购书优惠问题

#include<iostream>
using namespace std;
double num1(int a){
   double sum1;
   switch(a%5)
       {
       case 1:
           sum1=5*8*0.75+8;break;
       case 2:
           sum1=5*8*0.75+2*8*0.95;break;
       case 3:
           sum1=4*8*0.8+4*8*0.8;break;
       case 4:
           sum1=5*8*0.75+4*8*0.8;break;  
       }
return sum1;
}
 
int main(int argc, char* argv[])
{
    int num;
    double sum;
    cout<<"请输入您要购买的本数:";
    cin>>num;
    if(num<=5)
    {
       switch(num)
       {
       case 1:
           sum=8;break;
       case 2:
           sum=2*8*0.95;break;
       case 3:
           sum=3*8*0.9;break;
       case 4:
           sum=4*8*0.8;break;
       case 5:
           sum=5*8*0.75;break;
       }
    }
    if((num>5)&&(num<10))
    {
        sum=num1(num);
    }
    if(num>9)
    {
        switch(num%5)
       {
        case 0:
            sum=(num/5)*8*5*0.75;break;
       case 1:
           sum=(num/5-1)*5*8*0.75+num1(6);break;
       case 2:
           sum=(num/5-1)*8*5*0.75+num1(7);break;
       case 3:
           sum=(num/5-1)*8*5*0.75+num1(8);break;
       case 4:
           sum=(num/5-1)*8*5*0.75+num1(9);break;
       }
    }
 
   cout<<"最大折扣价为："<<sum;
   cout<<endl;
   return 0;
 
}