Combining two unordered_maps with overlapping keys

Question

I would like to combine the two std::unordered_map : mapA and mapB , while maintaining priority for the elements from mapA if both maps contain the same key.

Is there an elegant way to do this (and not check every key .. my cards contain a large number of elements)?

Example:

 mapA = {{"sugar",0.1},{"salt",0.2}} mapB = {{"sugar",0.3},{"pepper",0.4}} 

As a result, I would like to:

 result = {{"sugar",0.1},{"salt",0.2},{"pepper",0.4}} 

Ignoring the key value {"sugar",0.3} from mapB

Thanks.

Answer 1

Absolutely:

 auto result = mapA; result.insert(mapB.begin(), mapB.end()); 

The member functions of insert unordered_map do nothing if the key already exists in the container.

Demo version

Answer 2

Since std::unordered_map is unsorted, there is no other way. You will need to mapB over each element in mapB and try to insert it into mapA . Existing elements in mapA will not be inserted.