# Why hash tables should use a prime-number size

I read in several books and online pages that hash tables should use a prime number for the size. Nobody really justified this statement properly. Here’s my attempt!

I believe that it just has to do with the fact that computers work with in base 2. Just think at how the same thing works for base 10:

• 8 % 10 = 8
• 18 % 10 = 8
• 87865378 % 10 = 8
• 2387762348 % 10 = 8

It doesn’t matter what the number is: as long as it ends with 8, its modulo 10 will be 8. You could pick a huge power of 10 as modulo operator, such as 10^k (with k > 10, let’s say), but

1. you would need a huge table to store the values
2. the hash function is still pretty stupid: it just trims the number retaining only the first k digits starting from the right.

However, if you pick a different number as modulo operator, such as 12, then things are different:

• 8 % 12 = 8
• 18 % 12 = 6
• 87865378 % 12 = 10
• 2387762348 % 12 = 8

We still have a collision, but the pattern becomes more complicated, and the collision is just due to the fact that 12 is still a small number.

Picking a big enough, non-power-of-two number will make sure the hash function really is a function of all the input bits, rather than a subset of them.

For example, with 367:

• 8 % 367 = 8
• 18 % 367 = 18
• 87865378 % 367 = 73
• 2387762348 % 367 = 240

What is worth nothing is that there may be a pattern even with modulo 367, but it would be way less trivial than with modulo 10 (or with modulo 2 in binary). We don’t really need a prime number, just having a big non-power of two is enough. Having a prime number, obviously, is just a guaranteed way of satisfying those conditions.