Question

I’ve been reading a lot about Hash Tables and how to implement on in C and I think I have almost all the concepts in my head so I can start to code my own, I just have a couple of questions that I have yet to properly understand.

As a reference, I’ve been reading this:
http://eternallyconfuzzled.com/jsw_home.aspx

1) As I’ve read on the site above, a power of two or a prime number is recommended for the Hash Table size. This is basically an array and an array has a fixed size so I can quickly look up for the value I’m looking for. I can’t declare a small array if I have a large input as it won’t fit and I can’t declare a very large array if my input data is not that large cause it’s wasted memory.

What is the optimum size for the Hash Table? What should I base my decision on?

2) Also, on that site, there’s a couple of hashing functions which I have yet to read them all. It also states that it’s always best to use a good known algorithm and to roll my own. And I might do just that, I’ll pick one from that site and test it out on my code and see if it minimizes collisions based on my input data.

What’s bugging me is how I control the hash range? The hash can’t return and integer larger than the Hash Table size or we’ll have a serious problem. How do I deal with this?

Answer 1

1) What you are referring to is the load factor of the hash table – the percentage of buckets that are expected to be filled. Wikipedia has this to say:

With a good hash function, the average
lookup cost is nearly constant as the
load factor increases from 0 up to 0.7
or so. Beyond that point, the
probability of collisions and the cost
of handling them increases.

I believe the Java implementation (and probably others) resizes periodically to keep the load factor within an acceptable range.

2) Just use the modulo operator (%) to keep the bucket index legal. The second operator should be the size of your bucket array.