select函数如下：for循环中，为什么是i<(r-p-4)/5，而不是i<(r-p)/5  ????

求教各位一下。

int select(int p,int r,int k)   //寻找中位数

       {

              if(r-p<5){

                     bubble(p,r);

                     return a[p+k-1];

              }

              for(int i=0;i<(r-p-4)/5;i++)

              {

                     int s=p+5*i,t=s+4;

                     bubble(s,t);

                     int temp=a[p+i];

                     a[p+i]=a[s+2];

                     a[s+2]=temp;

              }

              int x=select(p,p+(r-p-4)/5,(r-p+6)/10);

              i=partition(p,r,x);

              int j=i-p+1;

              if(k<=j)

                     return select(p,i,k);

              else

                     return select(i+1,r,k-j);

       }