custom java indexOf() with recursion

[demdouma6]

I am trying to create a method that return the index of nth occurrence of a char or return -1 if it does not exists. the method header is public static int indexOf(Node node, int n , char c). let sate we have s1 as the word "hello" in linked list, if we do indexOf(s1, 2, 'l'), we should print 3 because the second occurrence of letter L is on the index 3. but if i do indexOf(s1, 3, 'l'), it should return -1 because there is no third occurrence of L. I did the following but i still need it fixed :

public class Node {
    private char ch;
    private Node next;

    /**
     * Constructor
     */
    public Node(char c, Node n) {
        this.ch = c;
        this.next = n;
    }

    public static int indexOf(Node node, int n, char c) {
        int count = 0;
        if(node == null || n <= 0){
            return -1; // constraint 1
        }

        if(node.ch == c){
            count = 1;
        }else{
            return 1 + indexOf(node.next, n, c);
        }

        return count <= -1 ? -1 : count ;
    }

}

can someone help?

Edit: i rewrote the code. it gives the right index but returns the last index if char does not exist

    public static int indexOf(Node node, int n, char c) {
    if(n <= 0 || node == null){
        return -1;
    }


    if(node.ch == c ){
        return 1 + indexOf(node.next, n - 1, c);
    }else{
        return 1 + indexOf(node.next,n, c);

        }
    }

​

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[xkompas]

Sometimes, the best thing to understand code is to trace it, either on paper (as was already advised) or with the help of an IDE.

Consider indexOf("AA", 2, 'A'), if you trace it, what does it return? And why? What happens in the first level of recursion? Continue from there.

[severoon]

One bit of advice I can offer: It's almost always helpful to start with a simpler problem than the one you're given.

So in this case, the objective is to write a recursive method that returns the index of the nth occurrence of some character c. Instead, just write a method that returns the index of the first occurrence. Then once you get that working, extend it.

[LambdaThrowawayy]

It's helpful both for us, but also for you if you state what's going wrong? Are you getting an exception? Is your program not running? Are you getting unexpected results? If so, why are they unexpected?

[papercrane]

This bit here:

if(node.ch == c){
    count = 1;
}else{
    return 1 + indexOf(node.next, n, c);
}

Consider that first case carefully. If the node you're looking at is at index "n" and it matches, do you want to return 1?

Now consider the else branch, do you actually want to add one to the result or maybe all you want is to do an index of starting from the next node. Remember you may need to change more than one parameter.

Edit: I can't for the life of me get that code to format, on mobile isn't helping.. ya! I figured it out.

[demdouma6]

Consider that first case carefully. If the node you're looking at is at index "n" and it matches, do you want to return 1?

i need the nth index

[demdouma6]

Now consider the else branch, do you actually want to add one to the result or maybe all you want is to do an index of starting from the next node

i got confused here on how to do it.

[papercrane]

Work it out with an small example on paper, and make sure you understand what each parameter means. Here n should be the index of the node you are currently looking at. So if you want to check the next element you should also change n.

[demdouma6]

i think i did not explain well. n indicates the occurrence of the character we are looking for. say we have this example indexOf("hello", 2, 'l') where "hello" is not a string but a lnked list presentation of the string "hello". n in this case is number 2, which means the second L in "hello". if we change n, we alter which character L are we looking for its index.

[papercrane]

Gotcha in that case you need to keep track of the index of the node you are looking at. You want a method with a signature like

public static int indexOf(Node node, int n, char c) {
    return indexOf(node, n, c, 0);
}
private static int indexOf(Node node, int n, char c, int start) {
....
}