C++ primer plus里的一个程序

#include<iostream>
#include<vector>
#include<array>//visual studio 2005不支持array
int main()
{
 using namespace std;
 double a1[4]={1.2,2.4,3.6,4.8};
 vector<double>a2(4);
 a2[0]=1.0/3.0;
 a2[1]=1.0/5.0;
 a2[2]=1.0/7.0;
 a2[3]=1.0/9.0;
 array<double,4> a3={3.74,2.72,1.62,1.41};
 array<double,4> a4;
 a4=a3;
 cout<<"a1[2]: "<<a1[2]<<"  at "<<&a1[2]<<endl;
 cout<<"a2[2]: "<<a2[2]<<"  at "<<&a2[2]<<endl;
 cout<<"a3[2]: "<<a3[2]<<"  at "<<&a3[2]<<endl;
 cout<<"a4[2]: "<<a4[2]<<"  at "<<&a4[2]<<endl;
 a1[-2]=20.2;//   a1[-2]=*(a1-2);这个之前很少注意到
 cout<<"a1[-2]: "<<a1[-2]<<"at "<<&a1[-2]<<endl;
 cout<<"a3[2]:  "<<a3[2]<<" at "<<&a3[2]<<endl;
 cout<<"a4[2]:  "<<a4[2]<<" at "<<&a4[2]<<endl;
 return 0;
 
}