如何在不更改元素顺序的情况下合并两个unordered_maps？

Question

I need to merge two unordered_map s without changing the order. For example,

unordered_map<int,int> map1 ,map2, map3;

map1 contains : <4,4> <2,2>

map2 contains : <3,3> <1,1>

map1 and map2 to be merged with map3.

so my map3 should contain <4,4><2,2><3,3><1,1>

map<int,int>::iterator it   = map3.begin();
std::merge(map1.begin(),map1.end(),map2.begin(),map2.end(),inserter(map3,it));

Still map3 order is changing. I have tried with std::merge and insert, but nothing worked as per the above req. Can someone help me on this. or am i doing some mistake on merge and insert?

Answer 1

std::unordered_map doesn't guarantee any kind of order, while std::map is always in sorted order by key (though you can specify your own comparison function). It looks like you want items in their order of insertion. In this case you can just push your data to a std::vector , although you will have to give up the sublinear operations that the mapped types provide.

Answer 2

There order of map elements dependent of its keys and balanced tree algorithm but not of source map elements order. If you want to save order use std::vector<std::pair<inr,int>> instead.

Answer 3

std::merge works assuming two already sorted ranges (in your case two unsorted maps) achieving linear time performance. Due to its implementation, using unsorted maps will not give the required result.