C++ primer 5th 7章4题

#include <iostream>
#include <cmath>
long double gailv(unsigned pick,unsigned num1,unsigned num2);
int main()
{
 using namespace std;
 unsigned pick,num1,num2;
 cout << "你打算从几个域号码中选择几个，及特选号码的数量：";
 while ((cin >> num1 >> pick >> num2) && pick<num1)
 {
  cout <<"你中头奖的概率是：" << gailv(pick,num1,num2)<<endl;
  cout << "再输入3个数字（按字母键退出）:";
 }
 cout <<"再见！/n";
 return 0;
}
long double gailv(unsigned pick,unsigned num1,unsigned num2)
{
 long double t=1.0;
 unsigned n,p;
 for (n=num1,p=pick;p>0;n--,p--)
        t=t*p/n;
 t=t/num2;
 return t;
}