如何应用std :: accumulate算法的联想容器？ [英] How to apply std::accumulate algorithm for associative containers?

问题描述

对于像std :: map的地图，我如何积累它的值的总和？

其实，我做了一个函数和std :: for_each算法。但我也想使用std :: accumulate算法。

我不知道如何应用它到std :: map。

这是可能吗？ / p>

For a map like std::map, how do I accumulate it's values' sum?
Actually, I made it with a functor and std::for_each algorithm. But I'd also like to make this using std::accumulate algorithm.
I have no idea how to apply it to std::map.
Is this even possible?

struct Accumurator
    : std::unary_function<std::pair<int, int>, void>
{
    Accumurator()
        : totalValue_(0)
    {
    } 

    void operator()(const std::pair<int, int>& p)
    {
        totalValue_ += p.second;
    }

    int result() const
    {
        return totalValue_;
    }

    int totalValue_; 
};

int _tmain(int argc, _TCHAR* argv[])
{
    std::map<int, int> m;
    m.insert(make_pair(1, 10));
    m.insert(make_pair(2, 10));
    m.insert(make_pair(3, 10));
    m.insert(make_pair(4, 10));
    m.insert(make_pair(5, 10));
    m.insert(make_pair(6, 10));

    int totalSum = std::for_each(m.begin(), m.end(), Accumurator()).result();

    // How can I apply accumulate algorithm for associative containers.
    // int totalSum = accumulate(m.begin(), m.end(), ???);

    return 0;
}

推荐答案

函数必须是一个二进制运算符，它将返回值类型作为第一个参数，并将范围类型作为第二个参数：

Almost. The functor must be a binary operator taking the return value type as the first and the range type as the second argument:

x = Functor(init, *it++);
x = Functor(x, *it++);
x = Functor(x, *it++);
// ... until it == end

有状态函数，一个简单的函数会做：

So you don't need a stateful functor at all, a simple function will do:

int map_acc(int lhs, const std::pair<int, int> & rhs)
{
  return lhs + rhs.second;
}

const int sum = std::accumulate(m.begin(), m.end(), 0, map_acc);

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