Random element in a map

Question

what is a good way to select a random element from a map? C++. It is my understanding that maps don\'t have random access iterators. The key is a long long and the map is

Answer 1

I like James' answer if the map is small or if you don't need a random value very often. If it is large and you do this often enough to make speed important you might be able to keep a separate vector of key values to select a random value from.

map<...> MyMap;
vector<...> MyVecOfKeys; // <-- add keys to this when added to the map.

map<...>::key_type key = MyVecOfKeys[ random_0_to_n(MyVecOfKeys.size()) ];
map<...>::data_type value = MyMap[ key ];

Of course if the map is really huge you might not be able to store a copy of all the keys like this. If you can afford it though you get the advantage of lookups in logarithmic time.