###### tags: `Java 學習筆記`
# Java 學習筆記 - 5-1: 陣列的應用

## 陣列的記憶體配置

### 記憶體管理
陣列一旦被創建，長度就不能被改變，同時也是一塊連續的記憶體

## 讓 Array 擴充的方法: 建立一個新的並將舊的資料放進去
通常會根據 2 倍逐漸擴大

    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        System.out.println(arr.length); // 5
        arr = doubleArr(arr);
        System.out.println(arr.length); // 10
    }
    public static int[] doubleArr(int[] arr) {
        int[] newArr = new int[arr.length * 2];
        for (int i = 0; i < arr.length; i++) {
            newArr[i] = arr[i];
        }
        return newArr;
    } 

### 範例: 輸入學生成績

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int[] students = new int[2];
        int count = 0;
        while (true) {
            System.out.println("請輸入第" + (count + 1) + "筆成績");
            int input = sc.nextInt();
            if (input == -1) {
                break;
            }
            if (input < 0 || input > 100) {
                continue;
            }
            if (count == students.length) {
                students = doubleArr(students);
            }
            students[count++] = input;
        }
        for (int i = 0; i < count; i++) {
            System.out.print(students[i] + " ");
        }
    }
    public static int[] doubleArr(int[] arr) {
        int[] newArr = new int[arr.length * 2];
        for (int i = 0; i < arr.length; i++) {
            newArr[i] = arr[i];
        }
        return newArr;
    } 

## 資料排序

### 氣泡排序法
* 第零個跟第一個比，每一格逐格比較，比較小的往前移
* 陣列有 n 個，連續交換　n - 1 次

for i = 1 ~ n - 1 // pass 次數
    for j = 0 ~ n - i - 1 // 交換次數
        if a[j] > a[j+1] then
            swap a[j] and a[j+1]
        end if
    end for
end for

## 查字典
1. 讓使用者輸入一堆字串並存到陣列

    Scanner sc = new Scanner(System.in);

    String[] strs = new String[2];
    int count = 0;
    
    String[] dictionary = new String[2]; // 出現過的字
    int[] dicNum = new int[2]; //出現過幾次
    int dicCount = 0;
    
    while(true) {
        String str =sc.nextLine();
        if (str.equals("-1")) { // 字串判斷要用equals
            break;
        }
        int index = findIndexWithKey(dictionary, str, dicCount);
        if (index == -1) {
            if (dicCount == dictionary.length) {
                dictionary = StringDoubleArr(dictionary);
                dicNum = NumberDoubleArr(dicNum);
            }
            dictionary[dicCount] = str;
            index = dicCount++;
        }
        dicNum[index]++;
    }
    for (int i = 0; i < dicCount++) {
        System.out.println("字串: " + dictionary[i] + "\t\t出現" + dicNum[i] + "次")
    }
    
public static int findIndexWithKey(String[] arr, String key, int count) {
    for (int i = 0; i < count; i++) {
        if (arr[i].equals(key)) {
            return i;
        }
    }
    return -1;
}
    
public static int[] doubleArr(int[] arr) {
    int[] newArr = new int[arr.length * 2];
    for (int i = 0; i < arr.length; i++) {
        newArr[i] = arr[i];
    }
    return newArr;
}