Merge two maps, summing values for same keys in C++

Question

I have two std::map maps and wish to merge them into a third map like this: if the same key is found in both maps, create a pair in the third map wi

Answer 1

I don't think it will be easy (if not impossible) to find a suitable std::algorithm that serves the purpose.

The easiest way would be to first make a copy of map1 to map_result.

Then iterate through map2 and see if any key already exists in map_result then add the values, else add the key_value pair to map_result.

std::map map_result( map1 );

for (auto it=map2.begin(); it!=map2.end(); ++it) {
  if ( map_result[it->first] )
    map_result[it->first] += it->second;
  else
    map_result[it->first] = it->second;
}