hash table

hash table, in computer science, a dictionary that maps keys to values using a hash function. A hash function is a mathematical function that maps data of arbitrary length to data of a fixed length. Hashing is a highly efficient way of performing certain operations, such as searches, insertions, and deletions. Hash functions are also widely used in cryptography, though not commonly in the form of hash tables.

A hash table allows stored data to be retrieved from a table more quickly than a simple binary search of the data would allow, because the key being searched for is used to directly identify the index (row, or record) in the table that should be returned. For example, suppose one has a hash table designed to allow one to input a name and return a phone number. The hash table uses an algorithm to convert the name to a value of a fixed length that serves as the index for the table. Thus, hashing the key directs you to a specific place in the table to find the desired information.

When more than one value is assigned to the same key, this is referred to as a collision. While collisions are not desirable, they are common and can be handled in different ways. One way is to have more than one value stored in a list at each index. This is called chaining. Multiple key values can be stored at the same index, and, when one of those keys is entered, it will be compared with the keys at the index until the correct one is found. Another method is called open addressing. In open addressing, when a collision occurs, one key is moved to a different open slot in the hash table based on a particular search algorithm.

With chaining, many keys can be divided among the indices. A key and its associated date can be deleted in the same way. Hashing can be effective at dividing up a large number of keys so that searches and other operations on even very large amounts of data go quickly.

A well-designed hash table can have a time complexity of O(1), or constant time, meaning that the same number of operations will be required to perform the search regardless of how many keys are in the table.