分数背包问题(贪心算法)

#include<iostream>
#include<iterator>
#include<vector>
#include<algorithm>
using namespace std;

/*
*分数背包问题(贪心算法)
*/

struct goods
{
	double  value;//物品的价值
	double  weight;//物品的重量
	double  ratio;//物品的性价比
	double in;//物品装入背包的重量
	int index;//物品的编号
	friend ostream & operator<<(ostream & o,const goods & g);
	friend istream & operator>>(istream & o,goods & g);
};

ostream & operator<<(ostream & o,const goods & g)
{
	o<<"物品编号:  "<<g.index<<" 物品装入背包的重量 "<<g.in<<endl;
	return o;
}
istream & operator>>(istream & o,goods & g)
{
	o>>g.index>>g.weight>>g.value;
	g.ratio = g.value/g.weight;
	return o;
}

bool cmp(goods g1,goods g2)
{
	if(g1.ratio>g2.ratio)
	{
	  return true;
	}else
	{
		return false;
	}
}

double knapsack(vector<goods>  & vec,double v)
{
	//先将货物按照性价比排序
	sort(vec.begin(),vec.end(),cmp);
	//从性价比高的开始装入背包
	double l = v;//背包的剩余容量
	double sum = 0;//背包的总价值
	for (int i = 0; i < vec.size(); i++)
	{
		if(l>0)
		{
			if(l>vec[i].weight)
			{
				l -= vec[i].weight;
				vec[i].in = vec[i].weight;
				sum += vec[i].value;
			}
			else
			{
				vec[i].in = vec[i].weight - l;
				sum += l*vec[i].ratio;
				l =0;
				break;
			}

		}
	}
	return sum;
}

int main()
{
	vector<goods> vec_goods;//可选物品向量
	//输入可选物品
	copy(istream_iterator<goods>(cin),istream_iterator<goods>(),back_inserter(vec_goods));
	double wei_bag;//背包的承载最大重量
	cin.clear();
	cin.sync();
	cin>>wei_bag;
	double max_v = knapsack(vec_goods,wei_bag);
	cout<<"背包可以装载的最大价值为：  "<<max_v<<endl;
	cout<<"分别承载的物品和重量为"<<endl;
	copy(vec_goods.begin(),vec_goods.end(),ostream_iterator<goods>(cout,"  "));
	cout<<endl;
}