Why use a prime number as the hash table size

A quick answer

Use a prime number as the hash table size will on average give you the best distribution of hash values to hash buckets [1].

An example

For the hash table, usually, we have more keys than the hash table size. We’d like to have a quick comparison by using this simple example

keys(even numbers)  : 2, 4, 6, 8, 10, 12, 14, 16, 18, 20
hashtable size = 10 : 2, 4, 6, 8, 0,   2,  4,  6,  8,  0
hashtable size = 11 : 2, 4, 6, 8, 10,  1,  3,  5,  7,  9

Detail explanations

See[1] and [2]

Reference

[1]https://www.quora.com/Why-should-the-size-of-a-hash-table-be-a-prime-number

[2]https://stackoverflow.com/questions/3980117/hash-table-why-size-should-be-prime