Hash Table

External

• http://en.wikipedia.org/wiki/Hash_table

Internal

• Data Structures
• Associative Array
• Distributed Hash Table
• Java HashMap

Overview

Hash tables are one of the most used data structures in programming. They don't have that many operations (INSERT(), DELETE() and SEARCH()), but what they do, they do really well. Conceptually, a hash table is an array that provides immediate random access for constant time insertion, deletion and lookup based on an arbitrary key and not an integral index, for a dynamic set of keys. The hash map implementations do store data in proper arrays, and the mapping between the arbitrary key and the integral position in the array is provided by a hash function. Hash maps are some times referred to as dictionaries, which is fine as long as the dictionary is not assumed to support queries on totally ordered sets: hash maps do not maintain the ordering of the elements that they contain.

Canonical Use

Hash tables are used in situations where we need to do fast lookups of arbitrary keys.

• The 2-SUM Problem

Hash Table Implementation Discussion

The purpose of a hash table is to maintain a dynamic set of records, each record identified by a unique key, and provide constant time insertion, deletion and search.

Constant Time Discussion

The constant time guarantees applies to properly implemented hash tables. Hash tables are easy to implement badly, on in those case, the constant time guarantee does not hold.

Supported Operations

INSERT(X)

Add a new key/value pair INSERT(X) to the hash map in O(1) running time.

DELETE(K)

Delete a key and the corresponding value DELETE(K) from the hash map in O(1) running time.

SEARCH(K)

Return the value corresponding to the provided key K SEARCH(K), or NULL if the key does not exist, in O(1) running time.

TO DEPLETE

HashTableTODEPLETE