数组遍历 for(int i = 0;i< arr1.length;i++){ System.out.print(arr1[i]+" "); } System.out.println(); for(int j = 0;j< arr2.length;j++){ System.out.print(arr2[j]+" "); } //数组默认值 int[] arr1 = new int[5];//默认0（byte、short、int、long类型的默认值都是0） double[] arr2 = new double[5];//默认0.0（float、double类型的默认值都是0） char[] arr3 = new char[5];//默认“空格”(0或'\u0000') boolean[] arr4 = new boolean[5];//默认false; String[] arr5 = new String[5];//默认null //练习一 int[] arr1 = new int[]{8,2,1,0,3}; int[] index = new int[]{2,0,3,2,4,0,1,3,2,3,3}; String tel = ""; for(int i = 0;i < index.length;i++){ tel += arr1[index[i]]; } System.out.println("联系方式："+tel); //学生成绩 Scanner input = new Scanner(System.in); System.out.print("请输入学生人数:"); int number = input.nextInt(); System.out.print("请输入"+number+"个学生成绩:"); int[] source = new int[number]; int max1 = 0; for (int i = 0; i < number; i++) { source[i] = input.nextInt(); if (source[i] >= max1) max1 = source[i]; } System.out.println("更高分为：" + max1); for (int j = 0; j <= 4; j++) { char grade = ' '; if (source[j] >= max1 - 10) grade = 'A'; else if (source[j] >= max1 - 20 && source[j] < max1 - 10) grade = 'B'; else if (source[j] >= max1 - 30 && source[j] < max1 - 20) grade = 'C'; else grade = 'D'; System.out.println("student " + j + " source is " + source[j] + ",grade is " + grade); } } }
多维数组
理解：数组构成的数组；数组的元素也是数组从内存角度来看，都是一维数组数据结构
数据与数据之间的逻辑关系：***、一对一、一对多、多对多数据的存储结构：线性表（一对一：）顺序表（数组）、链表（非连续的）、栈（先进后出）、队列（先进先出）树型结构（一对多）：二叉树图形结构：算法
排序算法搜索算法package www.wangwei.com; public class ArrayTwo { public static void main(String[] args) { System.out.println("二维数组的使用"); //静态数组声明和初始化 int[][] arr1 = new int[][]{{1},{2,3},{4,5,6}}; //动态数组声明和初始化 String[][] arr2 = new String[3][]; arr2[0] = new String[2]; arr2[1] = new String[2]; arr2[2] = new String[2]; String[][] arr3 = new String[3][2]; //调用二维数组指定位置的元素 System.out.println(arr1[0][0]);//1 System.out.println(arr2[2][1]);//null System.out.println(arr3[2][1]);//null //获取数组的长度 System.out.println(arr1.length);//3 System.out.println(arr1[0].length);//1 System.out.println(arr2.length);//3 //遍历数组的元素 for(int i = 0;i < arr1.length;i++){ for(int j = 0;j < arr1[i].length;j++){ System.out.print(arr1[i][j]); System.out.print(" "); } System.out.println(); } //默认初始化值 nt[][] arr1 = new int[4][3]; String[][] arr2 = new String[4][3]; double[][] arr3 = new double[4][]; System.out.println(arr1[0]);////输出地址值 System.out.println(arr1[0][0]);////0 System.out.println(arr3[0]);//null 内层y元素未赋值，不能指向地址 System.out.println(arr3[0][]);//报错空指针，为赋值 //数组遍历 for (int i = 0; i < arr1.length; i++){ for (int j = 0; j < arr1[i].length; j++) { System.out.print(arr1[i][j] + "\t"); } System.out.println(); } for (int i = 0; i < arr2.length; i++){ for (int j = 0; j < arr2[i].length; j++) { System.out.print(arr2[i][j] + "\t"); } System.out.println(); } //练习1 int[][] arr1 = {{3,5,8},{12,9},{7,0,6,4}}; int sum = 0; for (int i = 0; i < arr1.length ; i++) { for (int j = 0; j < arr1[i].length; j++) { sum += arr1[i][j]; } } System.out.println("总和为："+sum); //练习2 杨辉三角 int[][] arry = new int[10][10]; for (int i = 0; i <= 9; i++) { for (int j = 0; j <= i; j++) { if((i < 2) || (j == 0) || (j == i)) arry[i][j] = 1; else arry[i][j] = arry[i-1][j-1]+arry[i-1][j]; System.out.print(arry[i][j]); System.out.print(' '); } System.out.println(); } //练习3 生产随机数，不能重复 int[] arr = new int[6]; for (int i = 0; i < arr.length; i++) { arr[i] = (int)(Math.random()*30+1); for (int j = 0; j < i; j++) { if(arr[i] == arr[j]) { i--; break; } } System.out.print(arr[i] + "\t"); } //练习4 回形数 System.out.println("请输入一个整数（1-20）："); Scanner input = new Scanner(System.in); int num = input.nextInt(); System.out.println(num); int[][] array1 = new int[num][num]; int minx = 0; int miny = 0; int maxx = num - 1; int maxy = num - 1; int count = 0; while(minx <= maxx) { for (int i = minx; i <= maxx; i++) { ++count; array1[miny][i] = count; } miny++; for(int i = miny;i <= maxy; i ++){ ++count; array1[i][maxx] = count; } maxx--; for (int i = maxx; i >= minx ; i--) { array1[maxy][i] = count++; } maxy--; for (int i = maxy; i >= miny ; i--) { array1[i][minx] = count++; } minx++; } for (int i = 0; i < array1.length; i++) { for (int j = 0; j < array1.length; j++) { String space = (array1[i][j] + "").length() == 1 ? "0" : ""; System.out.print(space + array1[i][j] + " "); } System.out.println(); } } }
