c++谭浩强（第四版），课后习题7-2，输入年月日判断是该年的第几天

#include<iostream>

using namespace std;

struct Date

{

       int year ;

       int month ;

       int day ;

 } ;

 int main()

 {

      Date date ;

      int a, b, c, sum=0 ;

      cout << "year: " ;

      cin >> date.year ;

      cout << endl << "month: " ;

      cin >> date.month ;

      cout << endl << "day: " ;

      cin >> date.day ;

      cout<<endl ;

      a=date.year ;

      b=date.month ;

      c=date.day ;

      if(b<3)

             sum =(b-1)*31+c ;

      else{

             for(int i=3; i<=b; i++){

                    switch(i){

                           case 3: sum=59 ; break ;

                           case 4: sum+=31 ; break ;

                           case 5: sum+=30 ; break ;

                           case 6: sum+=31 ; break ;

                           case 7: sum+=30 ; break ;

                           case 8: sum+=31 ; break ;

                           case 9: sum+=31 ; break ;

                           case 10: sum+=30 ; break ;

                           case 11: sum+=31 ; break ;

                           case 12: sum+=30 ; break ;

                           default: break ;     

                     }

              }

              sum+=c ;

              if(a%400==0 || (a%4==0&&a%100!=0))

                     sum+=1 ;     

        }

        cout<<"it's the "<<sum<<"th day" ;

        return 0 ;

 }