#include<iostream>

using std::cout;
using std::cin;
using std::endl;

int main()
{
    int total = 0;
    int counter;
    double average;
    int a=0;      //a代表输入值
   
    cout << "Enter(9999 to quit):"<<endl;
     //此行为输入一个值，进行判断
    cin >> a;
    for ( counter = 1; a != 9999; ++counter )
    {
        
               
        total += a;
      
        cin >> a;
        //average = static_cast< double >( total )/counter;
    }
      average = static_cast< double >( total )/(counter-1);
    cout << " The average is:" << average << endl;
   
    system("pause");
    return 0;
}  

了解！