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A hash table is a data structure in computer science that is used to retrieve data quickly. It is a special type of associative array that uses a key value to access the value of an element.
A hash table consists of an array in which each element contains a key and an associated value. The key is used to calculate the index at which the element is stored in the array. This index is calculated using a so-called hash function that converts the key into an integer value.
When a new element is inserted into the hash table, the hash function is first applied to the key to calculate the index at which the element is stored in the array. If there is already an element stored at that index that has the same index, a so-called collision resolution procedure is applied to store the new element at a different location in the array.
When an element is to be retrieved from the hash table, the hash function is again applied to the key to calculate the index where the element is stored in the array. Since the hash function maps the keys to unique indexes, the element can be retrieved in constant time, regardless of the size of the hash table.
Hash tables are often used to implement databases, as a cache or as part of algorithms such as the search algorithm or the sorting algorithm.