Question

I am attempting to modify some QuickSort code to have a random number as the pivot.

However, while testing program it doesn’t sort correctly. I’m unsure why this is doing this.

int random (int num) {
    int random = rand() % (num - 1);
    return random;
}

int* partition (int* first, int* last);
void quickSort(int* first, int* last) {
    if (last - first <= 1) return;

    int* pivot = partition(first, last);
    quickSort(first, pivot);
    quickSort(pivot + 1, last);
}

int* partition (int* first, int* last) {
    //--int pivot = *(last - 1);    
    int* pos = (first + random(last - first));
    int pivot = *pos;
    int* i = first;
    int* j = last - 1;

    for (;;) {
        while (*i < pivot && i < last) i++;
        while (*j >= pivot && j > first) j--;
        if (i >= j) break;
        swap (*i, *j);
    }
    swap (*pos, *i);
    return i;
}

Answer 1

Your first implementation worked but had degenerate behavior with specific datasets. The pivot in your original working code was *(last - 1) so the simplest fix would be to swap a random element with *(last - 1). The rest of your original partition code would work unchanged.