Question

I have question about my code which does the Hoare Partition Method.

Here is the pseudo-code: (please correct something if it’s incorrect)

HOARE-PARTITION ( A, p, r)
 1 x ← A[ p]
 2 i ← p−1
 3 j ← r +1
 4 while TRUE
 5        do repeat j ← j − 1
 6        until A[ j ] ≤ x

 7        do repeat i ← i + 1
 8        until A[i] ≥ x

 9        if i < j
10             then exchange A[i] ↔ A[ j ]
11             else return j

And my code:

public class Hoare {
    public static  int partition(int a[],int p,int r) {
        int x = a[p];
        int i = p - 1;
        int j = r + 1;

        while (true) {    
            do
            {
                j = j - 1;
            } while(a[j] >= x);

            do
            {
                i = i + 1;
            } while(a[i] <= x);

            if (i < j) {
                int t = a[i];
                a[i] = a[j];
                a[j] = t;
            } else {    
                return j;
            }
        }        
    }

    public static void main(String[]args) {
        int a[] = {13, 19, 9, 5, 12, 8, 7, 4, 11, 2, 6, 21};
        partition(a, 0, a.length-1);
    }
}

And error is:

error: Class names, 'Hoare', are only accepted if annotation
processing is explicitly requested
1 error

Any ideas as to the cause?

Answer 1

You seem to have translated:

do repeat stmt
until A[ j ] ≤ x;

into

do {
    stmt;
} while (A[j] >= x);

This is not correct. The comparison which is the opposite of ≤ would be >, not ≥. Therefore, your comparison when translating from “until” to “while” would use > such as while (A[j] > x);.

The other errors in your possible homework are similar and I encourage you to think each translation through carefully on your own.