Hash table

Overview

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

Design

The main idea behind a hash table is to take a range of key values and transform them into index values, which are used to specify the exact location in the memory where the record should be stored. The transformation process is performed by a hash function.

Hash Function

A hash function is any function that can be used to map data of arbitrary size to fixed-size values. The values returned by a hash function are called hash codes, hash values, or simply hashes. If two keys map to the same index, a collision is said to occur.

Collision Resolution

When a collision occurs, there are several ways to handle it. The simplest method is to build a list of entries that hash to the same index, a technique known as separate chaining. Another method is open addressing, in which all entry records are stored in the bucket array itself.

Performance

The time complexity of hash table operations is a function of the load factor, which is a measure of how full the hash table is. A good hash function will result in a uniform distribution across the array, which will ensure that the load factor remains low, leading to efficient search, insert, and delete operations.

Applications

Hash tables are used in many different applications, including database indexing, caching, memory management, and many more. They are particularly useful in situations where rapid access to data is required.

Variants

There are many variants of hash tables, including cuckoo hashing, hopscotch hashing, and robin hood hashing. These variants offer different trade-offs in terms of performance and complexity.