difference between dictionary and hash table

What is the true difference between a dictionary and a hash table?

up vote 21 down vote favorite 5	I've always used dictionaries. I write in Python. python  data-structures  hash  dictionary  hashtable share improve this question edited Jan 13 '10 at 23:56 bernie 67.6k 8 116 128 asked  Jan 13 '10 at 23:53 TIMEX 36k 180 502 795
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up vote 80 down vote accepted

A dictionary is a general concept that maps keys to values. There are many ways to implement such a mapping. A hashtable is a specific way to implement a dictionary. Besides hashtables, another common way to implement dictionaries is red-black trees. Each method has it's own pros and cons. A red-black tree can always perform a lookup in O(log N). A hashtable can perform a lookup in O(1) time although that can degrade to O(N) depending on the input. share improve this answer edited Dec 20 '12 at 18:48 Garen 559 2 6 16 answered  Jan 14 '10 at 0:29 R Samuel Klatchko 54.1k 7 89 156

8   I wish I could vote this one up more than once. –  LJM  Jan 14 '10 at 0:41 1   I'll re-upvote it for you. –  Nicolás  Jan 14 '10 at 2:12 add a comment

up vote 19 down vote

A dictionary is a data structure that maps keys to values. A hash table is a data structure that maps keys to values by taking the hash value of the key (by applying some hash function to it) and mapping that to a bucket where one or more values is stored. IMO this is analogous to asking the difference between a list and a linked list. For clarity it may be important to note that it MAY be the case that Python currently implements their dictionaries using hash tables, and it MAY be the case in the future that Python changes that fact without causing their dictionaries to stop being dictionaries. share improve this answer edited Jan 14 '10 at 0:13 answered  Jan 13 '10 at 23:57 danben 45k 11 86 113

Wouldn't the main difference be that a dictionary also stores the keys? So you can query a dictionary for th elsit of keys - you can't for a hash table –  Martin Beckett  Jan 14 '10 at 0:24 1   @Martin Beckett: Nope. Both can store the keys. The dictionary is general. The hash table is a specific implementation of the general concept. –  S.Lott  Jan 14 '10 at 0:24 @Martin Beckett: Hm, interesting point. Is it necessarily the case that a dictionary stores keys and a hash table doesn't? Java's Hashtable stores the keys - is it not a hash table, then? –  danben  Jan 14 '10 at 0:28 3   Thinking a bit more, a hashtable necessarily has to store the keys in case of collisions! –  Martin Beckett  Jan 14 '10 at 0:54 2   @danben: No. Storage of keys is not a distinguishing feature. The distinguishing feature is that a dictionary is a generic concept where a hashtable is an actual implementation. –  S.Lott  Jan 14 '10 at 1:05 show 3 more comments

up vote 12 down vote

"A dictionary" has a few different meanings in programming, as wikipedia will tell you -- "associative array", the sense in which Python uses the term (also known as "a mapping"), is one of those meanings (but "data dictionary", and "dictionary attacks" in password guess attempts, are also important). Hash tables are important data structures; Python uses them to implement two important built-in data types, dict and set. So, even in Python, you can't consider "hash table" to be a synonym for "dictionary"... since a similar data structure is also used to implement "sets"!-) share improve this answer answered  Jan 14 '10 at 0:01 Alex Martelli 456k 80 834 1131

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up vote 5 down vote	A Python dictionary is internally implemented with a hashtable. share improve this answer answered  Jan 13 '10 at 23:55 Nicolás 5,610 19 27
	A subclass of dict is not the Python implementation of a dictionary; it's your own