Why are hash table expansions usually done by doubling the size?

Question

I\'ve done a little research on hash tables, and I keep running across the rule of thumb that when there are a certain number of entries (either max or via a load factor lik

Answer 1

Doubling the memory when expanding any type of collection is an oftenly used strategy to prevent memory fragmentation and not having to reallocate too often. As you point out there might be reasons to have a prime number of elements. When knowing your application and your data, you might also be able to predict the growth of the number of elements and thus choose another (larger or smaller) growth factor than doubling.

The general implementations found in libraries are exactly that: General implementations. They have to focus on being a reasonable choice in a variety of different situations. When knowing the context, it is almost always possible to write a more specialized and more efficient implementation.