###### tags: `Java`
# Java自學紀錄 - 進位制轉換
## 學習重點

* while迴圈
* 字串累加
* 字串反轉
* 進位置轉換
* 陣列

### while 迴圈

while(運算式){
    [敘述區塊];
}

### 字串累加

String str;
str=str + str.valueOf(資料型別);

### 字串反轉

String str = "abcde";
Strirng reverse = new StringBuffer(str).reverse().toString();
System.out.println(reverse);
//第一個reverse為新字串名稱
//將字串存入 StringBuffer()(字串緩衝區)
//並反轉成字串 reverse().toString()
輸出結果為 : edcba

### 進位制運用

若想把10進位制的數字轉n進位制(n<=9),就一直取除以n的餘數即可得其值

>```example```
>```10進位```的10轉```2進位```
>10/2=5...0
>5/2=2....1
>2/2=1....0
>1/2=0....1
>所以```2進位制```的10為1010

import java.util.Scanner;

public class Java進位制 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int a,b;
        System.out.print("請輸入10進位制 : ");
        a = in.nextInt();
        System.out.print("想轉換成幾進位制 : ");
        b = in.nextInt();
        String str="";
        while(a>0){
            int r;
            r=a%b;
            str=str+String.valueOf(r);
            a=a/b;
        }
        String reverse = new StringBuffer(str).reverse().toString();
        System.out.println("其"+b+"進位制為 : "+reverse);
    }
}

>下圖為執行結果


# 進位制轉換-加強版
>上面為2~9進位置的轉換,假如轉換的進位制超過10呢?
>這時候可用陣列來存```餘數```的值

import java.util.Scanner;

public class Java進位制進階 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        long a,b;
        System.out.print("請輸入10進位制 : ");
        a = in.nextLong();
        System.out.print("想轉換成幾進位制 : ");
        b = in.nextLong();
        String str="";
        String s[]={"0","1","2","3","4","5","6","7","8","9","A","B","C","D","E","F"};
        while(a>0){
            long r;
            r=a%b;
            str=s[(int) r].concat(str);
            //亦等於 str=s[(int) r]+str;
            a=a/b;
        }
        System.out.println("其"+b+"進位制為 : "+str);
    }
}

>下圖為執行結果