A recursive insertion method with a logic error

Question

I wrote a recursive insertion method that inserts a number in a list of ascending numbers and reserves the list order. It's simple and it works, the problem is that it doesn't return the right list and duplicates it when the number I want to insert is higher than all the numbers in the list.

This is my code:

public static String insertNumber(String list, int numberToInsert, int startOfList, int endOflist) {
    if (startOfList > endOflist) { // base case, reach end of the list 
        return list;
    }
    if (list.charAt(startOfList) == numberToInsert + '0') { // number already exist in list
        System.out.println("No changes because " + numberToInsert + " exist in the list");
        System.exit(0);
    }
    if (list.charAt(startOfList) > numberToInsert + '0') {
        String s = Character.toString((char) (numberToInsert + '0'));
        list = s + list.substring(startOfList);
        return list;
    }
    return list.charAt(startOfList) + insertNumber(list, numberToInsert, startOfList + 1, endOflist);
}

public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    System.out.println("Enter a numerical ascending list and a number to insert: ");

    String inputList = scan.next();
    int numberToInsert = scan.nextInt();

    String result = insertNumber(inputList, numberToInsert, 0, inputList.length() - 1);
    System.out.println("The new list is: " + result);
}

This is the output I get for the following inputs:

Enter a numerical ascending list and a number to insert: 
12567 3
The new list is: 123567 

----------------------------------------------
Enter a numerical ascending list and a number to insert: 
4567 1
The new list is: 14567

----------------------------------------------
Enter a numerical ascending list and a number to insert: 
3456 9 
The new list is: 34563456

I don't know where I went wrong.

score 0 · Answer 1 · answered Jan 19 '23 at 02:34

Your "base case" is returning the wrong result. The test case hits that case after processing the complete list, startOfList is 4, endOfList is 3. It returns the whole list, which is appended to the whole list that has already been processed.

If it were to return "numberToInsert" (as a String) instead, it would work correctly:

if (startOfList > endOflist) { // base case, reach end of the list 
    return ""+numberToInsert;
}

result:

Enter a numerical ascending list and a number to insert: 
3456 9
The new list is: 34569

full updated code:

    public static String insertNumber(String list, int numberToInsert, int startOfList, int endOflist) {
        if (startOfList > endOflist) { // base case, reach end of the list 
            return ""+numberToInsert;
        }
        if (list.charAt(startOfList) == numberToInsert + '0') { // number already exist in list
            System.out.println("No changes because " + numberToInsert + " exist in the list");
            System.exit(0);
        }
        if (list.charAt(startOfList) > numberToInsert + '0') {
            String s = Character.toString((char) (numberToInsert + '0'));
            list = s + list.substring(startOfList);
            return list;
        }
        return list.charAt(startOfList) + insertNumber(list, numberToInsert, startOfList + 1, endOflist);
    }

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter a numerical ascending list and a number to insert: ");

        String inputList = scan.next();
        int numberToInsert = scan.nextInt();

        String result = insertNumber(inputList, numberToInsert, 0, inputList.length() - 1);
        System.out.println("The new list is: " + result);
    }

A recursive insertion method with a logic error

1 Answers1