问题描述

我需要按值而不是键对 std::map 进行排序.有什么简单的方法吗?

I need to sort an std::map by value rather than by key. Is there an easy way to do it?

我从以下线程中得到了一个解决方案:
std::map 按数据排序?
有更好的解决方案吗?

I got one solution from the follwing thread:
std::map sort by data?
Is there a better solution?

map<long, double> testMap;
// some code to generate the values in the map.

sort(testMap.begin(), testMap.end());  // is there any function like this to sort the map?

推荐答案

即使正确答案已经发布，我还是想添加一个演示，说明如何干净利落地做到这一点:

Even though correct answers have already been posted, I thought I'd add a demo of how you can do this cleanly:

template<typename A, typename B>
std::pair<B,A> flip_pair(const std::pair<A,B> &p)
{
    return std::pair<B,A>(p.second, p.first);
}

template<typename A, typename B>
std::multimap<B,A> flip_map(const std::map<A,B> &src)
{
    std::multimap<B,A> dst;
    std::transform(src.begin(), src.end(), std::inserter(dst, dst.begin()), 
                   flip_pair<A,B>);
    return dst;
}

int main(void)
{
    std::map<int, double> src;

    ...    

    std::multimap<double, int> dst = flip_map(src);
    // dst is now sorted by what used to be the value in src!
}

<小时>

通用关联源(需要 C++11)

如果您为源关联容器使用 std::map 的替代品(例如 std::unordered_map)，您可以编写一个单独的重载，但最终动作还是一样的，所以使用可变参数模板的广义关联容器可以用于任一映射构造:

If you're using an alternate to std::map for the source associative container (such as std::unordered_map), you could code a separate overload, but in the end the action is still the same, so a generalized associative container using variadic templates can be used for either mapping construct:

// flips an associative container of A,B pairs to B,A pairs
template<typename A, typename B, template<class,class,class...> class M, class... Args>
std::multimap<B,A> flip_map(const M<A,B,Args...> &src)
{
    std::multimap<B,A> dst;
    std::transform(src.begin(), src.end(),
                   std::inserter(dst, dst.begin()),
                   flip_pair<A,B>);
    return dst;
}

这对于作为翻转源的 std::map 和 std::unordered_map 都有效.

This will work for both std::map and std::unordered_map as the source of the flip.

这篇关于使用值对 std::map 进行排序的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！