problems with merge sort in java

Question

i'm brand new to stackoverflow and i need some help with writing a program to mergesort an arraylist of comparables. i have worked on this code for hours to no avail. the program needs to work correctly, because i am doing it for a computer science class, and the very next assignment requires us to test the efficiency of different sorts. here's the code:

Merge Method:

public static void merge(ArrayList <Comparable> a, int first, int mid, int last){
    ArrayList <Comparable> b = new ArrayList();
    for(int k = first; k <= last; k++){
        b.add(a.get(k));
    }

    System.out.println("b now contains " + b);
    int middle =b.size() /2;
    for(int i = first; i <= last; i++){
        //System.out.println("mid: " + b.size() /2);
        //System.out.println("b: " + b);
        //System.out.println("a: " + a);
        //System.out.println("i: " + i);
        if(middle == b.size()){
            a.set(i, b.remove(0));
            middle--;
        }else if(middle == 0){
            a.set(0, b.remove(0));
        }else if(b.get(0).compareTo(b.get(middle)) < 0){
            System.out.println("moving " + b.get(0) + " from b[0] to a[" + 
                i + "] because " + b.get(0) + " is less than " + b.get(middle));
            a.set(i, b.remove(0));
            middle--;
            System.out.println("b now contains " + b);
        }else{
            System.out.println("moving " + b.get(middle) + " from b[" + 
                b.size() /2 + "] to a[" + i + "] because " + b.get(0) + 
                    " is greater than " + b.get(middle));
            a.set(i, b.remove(middle));
            //middle--;
            System.out.println("b now contains " + b);
        }
    }

    System.out.println();
    System.out.println("Merge");
    System.out.println(a);
    System.out.println();
}

Mergesort Method:

public static void mergeSort(ArrayList <Comparable> a, int first, int last){
    if(first < last){
        int mid = first + (last - first) /2;
        System.out.println("mergeSorting " + a.subList(first, last + 1));
        mergeSort(a, first, mid);
        System.out.println("mergeSorting " + a.subList(first, mid + 1));
        mergeSort(a, mid + 1, last);
        System.out.println("merging " + a.subList(first, mid + 1) + 
            " and " + a.subList(mid + 1, last + 1));
        merge(a, first, mid, last);
    }
    System.out.println();
    System.out.println("base case");
    System.out.println();
}

I think that there is a problem with the merge method, but i'm not sure.

my code seems to be sorting the list incorrectly, i.e:

Input:
[7,1,9,9,5,4,8,9,10,4] 

Output:
[4,8,9,4,10,10,5,9,9,7]

Answer 1

The mergeSort algorithm is ok. I just defined the type of ArrayList to avoid the warning

   public static void mergeSort(ArrayList <Comparable> a, int first, int last){
        if(first < last){
            int mid = first + (last - first) /2;
        //    System.out.println("mergeSorting " + a.subList(first, mid ));
            System.out.println("First"+first);
            System.out.println("Mid"+mid);
            System.out.println("Last"+last);
            System.out.println("MergeSortCall");
            System.out.println("firstVector " + a.subList(first, mid+1));
            System.out.println("secondVector " + a.subList(mid + 1,last+1));                
            mergeSort(a, first, mid);
            mergeSort(a, mid + 1, last);
            merge(a, first, mid, last);
            System.out.println("mergeVector " + a.subList(first,last+1));
        }

    }

The second part is really difficult to know exactly what is wrong so I replaced it for something much simpler using basically your notation. Note that the second part is just a "merge" algorithm. It is supposed to be really simple. Anyway, using this you can compare with your results (sorry it is difficult to understanding the logic of the second part and this is your homework, isn't it?). I'm using two vectors to improve the readability, but only one as you did is necessary.

public static void merge(ArrayList <Comparable> a, int first, int mid, int last){
            ArrayList <Comparable> vectorFirst = new ArrayList<Comparable>();
            ArrayList <Comparable> vectorSecond = new ArrayList<Comparable>();

            for(int k=first;k<=mid;k++){
                vectorFirst.add(a.get(k));
            }
            for(int k=mid+1;k<=last;k++){
                vectorSecond.add(a.get(k));
            }

            int i=first;
            int j=mid+1;
            int k=first-1;
            while((i<=mid)&(j<=last)){
                if(vectorFirst.get(i-first).compareTo(vectorSecond.get(j-mid-1))==-1){
                    k=k+1;
                    a.set(k,vectorFirst.get(i-first));
                    i=i+1;
                }
                else{
                     k=k+1;
                     a.set(k,vectorSecond.get(j-mid-1));
                     j=j+1;
                }
            }
            if(i==mid+1){
                while(j<=last){
                    k=k+1;
                    a.set(k,vectorSecond.get(j-mid-1));
                    j=j+1;
                }
            }
            else{
                while(i<=mid){
                    k=k+1;
                    a.set(k,vectorFirst.get(i-first));
                    i=i+1;
                }
            }


        }