使用比较器进行快速排序

Question

i'am working on project to Sort an array of Student objects with the following critiria:

1.Sort grades

2.If grades are equal, sort on StudentNumber.

now i got this working using the Arrays.Sort(T[] a, Comparator<> c ) method. since i think this sorting method is to slow, i want to use Quicksort.

i got the following piece of code:

public static void quickSort(Student[] arr, int low, int high) {
    if (arr == null || arr.length == 0) {
        return;
    }
    if (low >= high) {
        return;
    }
    // pick the pivot
    int middle = low + (high - low) / 2;
    Student pivot = arr[middle];
    // make left < pivot and right > pivot
    int i = low, j = high;
    while (i <= j) {
        while (arr[i].getCijfer() < pivot.getCijfer()) {
            i++;
        }
        while (arr[j].getCijfer() > pivot.getCijfer()) {
            j--;
        }
        if (i <= j) {
            Student temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
            i++;
            j--;
        }
    }
    // recursively sort two sub parts
    if (low < j) {
        quickSort(arr, low, j);
    }
    if (high > i) {
        quickSort(arr, i, high);
    }
}

UPDATE Comparator Code:

private static Comparator<Student> gradeComparator = new Comparator<Student>() {
    @Override
    public int compare(Student o1, Student o2) {
        int compareTo = 0;
        if (o1.cijfer > o2.cijfer) {
            compareTo = -1;
        } else {
            if (o1.cijfer < o2.cijfer) {
                compareTo = 1;
            } else {
                // als Studenten Grade gelijk, vergelijk op studenten Ldap
                if (o1.cijfer == o2.cijfer) {
                    if (o1.studentnummer > o2.studentnummer) {
                        compareTo = -1;
                    } else {
                        compareTo = 1;
                    }
                }
            }
        }
        return compareTo;
    }
};

this sorts the array on the Grades(cijfer) but i'am stuck on where to put my 2nd sorting criteria. please help me!

Kind regards

Answer 1

This expression: arr[i].getCijfer() < pivot.getCijfer() and this: arr[j].getCijfer() > pivot.getCijfer() are the source of your problem. You need to extract these into a single function which returns an integer, implemented as follows: return Integer.compare( a.getCijfer(), b..getCijfer() ); .

But then you will notice that this is just a comparator function, so you can go ahead and modify it to work exactly as it worked when you were using Arrays.sort() with comparator.

This addresses your question.

However, be aware of the following:

1. Your suspicion that Arrays.sort() will be slower than some quicksort is completely unfounded, and most likely false.

2. Your belief that you can do a home-brewed quicksort that will perform better than Arrays.sort() is most likely false.

3. If you do not know how to get your home-brewed quicksort to sort the way you want it to, and you have to ask on stackoverflow, there is nothing wrong with that, but then obviously, sorting is a topic which is far more advanced than what your professor expects from you.

Answer 2

You would need to put your second criterion anywhere you compare two Student instances, eg

if (arr[i].getCijfer() < pivot.getCijfer()
    || (arr[i].getCijfer() == pivot.getCijfer()
        && arr[i].getStudentNumber() < pivot.getStudentNumber()))

However, it is inconvenient to add multiple criteria here, so you would be better introducing a method in which the comparison is done:

if (compare(arr[i], pivot) < 0)

and then pushing the comparison logic into that method, so that it returns some number depending upon whether arr[i] is less than, equal to, or greater than pivot .

Note that this is basically what Comparator provides:

class MyComparator implements Comparator<Student> {
  @Override int compare(Student a, Student b) {
    if (a.getCijfer() != b.getCijfer()) {
      return // Some comparison of a.getCijfer() and b.getCijfer()
    }
    // Somehow compare student number.
  }
}