Hash tables are O(1) average and amortized case complexity, however it suffers from O(n) worst case time complexity. [And I think this is where your confusion is] Hash tables suffer from O(n) worst time complexity due to two reasons: If too many elements were hashed into the same key: looking inside this key may take O(n) time. Once a hash table has passed its load balance – it has … Read more
The use of “closed” vs. “open” reflects whether or not we are locked in to using a certain position or data structure (this is an extremely vague description, but hopefully the rest helps). For instance, the “open” in “open addressing” tells us the index (aka. address) at which an object will be stored in the … Read more
The Wikipedia article will have a lot of technical information, but a simplistic view of hashing is something like the following. Imagine that there’s a magical function that can give a number to any object. Given the same object, it always return the same number. Immediately now you have a quick way to test if … Read more
This whileloop is strange: If your input file is: Then scanner.nextLine() would be return a, then scanner.next() would return b, because nextLine returns the next end-line delimited String, and next returns the next token from the input file. Is this really what you want? I’d suggest trying this: Keep in mind that this would only … Read more
I’ve had nice results with djb2 by Dan Bernstein.
Let’s imagine we have a truly random hash function that hashes from strings to n-bit numbers. This means that there are 2n possible hash codes, and each string’s hash code is chosen uniformly at random from all of those possibilities. The birthday paradox specifically says that once you’ve seen roughly √(2k) items, there’s a 50% chance … Read more
First, it usually does not matter that much in practice. Most hash functions are “good enough”. But if you really care, you should know that it is a research subject by itself. There are thousand of papers about that. You can still get a PhD today by studying & designing hashing algorithms. Your second hash … Read more
For doing “normal” hash table lookups on basically any kind of data – this one by Paul Hsieh is the best I’ve ever used. http://www.azillionmonkeys.com/qed/hash.html If you care about cryptographically secure or anything else more advanced, then YMMV. If you just want a kick ass general purpose hash function for a hash table lookup, then … Read more
Usually hashes wouldn’t do sums, otherwise stop and pots will have the same hash. and you wouldn’t limit it to the first n characters because otherwise house and houses would have the same hash. Generally hashs take values and multiply it by a prime number (makes it more likely to generate unique hashes) So you … Read more
A lot of people are saying that once you get to the size where speed is actually a concern that HashSet<T> will always beat List<T>, but that depends on what you are doing. Let’s say you have a List<T> that will only ever have on average 5 items in it. Over a large number of cycles, if a single item … Read more