Selecting a random element for a specific key in a Multimap

问题

How can I select a random element for a specific keys in a multimap. For example:

multimap<string, string> map;
map.insert(pair<string, string>("Mammal", "Tiger"));
map.insert(pair<string, string>("Mammal", "Chicken"));
map.insert(pair<string, string>("Mammal", "Fox"));
map.insert(pair<string, string>("Fish", "Clown Fish"));
map.insert(pair<string, string>("Fish", "Ray"));

In the above, what would be the best way to get a random "Mammal"?

I know I can get the iterators for the "Mammal" so:

pair<MultiMapIt,MultiMapIt>iterators = mMultiMap.equal_range("Mammal");
// loop through each... and select one.

But I am sure there is a better solution... perhaps using the iterator as numbers..
Thanks

回答1:

Inserting comments as answer:

1. Get the iterator range - you have this already

2. Calculate the size of the range

   std::size_t sz = std::distance(iterators.first, iterators.second);

3. Now generate a random index:

   std::size_t idx = std::rand() % sz; // stupid example

4. Move the iterator to that position

   std::advance(iterators.first, sz);

Now iterators.first is pointing at a random mammal.

来源：https://stackoverflow.com/questions/11431631/selecting-a-random-element-for-a-specific-key-in-a-multimap