Arrays工具类
是否相等：Arrays.equals(arr1,arr2);输出数组：Arrays.toString(arr1)填充数组：Arrays.fill(arr1,10);排序数组：Arrays.sort(arr2);二分查找：Arrays.binarySearch(arr2,6);package www.wangwei.com; import java.util.Arrays; public class Arraystest { public static void main(String[] args){ int[] arr1 = new int[]{1,2,3,4}; int[] arr2 = new int[]{1,3,2,4}; //判断两个数组是否相等 boolean isEquals = Arrays.equals(arr1,arr2); System.out.println(isEquals); //输出数组信息 System.out.println(Arrays.toString(arr1)); //讲指定值填充到数组中 Arrays.fill(arr1,10); System.out.println(Arrays.toString(arr1)); //对数组进行排序 Arrays.sort(arr2); System.out.println(Arrays.toString(arr2)); //二分查找 int index = Arrays.binarySearch(arr2,6); //如果值没在所查找的数组中，那么返回值是一个负数 if(index > 0) System.out.println(index); else System.out.println("未找到"); } }
数组中常用算法
数组元素的赋值（杨辉三角、回形数）求数值型数组中的值数组的赋值、反转、查找数组元素的排序算法package www.wangwei.com; public class ArrayAlgo1 { public static void main(String[] args) { //练习一 求一维数组的更大值、最小值、和、平均数 //获取随机数的一维数组 int[] arr = new int[10]; for (int i = 0; i < arr.length; i++) { arr[i] = (int) (Math.random() * 90 + 10); System.out.print(arr[i] + "\t"); } System.out.println(); //求更大值、最小值、和、平均数 int maxnum = arr[0]; int minnum = arr[0]; int sum = 0; int avg = 0; for (int i = 0; i < arr.length; i++) { if(arr[i] > maxnum) maxnum = arr[i]; if(arr[i] < minnum) minnum = arr[i]; sum+=arr[i]; } System.out.println("更大值是："+maxnum); System.out.println("最小值是："+minnum); System.out.println("总和值是："+sum); System.out.println("平均值是："+(sum/arr.length)); System.out.println(); //练习一 ***数组与赋值数组 int[] array1 = new int[]{2,3,5,7,11,13,17,19}; int[] array2 = new int[8]; System.out.print("数组array1原始值："); for (int i = 0; i < array1.length; i++) { System.out.print(array1[i]+"\t"); } System.out.println(); //数组*** 修改array2时 array1不会发生改变 System.out.print("数组array2***值："); for (int i = 0; i < array1.length; i++) { if(i % 2 == 0) array2[i] = i; else array2[i]= array1[i]; System.out.print(array2[i]+"\t"); } System.out.println(); /* 以下操作相当于把array赋值给了array3 地址值相同，修改array3时，array1也会相应发生变化 int[] array3 = new int[8]; array3 = array1; */ //数组的反转 System.out.print("数组array1反转值："); for (int i = 0; i < (array1.length / 2); i++) { int temp = array1[i]; array1[i] = array1[array1.length - 1 -i]; array1[array1.length - 1 -i] = temp; } for (int i = 0; i < array1.length; i++) { System.out.print(array1[i]+"\t"); } System.out.println(); System.out.println(); //查找 线性查找 int num = 13; boolean isflag = true; for (int i = 0; i < array1.length; i++) { if(num == array1[i]) { //if后面跟了好几行操作时，更好加上{} System.out.println("找到了指定的元素"+num+"，位置是" + i); isflag = false; break; } } if(isflag) { System.out.println("很遗憾，没有找到指定的元素"); } //查找，二分法查找，前提是数组有序 int num2 = 11; int head = 0; int end = array1.length - 1; boolean isflag1 = true; while(head <= end){ int middle = (head + end)/2; if(num2 == array1[middle]){ System.out.println("找到了指定的元素"+num2+"，位置是" + middle); isflag1 = false; break; } else if(num2 > array1[middle]){ head = middle + 1; } else{ end = middle - 1; } } if(isflag1) System.out.println("很遗憾没有找到元素"+num2); } }
算法五大特征
输入输出有穷性确定性可行性十大内部排序算法
选择排序插入排序堆排序归并排序基数排序计数排序快速排序冒泡排序桶排序希尔排序排序算法优劣
时间复杂度空间复杂度稳定性package www.wangwei.com; public class BubbleSort { public static void main(String[] args){ //冒泡排序算法的实现 int[] array = new int[]{-4,67,92,-54,6,40,102,-906}; for (int i = 0; i < array.length; i++) { System.out.print(array[i] + "\t"); } System.out.println(); for(int i = 0;i < array.length - 1; i++){ for (int j = 0; j < array.length - 1 - i; j++) { if(array[j] > array[j + 1]) { int temp = array[j]; array[j] = array[j + 1]; array[j + 1] = temp; } } } for (int i = 0; i < array.length; i++) { System.out.print(array[i] + "\t"); } } }
数组常见异常
数组脚标越界异常：ArrayIndexOutOfBoundsException空指针异常：NullPointerException
