循环不变式

        假设a是浮点数，n是正整数，计算a的n次方。

                      double r=1;

                      double b=a;

               int i=n;

             while(i>0)

             {

                  if(i%2==0)

                  {

                       b=b*b;

                       i=i/2;

                   }

                   else

                   {

                            r=r*b;

                             i--;

                    }

             }