Hash Table: Computer science, Data structure, Hash function, Unique key, Value (mathematics), Array data type, Collision (computer science), Instruction ... science), Amortized analysis, Search tree

In ? computer science, a hash table or hash map is a ? data structure that uses a ? hash function to efficiently map certain identifiers or ? keys (e.g., person names) to associated ? values (e.g., their telephone numbers). The hash function is used to transform the key into the index (the hash) of an ? array element (the slot or bucket) where the corresponding value is to be sought. Ideally the hash function should map each possible key to a different slot index, but this ideal is rarely achievable in practice (unless the hash keys are fixed; i.e. new entries are never added to the table after creation). Most hash table designs assume that hash collisions ? pairs of different keys with the same hash values ? are normal occurrences and must be accommodated in some way.