Question

I need help with using merge sort with a string array. I’ve seen many examples of this with an int array but I need help using a string array. I can only use the .compareTo() method. This is my main method:

public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);

    String[] nameList = {"John", "Mark", "Amber", "Tony", "Matt", "George", 
    "Will", "Bob", "Paul", "Mary Ellen", "Kate", "Joe", "Fred", "Joe", 
    "Anne", "Amber", "Kimberly", "Kelsey", "Matthew"};

    //print original
    System.out.println("Before sorting the names: ");
    for(String element: nameList)
    System.out.print(element + " ");
    System.out.println("\n");
    //Merge Sort
    System.out.println("After sorting the names: ");
    mergesort(nameList, 0, nameList.length);

}

These are my methods:

private static void merge(String[] data, int first, int n1, int n2) {
    String[] temp = new String[n1 + n2];
    int copied = 0;
    int copied1 = 0;
    int copied2 = 0;

    while((copied1 < n1) && (copied2 < n2)) {
        if(data[first + copied].compareTo(data[first + n1 + copied2]) < 0)
            temp[copied++] = data[first + (copied1++)];
        else
            temp[copied++] = data[first + n1 +(copied2++)];
    }

    while(copied1 < n1)
        temp[copied++] = data[first + (copied1++)];

    for(int i = 0; i < copied; i++)
        data[first +i] = temp[i];

}


public static void mergesort(String[] data, int first, int n) {
    int n1 = 0;
    int n2 = 0;

    if(n > 1) {
        n1 = n/2;
        n2 = n-n1;

        mergesort(data, first, n1);
        mergesort(data, first + n1, n2);
    }

    merge(data, first, n1, n2);

    for(String element: data)
        System.out.print(element +" ");
}

When I run the program it sorts some of it correctly but over all it is not inorder.

Answer 1

You have one typo and two forgotten lines. Below is fixed function. Compare it with your one.

    private static void merge(String[] data, int first, int n1, int n2)
    {
        String[] temp = new String[n1 + n2];
        int copied = 0;
        int copied1 = 0;
        int copied2 = 0;

        while ((copied1 < n1) && (copied2 < n2))
        {
            if (data[first + copied1].compareTo(data[first + n1 + copied2]) < 0)
                temp[copied++] = data[first + (copied1++)];
            else
                temp[copied++] = data[first + n1 + (copied2++)];
        }

        while (copied1 < n1)
            temp[copied++] = data[first + (copied1++)];
        while (copied2 < n2)
            temp[copied++] = data[first + n1 + (copied2++)];

        for (int i = 0; i < copied; i++)
            data[first + i] = temp[i];

    }