# JAVA Arrays陣列 For&For-each
###### tags: `Java`
---
## 陣列表示方法
<span class="code1">String[] name;</span>
<span class="code1">byte []b;</span>
<span class="code1">Char hex[];</span>
## 提取陣列資料
#### For
* 適合提取部分資料
>練習使用For迴圈提取陣列元素
```java=
package Ex08;
public class Ex08_Arrays_For {
public static void main(String[] args) {
int[] n = new int[]{56, 45, 68, 32}; //設定陣列n的值是n[0]=56, n[1]=45, n[2]=68, n[3]=32
int sum = 0; //設定一個整數sum來計算總額
//提取陣列元素JAVA使用For與For-each的迴圈
for (int i =0; i<4; i++) //for迴圈設定當i小於4,i會重複執行加1到條件不符中斷迴圈
{
System.out.println("n[" +i+ "]=" +n[i]); //印出陣列n的值
sum+= n[i]; //總和等於sum+n[0~3]的值
}
System.out.println("n陣列元素總合為" +sum); //印出總和
}
}
```
>結果:
n[0]=56
n[1]=45
n[2]=68
n[3]=32
n陣列元素總合為201
---
#### For-each
* 適合提取全部資料
```java=
package Ex08;
public class Ex08_Arrays_For_each {
public static void main(String[] args) {
int[] n = new int[]{56, 45, 68, 32}; //設定陣列n的值是n[0]=56, n[1]=45, n[2]=68, n[3]=32
int sum = 0; //設定一個整數sum來計算總額
//提取陣列元素JAVA使用For與For-each的迴圈
for (int i :n) //for-each迴圈(資料型別+變數:陣列名稱)
{
System.out.println(i); //印出陣列n的值
sum+= i; //總和等於sum+n[0~3]的值
}
System.out.println("n陣列元素總合為" +sum); //印出總和
}
}
```
>結果:
56
45
68
32
n陣列元素總合為201
---
## 多維陣列表示方法
寫法一
<span class="code1">int[][] n;</span>
<span class="code1">n = new int[3][4];</span> //建立3x4的陣列
寫法二(合併成一行)
<span class="code1">int[][] n = new int [3][4];</span>
>For的寫法:
```java=
package Ex08;
public class Ex08_Arrays_many {
public static void main (String[] args){
int[][] n = {{0,1,2,3},{4,5,6,7},{8,9,10,11}};
int sum = 0;
for (int i=0; i<n.length; i++) {
for (int j=0; j<n[0].length; j++) {
System.out.println("n[" +i+ "][" +j+ "]=" +n[i][j]+ "," );
sum += n[i][j];
}
System.out.println();
}
System.out.println("n陣列元素總和為" +sum);
}
}
```
>For-each的寫法:
```java=
package Ex08;
public class Ex08_Arrays_many {
public static void main (String[] args){
int[][] n = {{0,1,2,3},{4,5,6,7},{8,9,10,11}};
int sum = 0;
for (int[] r :n) { //設定陣列r為提取陣列n的變數值
for (int i : r) { //設定變數i提取陣列r的值
sum += i;
}
System.out.println();
}
System.out.println("n陣列元素總和為" +sum);
}
}
```
---
## 提取非對稱型陣列
>使用For迴圈提取不對稱的陣列~
>
```java=
package Ex08;
public class Ex08_Arrays_asymmetric {
public static void main(String[] args) {
int[][] n = new int[3][];
n[0] = new int[]{1};
n[1] = new int[]{2,3};
n[2] = new int[]{4,5,6};
for(int i = 0; i<n.length; i++){
for(int j=0; j<n[i].length; j++){
System.out.println("n["+i+"]["+j+"]=" +n[i][j]);
};
};
}
}
```
>結果:
n[0][0]=1
n[1][0]=2
n[1][1]=3
n[2][0]=4
n[2][1]=5
n[2][2]=6
## 氣泡排序法
```java=
package Ex08;
public class Ex08_Arrays_BubbleSort {
public static void main(String[] args) {
int[] x = {32,24,11,48,15};
System.out.println("【排序前】");
for(int a=0; a<x.length;a++) {
System.out.print(" " + x[a]);
}
System.out.println();
//氣泡排序法的公式，也可以寫成 for(int i = x.length-1; i>0; i--) {
// for (int j = 0; j < i; j++) { ... }}
for(int i = x.length-2; i>=0; i--) {
for (int j = 0; j <= i; j++) {
if (x[j] > x[j+1]) {
int t = x[j];
x[j] = x[j+1];
x[j+1] = t;
}
}
System.out.println("第" + (4 - i) + "次循環：");
for (int a = 0; a < x.length; a++)
System.out.print(" " + x[a]);
System.out.println();
}
System.out.println("【排序後】");
for (int a = 0; a < x.length; a++)
System.out.print(" " + x[a]);
}
}
```
>結果:
【排序前】
32 24 11 48 15
第1次循環：
24 11 32 15 48
第2次循環：
11 24 15 32 48
第3次循環：
11 15 24 32 48
第4次循環：
11 15 24 32 48
【排序後】
11 15 24 32 48
Process finished with exit code 0
~~小小OS:希望不要再用i跟j做講義了，截圖模糊i跟j傻傻分不清唷!!~~
---
## 循序搜尋法
* Step by step一個接一個地找~
