void quicksort(float tab[], int left, int right){
   int pivot=(left+right)/2;
   if(left>=right)return;
   int p=left;
   int q=right;
   do{
         while(p<=right && tab[p] < pivot) p++;
         while(q>=left && tab[q] > pivot) q--;
         if(p<= q){
         		float z=tab[p];
                 tab[p]=tab[q];
                 tab[q]=z;
                 p++;
                 q--;
         }
   }while(p<=q);
   quicksort(tab, left, q);
   quicksort(tab, p, right);
}