下面这段程序是我copy来的  我有些疑问 就是 如果不先选择运算方法（即下面的1，2，3，4，5，0）。而是直接输入数字，运算符，数字的程序该怎么做  只求一点思路  不求结果



#include <iostream.h>
#include <stdlib.h>
double cf(double x,double n)
{
    double a=1;
    for(int i=1;i<=n;i++)
        a*=x;
        return a;
}

void main()
{
  double a,b,c;                                 
  int hd;                                    
  do
  {
     
     cout<<"\t\t请选择您需要的操作："<<endl;
     cout<<"\t**********************************"<<endl;
     cout<<"\t1.执行加运算"<<endl;
     cout<<"\t2.执行减运算"<<endl;
     cout<<"\t3.执行乘运算"<<endl;
     cout<<"\t4.执行除运算"<<endl;
     cout<<"\t5.执行a的b次方运算"<<endl;
     cout<<"\t0.退出程序"<<endl<<endl;
     cout<<"\t请选择序号:";
     cin>>hd;
     
     switch(hd)
     {
         case 1:
              cout<<"\t请输入数据(以空格分开)：";
              cin>>a;cin>>b;            
              c=a+b;                                      
              cout<<"\t运算的结果为 "<<c<<endl;        
             break;
         case 2:
              cout<<"\t请输入数据(以空格分开)：";
              cin>>a>>b;            
              c=a-b;
              cout<<"\t运算的结果为 "<<c<<endl;         
             break;
         case 3:
              cout<<"\t请输入数据(以空格分开)：";
              cin>>a>>b;     
              c=a*b;
              cout<<"\t运算的结果为 "<<c<<endl;         
             break;
         case 4:
              cout<<"\t请输入数据(以空格分开)：";
              cin>>a>>b;
              if(b==0||a==0)
              {
                  cout<<"\t被除数或除数等于0,计算没有意义,请重新输入!"<<endl;
              }
              else
              {
                c=a/b;
                cout<<"\t运算的结果为 "<<c<<endl;
              }     
             break;
         case 5:
              cout<<"\t请输入数据(以空格分开)：";
              cin>>a>>b;     
              c=cf(a,b);
              cout<<"\t运算的结果为 "<<c<<endl;
             break;
         case 0:
             break;
         default:
                cout<<"\t选择错误！请重新选择！"<<endl;
                break;
     }
            system("pause");
            system("cls");
  }while(1);
}