There is no hashtable in the standard C library because either: no-one has submitted a proposal to the working group; or the working group has deemed it unnecessary. That’s the way ISO works. Proposals are put forward and accepted or rejected. You have to be careful what you add to the standard library since you … Read more
I made a program recently that dealt with a lot of if/else statements to return particular values. Someone recommended using lookup tables instead. My question is, How do they work and how do you implement them? What is the difference between a map, hash table, and lookup table.
Wikipedia’s article on hash tables gives a distinctly better explanation and overview of different hash table schemes that people have used than I’m able to off the top of my head. In fact you’re probably better off reading that article than asking the question here. 🙂 That said… A chained hash table indexes into an … Read more
To add to Irfan’s answer, here is a shorter and faster version of get() since it requires no iteration over the map contents:
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
I’ve had nice results with djb2 by Dan Bernstein.
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
In the updated code you use an uninitialized pointer: As people had wrote already, this will only work if you had pre-set all the pointers to be NULL before entering this loop:
What is the complexity of creating a trie of a list of words and what is complexity of searching other set of word in that trie? Should I use trie for string searching, when i have hashtable?
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