As an exercise, I've written quicksort algorithm in C using pointers. Please comment and help me find the cases where it breaks (if any).

void qsort(int *, int, int);
void swap(int *, int *);

void qsort(int *v, int left, int right)
    int i, *last;

    if (right <= left)
    last = v + left; //choosing the first element as the pivot
    for (i = left + 1; i <= right; i++)
        if (*(v + i) < *(v + left))
            swap(++last, v + i); //swapping the numbers < pivot
    swap(v + left, last);
    qsort(v, left, last - v - 1); //sub-array containing numbers < pivot
    qsort(v, last - v + 1, right); //sub-array containing numbers >= pivot

void swap(int *i, int *j)
    int temp;

    temp = *i;
    *i = *j;
    *j = temp;

1 Answer 1


I'd make right an exclusive instead of inclusive upper bound, that is write i < right instead of i <= right and last - v instead of last - v - 1.

This has the advantage of making the base call qsort(v, 0, length) instead of qsort(v, 0, length - 1).


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