Optimizing Lookups: Dictionary key lookups vs. Array index lookups

Question

I\'m writing a 7 card poker hand evaluator as one of my pet projects. While trying to optimize its speed (I like the challenge), I was shocked to find that the performance o

Answer 1

Don't forget that Big-O notations only says how the complexity grows with respect to the size (etc) - it doesn't give any indication of the constant factors involved. That's why sometimes even a linear search for keys is faster than a dictionary lookup, when there are sufficiently few keys. In this case you're not even doing a search with the array though - just a straight indexing operation.

For straight index lookups, arrays are basically ideal - it's just a case of

pointer_into_array = base_pointer + offset * size

(And then a pointer dereference.)

Performing a dictionary lookup is relatively complicated - very fast compared with (say) a linear lookup by key when there are lots of keys, but much more complicated than a straight array lookup. It has to calculate the hash of the key, then work out which bucket that should be in, possibly deal with duplicate hashes (or duplicate buckets) and then check for equality.

As always, choose the right data structure for the job - and if you really can get away with just indexing into an array (or List<T>) then yes, that will be blindingly fast.