Hashtable implementation in C and Java

Question

In Java, HashMap and Hashtable, both implement map interface and store key/value pairs using hash function and Array/LinkedList implementation. In C also, Hash table can be implemented using Array/LinkedList functionality but there is no concept of key/value pair like map.

So my question is, whether Hash table implementation in C, similar to Hashtable in Java? or its more closer to HashSet in java (except unique elements only condition)?

Answer 1

Both semantics ( Hashtable and HashSet ) can be implemented in C, but neither comes in the Standard C library. You can find many different has table implementation on the Internet, each with its own advantages and drawbacks. Implementing this yourself may prove difficult as there are many traps and pitfalls.

Answer 2

I previously used BSD's Red-Black trees implementation. It's relatively easy to use when you start to understand how it works.

The really great thing about is that you only have to copy one header file and then just include that where it's needed, no need to link to libraries.

It has similar functionallity to HashSet s, you can find by keys with the RB_FIND() macro, enumerate elements with RB_FOREACH() , insert new ones with RB_INSERT() and so on.

You can find more info in it's MAN page or the source code itself .

Answer 3

The difference (in Java) between a HashTable and a HashSet is in how the key is selected to calculate its hash value. In the HashSet the key is the instance stored itself, and the hashCode() method is applied to the complete instance, ( Object provides both, hashCode() and equals(Object) methods. In the case of an external key, the equals(Object) and hashCode() are selected now from the separate key instance, instead of from the stored data value. For that reason, HashTable is normally a subclass of HashSet (and every Java table is actually derived from its corresponding *Set counterpart), by publishing an internal implementation of the Map.Entry<K,V> interface)

Implementing a hash table in C is not too difficult, but you need to understand first what's the key (if external) and the differences between the Key and the Value, the differences between calculating a hashCode() and comparing for equality, how are you going to distinguish the key from the value, and how do you manage internally keys and hashes, in order to manage collisions.

I recently started an implementation of a hash table in C (not yet finished) and my hash_table constructor need to store in the instance record a pointer to an equals comparison routine (to check for equality, the same as Java requires an Object 's compareTo() method, this allows you to detect collisions (when you have two entries with the same hash but they compare as different) and the hash function used on keys to get the hash. In my implementation probably I will store the full hash value returned by the hash function (before fitting it on the table's size), so I can grow the table in order to simplify the placement of the elements in the new Z0800FC577294C34E0B28A D2839435945Z table once growed, without having to recalculate all hashes again.

I don't know if this hints can be of help to you, but it's my two cents. :)

My implementation uses a prime numbers table to select the capacity (aprox, doubling the size on each entry) to redimension the table when the number of collisions begin to be unacceptable (whatever this means to you, I have no clear idea yet, this is a time consuming operation, but happens scarcely, so it must be carefully specified, and it is something that Java's HashTable does) But if you don't plan to grow your hash table, or to do it manually, the implementation is easier (just add the number of entries in the constructor parameter list, and create some grow_hash_table(new_cap) method.)