仅通过使用charAt（lastIndexOf，concatenate，substring，contain）进行Java String操作

Question

I wanted to find the lastIndexOf a character in a String by only using charAt, but my code only finds the first occurrence. What do I have to change?

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);

    }

I concatenated two String successfully with this:

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);
        }
    }

The problem is, that I actually want to print this

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

But I haven't found a way to print this because x and y are only defined in the for loops and if I try to print it, it will be printed multiple times and not once.

This is also a problem when I implemented substring through charAt:

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);

        }
}

I want this to be printed:

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

But neither do I know here how to achieve this.

Answer 1

Very simple: when looking for the last occurrence of a certain character - simply walk the string backwards .

Start with the last character, and then move "forward" within the string. The first match ... is the last occurrence.

If you want to keep that loop that walks from 0 to the end of the string: simply remember the index of that char you are looking for. Initially, that index is -1, and each time you have a match - you update it to the corresponding index.

Answer 2

As @GhostCat said
For finding the last index of a character simply replace your code

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);

with this

        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);
        }

For substring operation the code should be like this

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();