C++ primer plus第六版课后编程练习答案：7.4

#define  _CRT_SECURE_NO_WARNINGS 
#include <iostream>

long double probability(unsigned numbers, unsigned picks);


int main()
{
    using namespace std;
    double total1, choices1, total2, choices2;
    cout << "Enter the total number of choices on the game card and the number of "
        <<"picks allowed in section 1:\n";
    while ((cin >> total1 >> choices1) && choices1 <= total1)
    {
        long double probability1 = probability(total1, choices1);
        cout << "You have one chance in " << probability1 << " of winning in this section.\n";
        long double probability2;
        if ((cin >> total2 >> choices2) && choices2 <= total2)
        {
            probability2 = probability(total2, choices2);
            cout << "You have one chance in " << probability2 << " of winning in this section.\n";
        }
        else break;
        cout << "So you have one chance in " << probability1*probability2 << " of winning.\n";
        cout << "Next group(q to quit):";
    }
    cout << "Bye~!\n";
    return 0;
}

long double probability(unsigned numbers, unsigned picks)
{
    long double results = 1.0;
    long double n;
    unsigned p;
    for (n = numbers, p = picks; p > 0; n--, p--)
        results = results*n / p;
    return results;
}