HashTables-handouts-2

HashTables-handouts-2 - Dictionaries 1/22/2006 10:17 AM...

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Dictionaries 1/22/2006 10:17 AM 1 Dictionaries and Hash Tables 1 Dictionaries and Hash Tables 0 1 2 3 4 451-229-0004 981-101-0002 025-612-0001 Dictionaries and Hash Tables 2 Dictionary ADT (§2.5.1) The dictionary ADT models a searchable collection of key- element items The main operations of a dictionary are searching, inserting, and deleting items Multiple items with the same key are allowed Applications: ± address book ± credit card authorization ± mapping host names (e.g., cs16.net) to internet addresses (e.g., 128.148.34.101) Dictionary ADT methods: ± findElement (k): if the dictionary has an item with key k, returns its element, else, returns the special element NO_SUCH_KEY ± insertItem (k, o): inserts item (k, o) into the dictionary ± removeElement (k): if the dictionary has an item with key k, removes it from the dictionary and returns its element, else returns the special element NO_SUCH_KEY ± size (), isEmpty () ± keys (), Elements ()
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Dictionaries 1/22/2006 10:17 AM 2 Dictionaries and Hash Tables 3 Log File (§2.5.1) A log file is a dictionary implemented by means of an unsorted sequence ± We store the items of the dictionary in a sequence (based on a doubly-linked lists or a circular array), in arbitrary order Performance: ± insertItem takes O (1) time since we can insert the new item at the beginning or at the end of the sequence ± findElement and removeElement take O ( n ) time since in the worst case (the item is not found) we traverse the entire sequence to look for an item with the given key The log file is effective only for dictionaries of small size or for dictionaries on which insertions are the most common operations, while searches and removals are rarely performed (e.g., historical record of logins to a workstation) Dictionaries and Hash Tables 4 Hash Functions and Hash Tables (§2.5.2) A hash function h maps keys of a given type to integers in a fixed interval [0, N 1] Example: h ( x ) = x mod N is a hash function for integer keys The integer h ( x ) is called the hash value of key x A hash table for a given key type consists of ± Hash function h ± Array (called table) of size N
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This note was uploaded on 09/10/2009 for the course CS 401 taught by Professor Cs401 during the Spring '08 term at Illinois Tech.

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HashTables-handouts-2 - Dictionaries 1/22/2006 10:17 AM...

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