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

Continuing ryan_s theme of preconstructed maps and fast random lookup: instead of vector we can use a parallel map of iterators, which should speed up random lookup a bit.

map const original;
...

// construct index-keyed lookup map   
map::const_iterator> fast_random_lookup;
map::const_iterator it = original.begin(), itEnd = original.end();
for (unsigned i = 0; it != itEnd; ++it, ++i) {
  fast_random_lookup[i] = it;
}

// lookup random value
V v = *fast_random_lookup[random_0_to_n(original.size())];