Question

I was asked to improve given quick-sort algorithm:

public void quickSort(Comparable[] a, int left, int right) {
// Sort a[left…right] into ascending order.
    if (left < right) {
        int p = partition(a, left, right);
        quickSort(a, left, p-1);
        quickSort(a, p+1, right);   
    }
}    




public int partition(Comparable[] a, int left, int right) {
// Partition a[left…right] such that  
// a[left…p-1] are all less than or equal to a[p] 
// and a[p+1…right] are all greater than or equal to 
// a[p]. Return p.
    Comparable pivot = a[left];
    int p = left;
    for (int r = left+1; r <= right; r++) {
        int comp = a[r].compareTo(pivot);
        if (comp < 0) {
            a[p] = a[r];  a[r] = a[p+1];
            a[p+1] = pivot;  p++;
        }
    }
    return p;
}

… using psude-code instructions below so it could work using less number of copies than initial algorithm:

To partition a[left…right] such that a[left…j–1] are all less than or equal to a[j], 
and a[j+1…right] are all greater than or equal to a[j]:
1.  Set pivot to a[left].
2.  Set i = left + 1 and j = right.
3.  While i <= j, repeat:
    3.1.    While i <= j and a[i] <= pivot, increment i. 
    3.2.    While j >= i and a[j] >= pivot, decrement j.
    3.3.    If i < j, swap a[i] and a[j].
4.  If j != left, set a[left] to a[j], and a[j] to pivot. 
5.  Terminate with answer j.

The problem is I cannot sort out this bit:

While i <= j and a[i] <= pivot,

as I’m getting error message saying I can’t use < and > signs with Comparable. I’ve found few solutions online but none of them worked.

Any ideas?
I’d really appreciate quick clues as I’m short of time for this project.

Thanks!
Marcepan

CODE AFTER EDITION:

public int partition(Comparable[] a, int left, int right) {
 // Partition a[left…right] such that

 // a[left…p-1] are all less than or equal to a[p]

 // and a[p+1…right] are all greater than or equal to

 // a[p]. Return p.

 Comparable pivot = a[left];
 int i = left +1;
 int j = right;
 while (i<=j){
     while (i<=j && a[i].compareTo(pivot) <= 0){
         i++;
     }
     while (i>=j && a[j].compareTo(pivot) >= 0){
         j--;
     }
     if (i<j){
     a[i] = a[j];
     }
 }
 if ( j != left){
 a[left] = a[j];
 a[j] = pivot;
 }

 return j;

}

Answer 1

Instead of a < b, you write a.compareTo(b) < 0.

Other comparison operators are written similarly, so

a[i] <= pivot

becomes

a[i].compareTo(pivot) <= 0