Moving from Linear Probing to Quadratic Probing (hash collisons)

Question

My current implementation of an Hash Table is using Linear Probing and now I want to move to Quadratic Probing (and later to chaining and maybe double hashing too). I\'ve re

Answer 1

You don't have to modify the hash function for quadratic probing. The simplest form of quadratic probing is really just adding consequent squares to the calculated position instead of linear 1, 2, 3.

There's a good resource here. The following is taken from there. This is the simplest form of quadratic probing when the simple polynomial c(i) = i^2 is used:

In the more general case the formula is:

And you can pick your constants.

Keep, in mind, however, that quadratic probing is useful only in certain cases. As the Wikipedia entry states:

Quadratic probing provides good memory caching because it preserves some locality of reference; however, linear probing has greater locality and, thus, better cache performance. Quadratic probing better avoids the clustering problem that can occur with linear probing, although it is not immune.

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EDIT: Like many things in computer science, the exact constants and polynomials of quadratic probing are heuristic. Yes, the simplest form is i^2, but you may choose any other polynomial. Wikipedia gives the example with h(k,i) = (h(k) + i + i^2)(mod m).

Therefore, it is difficult to answer your "why" question. The only "why" here is why do you need quadratic probing at all? Having problems with other forms of probing and getting a clustered table? Or is it just a homework assignment, or self-learning?

Keep in mind that by far the most common collision resolution technique for hash tables is either chaining or linear probing. Quadratic probing is a heuristic option available for special cases, and unless you know what you're doing very well, I wouldn't recommend using it.