杭电1009 FatMouse' Trade

Problem Description
FatMouse prepared M pounds of cat food, ready to trade with the cats guarding the warehouse containing his favorite food, JavaBean.
The warehouse has N rooms. The i-th room contains J[i] pounds of JavaBeans and requires F[i] pounds of cat food. FatMouse does not have to trade for all the JavaBeans in the room, instead, he may get J[i]* a% pounds of JavaBeans if he pays F[i]* a% pounds of cat food. Here a is a real number. Now he is assigning this homework to you: tell him the maximum amount of JavaBeans he can obtain.

Input
The input consists of multiple test cases. Each test case begins with a line containing two non-negative integers M and N. Then N lines follow, each contains two non-negative integers J[i] and F[i] respectively. The last test case is followed by two -1's. All integers are not greater than 1000.

Output
For each test case, print in a single line a real number accurate up to 3 decimal places, which is the maximum amount of JavaBeans that FatMouse can obtain.

Sample Input
5 3
7 2
4 3
5 2
20 3
25 18
24 15
15 10
-1 -1

Sample Output
13.333
31.500

#include<iostream>
#include<vector>
#include<algorithm>//sort()
#include<iomanip>//格式控制
using namespace std;

class Cost
{
public:
    int JavaBeans;
    int cat_food;
    double rate;
    Cost()
    {
        JavaBeans = 0;
        cat_food = 0;
        rate = 0.0;
    }
    friend istream& operator>>(istream& , Cost&);
};

inline istream& operator>>(istream& in, Cost& x)
    {
        //操作符重载
        in >> x.JavaBeans >> x.cat_food;
        if(in)
            x.rate = 1.0*x.JavaBeans/x.cat_food;
        return in;
    }

bool compare(Cost a, Cost b)
{
    return a.rate > b.rate;
}

double trade(vector<Cost> vec, int m)
{
    double max = 0;
    for(int i = 0; i != vec.size(); ++i)
        if(m >= vec[i].cat_food)
        {
            max += vec[i].JavaBeans;
            m -= vec[i].cat_food;
        }
        else
        {
            max += vec[i].JavaBeans*(1.0*m/vec[i].cat_food);
            break;
        }
    return max;
}
int main()
{
    int m, n;
    while(cin >> m >> n, m != -1&& n != -1)
    {
        vector<Cost> vec;
        Cost x;
        while(n--)
        {
            cin >> x;
            vec.push_back(x);
        }
        sort(vec.begin(), vec.end(), compare);
        cout.precision(3);
        cout.setf(ios::fixed);
        cout << trade(vec, m) << endl;
    }
}