* 未排序與資料量小適合使用
```java=
package Ex08;
import java.util.Scanner;
public class Ex08_Arrays_Step_by_step {
public static void main(String[] args) {
int[] a = new int[] {5,3,1,2,10,9,4,8,7,6};
for(int i=0; i<a.length;i++){
System.out.println("第" +(i+1)+ "值數=" +a[i]);
if (i==4 || i==9){
System.out.println();
}
}
Scanner scn = new Scanner(System.in);
System.out.println("請輸入要搜尋的數字:");
int search = scn.nextInt();
int num = -1;
for(int j=0; j<a.length;j++){
if(a[j] == search){
num=j;
break;
}
}
System.out.println("===================");
if(num==-1){
System.out.println("查無此數字-->"+search);
}else{
System.out.println(search+"是第" +(num+1)+ "個數字");
}
scn.close();
}
}
```
>結果:
第1值數=5
第2值數=3
第3值數=1
第4值數=2
第5值數=10
>第6值數=9
第7值數=4
第8值數=8
第9值數=7
第10值數=6
>請輸入要搜尋的數字:
---
## 二分搜尋法
* 必須先將陣列進行排序。
* 二分搜尋法執行效率比循序搜尋法好。
* N 筆資料進行二分搜尋法，平均做 Log2N+1 次比較。
```java=
package Ex08;
import java.util.Scanner;
public class Ex08_Arrays_Binary_search {
public static void main(String[] args) {
int[] aNum = {23,100,58,11,67,12,44,101,75};
System.out.println("【排序前】");
for (int i=0; i<aNum.length; i++) {
System.out.println(" " + aNum[i]);
}
for (int i=aNum.length-1; i>0; i--){
for (int j=0; j<i; j++){
if(aNum[j] > aNum[j+1]){
int t=aNum[j];
aNum[j]=aNum[j+1];
aNum[j+1]=t;
}
}
}
System.out.println("【排序後】");
for(int i=0; i<aNum.length; i++) {
System.out.print(" " + aNum[i]);
System.out.println();
}
Scanner scn = new Scanner(System.in);
System.out.println("請輸入要搜尋的數字");
int sNum = scn.nextInt();
int num = -1, low=0, high=aNum.length-1, midNum=0;
do {
midNum=(low+high)/2;
if (aNum[midNum]==sNum){
num=midNum;
break;
}
if (aNum[midNum]>sNum){
high=midNum-1;
}else {
low=midNum+1;
}
}while (low<=midNum+1);
if(num==-1){
System.out.println("沒有這個數字");
}else{
System.out.println("排序後找到" +sNum+ "是第" +(num+1)+ "個數字");
}
scn.close();
}
}
```
---
## Arrays 類別的應用
#### Arrays.sort()
遞增排序：<span class="code1">Arrays.sort(*陣列名稱*);</span>
遞減排序：<span class="code1">Arrays.sort(*陣列名稱*, Collections.reverseOrder());</span>
```java=
package Ex08;
import java.util.Arrays;
public class Ex08_sort {
public static void main(String[] args) {
int[] num = new int[]{1,9,6,2,8,4};
Arrays.sort(num);
for(int n :num){
System.out.print(n+", ");
}
}
}
```
>結果:
1, 2, 4, 6, 8, 9,
#### Arrays.binarySearch();
<span class="code1">Arrays.binarySearch(*陣列名稱*, *搜尋值*);</span>
```java=
package Ex08;
import java.util.Arrays;
import java.util.Scanner;
public class Ex08_Arrays_binarySearch {
public static void main(String[] args) {
Scanner scn = new Scanner(System.in);
int[] arr = {83, 25, 31, 5, 52, 17, 42, 63, 11, 9};
System.out.println("【排序前】");
for (int i =0; i<arr.length; i++){
System.out.print(arr[i]+ ", ");
}
System.out.println();
Arrays.sort(arr);
System.out.println("【排序後】");
for (int i = 0; i<arr.length;i++){
System.out.print(arr[i]+ ", ");
}
System.out.println();
System.out.print("請輸入搜尋值");
int sNum = scn.nextInt();
int find = -1;
if((find = Arrays.binarySearch(arr, sNum))>-1){
System.out.println("找到" +sNum+ "位於索引" +find+ "處");
}else{
System.out.println("找不到" +sNum+ "處" +find);
}
scn.close();
}
}
```
>結果:
【排序前】
83, 25, 31, 5, 52, 17, 42, 63, 11, 9,
【排序後】
5, 9, 11, 17, 25, 31, 42, 52, 63, 83,
請輸入搜尋值
---
<span class="code1"></span>
<style>
h2 {
color: #2383B8;
}
h3 {
color: #1AA340;
}
h4 {
color: white;
background-color: #2383B8;
padding:8px;
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.code1 {
padding: 2px 4px;
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