Select random element in an unordered_map

Question

I define an unordered_map like this:

std::unordered_map edges;

Is there a efficient way to choose a rando

Answer 1

Is there a efficient way to choose a random Edge from the unordered_map edges ?

If by efficient you mean O(1), then no, it is not possible.

Since the iterators returned by unordered_map::begin / end are ForwardIterators, the approaches that simply use std::advance are O(n) in the number of elements.

If your specific use allows it, you can trade some randomness for efficiency:

You can select a random bucket (that can be accessed in O(1)), and then a random element inside that bucket.

int bucket, bucket_size;
do
{ 
    bucket = rnd(edges.bucket_count());
}
while ( (bucket_size = edges.bucket_size(bucket)) == 0 );

auto element = std::next(edges.begin(bucket), rnd(bucket_size));

Where rnd(n) returns a random number in the [0,n) range.

In practice if you have a decent hash most of the buckets will contain exactly one element, otherwise this function will slightly privilege the elements that are alone in their buckets.