c - Strange things happen when swapping two vars in QuickSort -

i'm implementing quicksort in c.

here's swap procedure:

void swap(int *x, int *y) {     *x += (*y);     *y = (*x) - (*y);     *x = (*x) - (*y); } 

and here's partition procedure:

int partition(int a[], int sx, int dx) {     int indice_pivot = (rand()%(dx-sx+1))+sx;     int = sx-1, j;      swap(&a[indice_pivot],&a[dx]);       for(j=sx;j<dx;j++)     {         if(a[j] <= a[dx])         {             i++;             swap(&a[j],&a[i]);         }     }      i++;     swap(&a[i],&a[dx]);      return i; } 

the problem when swapping 2 variables, magically (?) become 0. did debugging , seems working fine inside swap procedure. array contains zeros @ end of partitions (not of them). strange thing if replace swap procedure

void swap(int *x, int *y) {     int temp = *y;     *y = *x;     *x = temp; } 

everything works fine. why?

your swap function not work if both pointers point same element. if second step *y = (*x) - (*y); sets element 0, since equivalent *x = (*x) - (*x);

the second swap function temp variable preserves values.

at glance looks swap(&a[indice_pivot],&a[dx]); hitting same element. can use assert( indice_pivot != dx ) determine that( or of course put 1 in swap function ).