How to count and return swaps in algorithms - MergeSort and QuickSort?

Question

I have this code for quicksort

int sum = 0;
int partition(int *L, int left, int right) {
        int pivot = left;
        int p_val = L[pivot];
        while (left < right) {
            while (L[left] <= p_val)
                left++;
            while (L[right] > p_val)
                right--;
            if (left < right) {
                swap(&L[left], &L[right]);
                sum++;
                }
            }
        swap(&L[pivot], &L[right]);
        sum++;
        return right;
        }

    void quicksort(int *L, int start, int end) {
        if (start >= end)
            return;
        int splitPoint = partition(L, start, end);
        quicksort(L, start, splitPoint - 1);
        quicksort(L, splitPoint + 1, end);
        }

And it works fine, I think. For given array {8,4,2,1} I get that 3 swaps have been made. BUT, I need to modify the code so that function RETURNS number of swaps. Is this possible, and if so, how? Please explain as simply as you can because I'm a novice in this. I need to do the same with mergeSort but I still haven't found working code that isn't too complicated. When I do, I'll update the question. Thank you.

Answer 1

You can use the int pointer as the argument of partition and quicksort functions:

int partition(int *num, int *L, int left, int right) {
    ...
    if (left < right) {
       swap(&L[left], &L[right]);
       (*num)++; // increment number of swap
       sum++;
       }
    }
    ...
    swap(&L[pivot], &L[right]);
    (*num)++;
    sum++;
    return right;
}

And in the quicksort function:

void quicksort(int *num, int *L, int start, int end) {
  if (start >= end)
      return;
  int splitPoint = partition(num, L, start, end);
  quicksort(num, L, start, splitPoint - 1);
  quicksort(num, L, splitPoint + 1, end);
}

Then in main function:

int main() 
{ 
    int arr[] = {8, 4, 2, 4, 5, 7, 8, 9, 1}; 
    int n = sizeof(arr)/sizeof(arr[0]); 
    int num = 0;
    quicksort(&num, arr, 0, n-1); 
    printf("sum = %d num = %d\n", sum, num);
    return 0; 
}

Answer 2

Okay now I have this:

int sum = 0;
int partition(int *L, int left, int right) {
    int pivot = left;
    int p_val = L[pivot];

    while (left < right) {
        while (L[left] <= p_val)
            left++;
        while (L[right] > p_val)
            right--;
        if (left < right) {
            swap(&L[left], &L[right]);
            sum++;
            }
        }
    swap(&L[pivot], &L[right]);
    sum++;
    return right;
    }

int quicksort(int *L, int start, int end) {
    if (start >= end)
        return;
    int splitPoint = partition(L, start, end);
    quicksort(L, start, splitPoint - 1);
    quicksort(L, splitPoint + 1, end);
    return sum;
    }

for QuickSort, and this:

int sumMerge = 0;
void merge(int arr[], int l, int m, int r) {
    int i, j, k;
    int n1 = m - l + 1;
    int n2 = r - m;

    int L[n1], R[n2];

    for (i = 0; i < n1; i++)
        L[i] = arr[l + i];
    for (j = 0; j < n2; j++)
        R[j] = arr[m + 1 + j];

    i = 0;
    j = 0;
    k = l;
    while (i < n1 && j < n2) {
        if (L[i] <= R[j]) {
            arr[k] = L[i];
            i++;
        }
        else {
            arr[k] = R[j];
            j++;
        }
        k++;
    }
    while (i < n1) {
        arr[k] = L[i];
        i++;
        k++;
    }
    while (j < n2) {
        arr[k] = R[j];
        j++;
        k++;
    }
}

int mergeSort(int arr[], int l, int r){
    if (l < r) {
        int m = l + (r - l) / 2;
        mergeSort(arr, l, m);
        mergeSort(arr, m + 1, r);
        merge(arr, l, m, r);


    }
return sumMerge;
}

for MergeSort. Is this correct? I mean, if I put sumMerge++ somewhere in the code, will this be ok? Also, what part of code actually swaps the values in merge? In quicksort I figured it was kind of obvious, wherever swap gets called, that's where I increase the counter. But what about merge?