Hash table: When is it suitable to use modulo power of 2?

Question

I am implementing a hash table that may need to grow and shrink as the number of stored keys varies. I have a hashcode function that uniformly hashes keys to positive 32-bit integers. The table itself will use a smaller array of approximate size M.

If I wanted to use the hash function to produce a value between 0 and P-1 (if P is close to M), can I use mod P if P is a power of 2?

What about using mod P, where P is a prime?

I don't understand your question. Why are you reducing your hash value modulo anything other than the size of the table? If you do that, the top elements of the table ($P\dots M-1$) will never be used. — David Richerby, Sep 22 '17 at 11:22
Possible duplicate of Why is it best to use a prime number as a mod in a hashing function? — Vladimir Panteleev, Sep 22 '17 at 12:11

score 2 · Answer 1 · answered Sep 22 '17 at 11:45

2

The exact size of the backing array is not that important only that it can hold enough buckets.

The reason why power of 2 is often used is because this can be implemented as a bitmask instead of a integer divide. Integer divides are pretty slow compared to other operation so they are often avoided by people trying to optimize data structures.

answered Sep 22 '17 at 11:45

ratchet freak

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Alright. So would power of 2 be appropriate for the hash table described above? – TheSaviour Sep 22 '17 at 14:46

Hash table: When is it suitable to use modulo power of 2?

1 Answers1