如何通过价值排序**的boost :: unordered_map **，并按此顺序仅返回键？ [英] How to sort **boost::unordered_map** by value and return only keys in that order?

问题描述

如何排序的的boost :: unordered_map 按价值计算，该订单只返回键？
我已经像地图的boost :: unordered_map ，我需要，我需要在递增/递减INT值进行排序枚举ONY列表。

How to sort boost::unordered_map by value and return only keys in that order ? I have map like boost::unordered_map and I need I need ony list of enums sorted by int values in asc/desc.

推荐答案

这是 unordered_map 时，顾名思义，本身没有排序或排序就地。你可以插入值对成设置（使用Boost.Range使这种东西更容易），它的价值排序，并得到从那里的钥匙。我用的是的std ::设为＆LT; T *＆GT; 不支付复制对对象的成本

An unordered_map is, as the name implies, inherently not sorted or sortable in-place. You could insert the value pairs into a set that is sorted on the value and get the keys from there (using Boost.Range to make this stuff easier). I use a std::set<T*> to not pay the cost of copying the pair objects.

#include <iostream>
#include <set>
#include <unordered_map>
#include <boost/range/adaptor/map.hpp>
#include <boost/range/adaptor/indirected.hpp>
#include <boost/range/adaptor/transformed.hpp>

struct compare_second{
  template<class Pair>
  bool operator()(Pair* lhs, Pair* rhs) const{
    return lhs->second < rhs->second;
  }
};

template<class T>
struct make_pointer{
  typedef T* result_type;
  T* operator()(T& v) const{ return &v; }
};

int main(){
  using namespace boost::adaptors;
  std::unordered_map<int, int> m{{0,4},{1,3},{2,2},{3,1},{4,0}};
  typedef std::unordered_map<int,int>::value_type pair_type;
  auto p = m | transformed(make_pointer<pair_type>());
  std::set<pair_type*, compare_second> value_ordered(p.begin(), p.end());
  for(auto x : value_ordered | indirected | map_keys)
    std::cout << x << " ";
}

活生生的例子。

这篇关于如何通过价值排序**的boost :: unordered_map **，并按此顺序仅返回键？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IT屋！