"A hash table, also known as hash map, is a data structure that implements an associative array, also called a dictionary, which is an abstract data type that maps keys to values."
A data structure that allows efficient insertion, deletion, and retrieval of elements. Hash tables use a hash function to map keys to index values.
Key-value pairs: The basic structure of hash tables involves pairing a unique key with a corresponding value.
Hash functions: Hash functions are used to convert a given key into a unique index within a hash table.
Collision resolution: When two or more keys are mapped to the same index within a hash table, collision resolution techniques must be used to avoid data loss or inefficiency.
Load factor: The load factor of a hash table refers to the ratio of stored elements to total table size, and can be used to optimize hash table performance.
Chaining: One of the most common collision resolution techniques, chaining involves linking elements that map to the same hash index using a linked list.
Open addressing: Another collision resolution technique, open addressing involves finding an available slot in the hash table when a collision occurs.
Linear probing: A specific form of open addressing, linear probing involves searching for the next available slot in the hash table when a collision occurs.
Quadratic probing: Another form of open addressing, quadratic probing is similar to linear probing but uses a quadratic function to calculate the next available slot.
Double hashing: A more complex form of open addressing, double hashing involves using a second hash function to calculate the next available slot when a collision occurs.
Hash table design considerations: Considerations such as table size, hash function choice, and collision resolution technique should be taken into account when designing and implementing a hash table.
Linear Probing Hash Tables: In this type of Hash Table, data that hashes to the same index is stored in adjacent cells of an array.
Quadratic Probing Hash Tables: Similar to Linear Probing, Quadratic Probing stores data that hashes to the same index in a nearby cell. However, it uses a quadratic function to find the next empty cell.
Chaining Hash Tables: Chaining Hash Tables keep a linked list in each cell of an array, for storing data that shares the same index.
Double Hashing Hash Tables: A Double Hashing Hash Table uses two hash functions to determine the index for each element, resolving any collisions by finding an empty cell using one of the hash functions.
Robin Hood Hashing: In Robin Hood Hashing, collisions are resolved by moving entries around to make room for new data. Entries with higher priority are moved further around until their ideal slot is found.
Cuckoo Hashing: In this type of Hash Table, two or more tables are used, and each element is inserted in one of the tables, if there is a collision, the element is moved to the alternate table.
Hopscotch Hashing: Hopscotch Hashing hashes elements to an index in an array, and stores it in that cell. It then searches for the nearest empty cell and starts a search in the nearby slot. If it doesn't find an empty cell within a certain range, it triggers a rehash that may increase the size of the array.
Dynamic Perfect Hashing: This type of Hash Table is unique in that it eliminates any need for probing or collisions. A perfect hash function is used to determine the index for each element, and the size of the array is automatically adjusted to fit the data.
"A hash table uses a hash function to compute an index, also called a hash code, into an array of buckets or slots, from which the desired value can be found."
"The resulting hash indicates where the corresponding value is stored."
"Hash collisions occur when the hash function generates the same index for more than one key."
"Such collisions are typically accommodated in some way."
"In a well-dimensioned hash table, the average time complexity for each lookup is independent of the number of elements stored in the table."
"Many hash table designs also allow arbitrary insertions and deletions of key-value pairs, at amortized constant average cost per operation."
"Hashing is an example of a space-time tradeoff."
"If memory is infinite, the entire key can be used directly as an index to locate its value with a single memory access."
"On the other hand, if infinite time is available, values can be stored without regard to their keys."
"In many situations, hash tables turn out to be on average more efficient than search trees or any other table lookup structure."
"They are widely used in many kinds of computer software, particularly for associative arrays, database indexing, caches, and sets."
"An associative array, also called a dictionary, is an abstract data type that maps keys to values."
"Hash tables are widely used in...database indexing."
"Many hash table designs also allow arbitrary insertions and deletions of key-value pairs, at amortized constant average cost per operation."
"Most hash table designs employ an imperfect hash function, which might cause hash collisions."
"In a well-dimensioned hash table, the average time complexity for each lookup is independent of the number of elements stored in the table."
"A hash table...maps keys to values."
"No, ideally, the hash function will assign each key to a unique bucket."
"In many situations, hash tables turn out to be on average more efficient than search trees or any other table lookup structure. For this reason, they are widely used in many kinds of computer software."