1 / 15

Exploring External Dictionaries: B-Trees and Hashing

Dive into external dictionaries and data structures like B-trees and hashing in external memory applications, analyzing disk accesses, search trees, B+-trees, hashing concepts, and collision handling options.

dixie
Download Presentation

Exploring External Dictionaries: B-Trees and Hashing

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. For Wednesday • Finish Weiss, chapter 4 • Homework • Weiss, chapter 4, exercise 27

  2. Paper 1 • Any questions?

  3. Review Chance • Any questions on: • AVL trees • Splay trees

  4. External Dictionaries • We’ve talked so far about dictionaries small enough to reside in memory • However, many applications require dictionaries much larger than will easily fit in memory • Biggest issue for external dictionaries is the number of disk accesses required for an operation • Each disk access retrieves a block of memory

  5. m-way Search Trees • Empty tree or • Each internal node has up to m children and between 1 and m-1 elements • A node with p elements has exactly p+1 children • Elements are ordered

  6. B-tree of Order m • an m-way search tree • If non-empty • The root has at least two children • All internal nodes other than the root have at least ceiling of m/2 children. • All external nodes are at the same level • Thus we have guarantees on the height of the tree • Book technically covers structure called B+-tree—items in internal nodes also appear in external nodes

  7. Operations • Searching • Insertion • Deletion

  8. Dictionaries • What’s one of our biggest goals in creating a dictionary?

  9. Answer • Fast search • How do we do with trees?

  10. New Search Goal • Constant time search

  11. Hashing • What’s the basic idea?

  12. Perfect Hashing • What would constitute a perfect hash?

  13. Imperfect Hashing • Two basic issues: • How do we determine the hash bucket from the key? • How do we deal with collisions?

  14. Computing the Hash Index • Typically use mod

  15. Collision Handling Options • Separate chaining • Open addressing • Linear probing • Quadratic probing • Double hashing

More Related