僅通過使用charAt（lastIndexOf，concatenate，substring，contain）進行Java String操作

Question

我想僅通過使用charAt在String中找到字符的lastIndexOf，但是我的代碼僅找到第一個匹配項。 我必須改變什么？

Scanner sc = new Scanner(System.in);
char operation = sc.nextLine().charAt(0);
if (operation == 'l' ) {
        System.out.print("Please enter a string: ");
        String enteredString = sc.next();
        System.out.print("Please enter a character: ");
        char char1 = sc.next().charAt(0);
        int index = 1;
        for (int i = 0; i < enteredString.length(); i++) {
            if (enteredString.charAt(i) == char1) {
                index = i;
                break;
            }
        }
        System.out.println("The index of character " + char1 + " in string " + enteredString + " is: " + index);

    }

我成功連接了兩個String：

Scanner sc = new Scanner(System.in);
char operation = sc.nextLine().charAt(0);
String c = "concatenation";
    if (operation == 'c' ) {
        System.out.println("Please enter the first string: ");
        String firstString = sc.next();
        System.out.println("Please enter the scond string: ");
        String secondString = sc.next();
        for (int i = 0; i < firstString.length(); i++) {
            char x = firstString.charAt(i);
            System.out.print(x);
        }
        for (int i = 0; i < secondString.length(); i++) {
            char y = secondString.charAt(i);
            System.out.print(y);
        }
    }

問題是，我實際上要打印此

System.out.println("The result of concatenating " + firstString + " and " + secondString + " is " + x + y);

但是我還沒有找到一種打印方法，因為x和y僅在for循環中定義，如果我嘗試打印它，它將被打印多次而不是一次。

當我通過charAt實現子字符串時，這也是一個問題：

if (operation == 's' ) {
        System.out.print("Please enter the string: ");
        String enteredString = sc.next();
        System.out.print("Please enter the first index: ");
        int index1 = sc.nextInt();
        System.out.print("Please enter the second index: ");
        int index2 = sc.nextInt();

        for (int i = index1; i < index2; i++) {
            char substring = enteredString.charAt(i);
            System.out.print(substring);

        }
}

我希望將其打印出來：

System.out.println("The resulting substring is: " + substring);

但是我也不知道如何實現這一目標。

Answer 1

非常簡單：尋找某個字符的最后一次出現時-只需向后移動字符串即可。

從最后一個字符開始，然后在字符串內“向前”移動。 第一個匹配項是最后一個匹配項。

如果要保留從0到字符串末尾的循環：只需記住要查找的char的索引。 最初，該索引為-1，並且每次匹配時，您都將其更新為相應的索引。

Answer 2

正如@GhostCat所說
要查找字符的最后一個索引，只需替換代碼

int index = 1;
    for (int i = 0; i < enteredString.length(); i++) {
        if (enteredString.charAt(i) == char1) {
            index = i;
            break;
        }
    }
    System.out.println("The index of character " + char1 + " in string " + enteredString + " is: " + index);

有了這個

        int index = -1;
        for (int i = enteredString.length() - 1; i >= 0; i--) {
            if (enteredString.charAt(i) == char1) {
                index = i;
                break;
            }
        }
        if (index == -1) {
            System.out.println("Character Not Found");
        } else {
            System.out.println("The index of character " + char1 + " in string " + enteredString + " is: " + index);
        }

對於子字符串操作，代碼應如下所示

System.out.print("The resulting substring is: ");
    for (int i = index1; i < index2; i++) {
        char substring = enteredString.charAt(i);
        System.out.print(substring);
    }
    System.out.println();