Question

Currently trying to figure out what the issue is with this sort. Built directly from the in-place Quicksort pseudocode from Wikipedia, which I am going to assume is reliable. I am attempting to sort an array of structs by a null-terminated 3-character “code” field.

The sort mostly works, but there are always a few elements out of place. I can only assume this has to do with the pivot somehow, but I have spent a few hours staring at it and have gotten nowhere. Thanks!

void quicksort(Cdir *directory, int left, int right) {

    if (left < right) {
        int pivotIdx = left;
        pivotIdx = partition(directory, left, right, pivotIdx);
        quicksort(directory, left, pivotIdx - 1);
        quicksort(directory, pivotIdx + 1, right);
    }
}

int partition(Cdir *directory, int left, int right, int pivot) {

    char *pivotVal = directory[pivot].code;
    int curIdx = left;
    swap(&directory[pivot], &directory[right]);

    int i;
    for (i = left; i < right; i++) {
        if (strncmp(directory[i].code, pivotVal, 3) < 0) {
            swap(&directory[i], &directory[curIdx]);
            curIdx++;
        }
    }
    swap(&directory[curIdx], &directory[right]);
    return curIdx;
} 

void swap(Cdir *s1, Cdir *s2) {

    Cdir temp = *s1; 
    *s1 = *s2;
    *s2 = temp;
}

Answer 1

I finally figured it out. When I replaced “pivotVal” in my string comparison with a direct reference to the pivot value (directory[right]), the sort works fine. Still trying to decide why that is, but it is fixed!

void quicksort(Cdir directory[], int left, int right) {

    if (left < right) {
        int pivotIdx = left;
        pivotIdx = partition(directory, left, right, pivotIdx);
        quicksort(directory, left, pivotIdx - 1);
        quicksort(directory, pivotIdx + 1, right);
    }
}

int partition(Cdir directory[], int left, int right, int pivot) {

    int curIdx = left;
    swap(&directory[pivot], &directory[right]);

    int i;
    for (i = left; i < right; i++) {
        if (strncmp(directory[i].code, directory[right].code, 3) < 0) {
            swap(&directory[i], &directory[curIdx]);
            curIdx++;
        }
    }
    swap(&directory[curIdx], &directory[right]);

    return curIdx;
} 

void swap(Cdir *s1, Cdir *s2) {

    Cdir temp = *s1; 
    *s1 = *s2;
    *s2 = temp;
}