Question

What is the most efficient way of obtaining lists (as a vector) of the keys and values from an unordered_map?

For concreteness, suppose the map in question is a unordered_map<string, double>.
I’d then like to obtain the keys as a vector<string>, and the values as a vector<double>.

unordered_map<string, double> um;

vector<string> vs = um.enum_keys();
vector<double> vd = um.enum_values(); 

I can just iterate across the map and collect the result, but is there a more
efficient method? It would be nice to have a method that also works for regular map,
since I might switch to that.

Answer 1

Okay, here you go:

std::vector<Key> keys;
keys.reserve(map.size());
std::vector<Val> vals;
vals.reserve(map.size());

for(auto kv : map) {
    keys.push_back(kv.first);
    vals.push_back(kv.second);  
} 

Efficiency can probably be improved, but there it is. You’re operating on two containers though, so there’s not really any STL magic that can hide that fact.

As Louis said, this will work for any of the STL map or set containers.