What is a hash table?

A hash table, also known as a hash map, is a data structure used to implement an associative array, a structure that can map keys to values. It uses a hash function to compute an index into an array of slots, from which the desired value can be found.

Ideally, the hash function will assign each key to a unique slot in the array. However, most hash table designs assume that hash collisions—different keys that are assigned by the hash function to the same slot—can occur and provide some method for handling them. Common collision resolution strategies include open addressing (where a collision leads to probing or searching the table for a free slot according to a deterministic sequence) and chaining (where each slot in the table is the head of a linked list of entries that collide at that slot).

Hash tables are known for their efficiency in performing lookup operations. They allow for average-case constant-time complexity (O(1)) for lookups, inserts, and deletions, assuming the hash function spreads the entries uniformly across the table. However, in the worst case, such as when all keys collide at a single slot, these operations can degrade to (O(n)) where (n) is the number of entries in the table.

Hash tables are widely used because they offer fast retrieval and insertion of data and can efficiently support operations such as search, delete, and insert. They are key components of many software systems, including database indexing, caches, and sets data structures.