1 / 26

Hash Tables

Hash Tables. Chapter 11 discusses several ways of storing information in an array, and later searching for the information. Hash tables are a common approach to the storing/searching problem. This presentation introduces hash tables. Data Structures and Other Objects Using Java.

bernard-ellison
Download Presentation

Hash Tables

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. Hash Tables • Chapter 11 discusses several ways of storing information in an array, and later searching for the information. • Hash tablesare a common approach to the storing/searching problem. • This presentation introduces hash tables. Data Structures and Other Objects Using Java

  2. The simplest kind of hash table is an array of Entrys. This example has 701 Entrys. What is a Hash Table ? [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700] . . . An array of Entrys

  3. Each Entry has a special field, called its key. In this example, the key is a long integer field called Number. What is a Hash Table ? [ 4 ] Number 506643548 [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700] . . .

  4. The number might be a person's identification number, and the rest of the entry has information about the person. What is a Hash Table ? [ 4 ] Number 506643548 [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700] . . .

  5. When a hash table is in use, some spots contain valid entries, and other spots are "empty". Number 281942902 Number 233667136 Number 506643548 Number 155778322 . . . What is a Hash Table ? [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  6. In order to insert a new Entry, the key must somehow be converted toan array index. The key is too large. The index is called the hash valueof the key. Number 281942902 Number 233667136 Number 506643548 Number 155778322 . . . Inserting a New entry Number 580625685 [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  7. Typical way create a hash value: Number 281942902 Number 233667136 Number 506643548 Number 155778322 . . . Inserting a New Entry Number 580625685 (Number mod 701) What is (580625685 mod 701) ? [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  8. Typical way to create a hash value: Number 281942902 Number 233667136 Number 506643548 Number 155778322 . . . Inserting a New Entry Number 580625685 (Number mod 701) 3 What is (580625685 mod 701) ? [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  9. The hash value is used for the location of the new Entry. [3] Number 281942902 Number 233667136 Number 506643548 Number 155778322 . . . Inserting a New Entry Number 580625685 [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  10. The hash value is used for the location of the new Entry. Number 281942902 Number 233667136 Number 580625685 Number 506643548 Number 155778322 . . . Inserting a New Entry [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  11. Here is another new Entry to insert, with a hash value of 2. Number 281942902 Number 233667136 Number 580625685 Number 506643548 Number 155778322 . . . Collisions Number 701466868 My hash value is [5]. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  12. This is called a collision, because there is already another valid Entry at [2]. Number 281942902 Number 233667136 Number 580625685 Number 506643548 Number 155778322 . . . Collisions Number 701466868 When a collision occurs, move forward until you find an empty spot. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  13. This is called a collision, because there is already another valid Entry at [2]. Number 281942902 Number 233667136 Number 580625685 Number 506643548 Number 155778322 . . . Collisions Number 701466868 When a collision occurs, move forward until you find an empty spot. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  14. This is called a collision, because there is already another valid Entry at [2]. Number 281942902 Number 233667136 Number 580625685 Number 506643548 Number 155778322 . . . Collisions Number 701466868 When a collision occurs, move forward until you find an empty spot. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  15. This is called a collision, because there is already another valid Entry at [2]. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . Collisions The new Entry goes in the empty spot. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  16. Where would you be placed in this table, if there is no collision? Use your social security number or some other favorite number. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . A Quiz [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  17. The data that's attached to a key can be found fairly quickly. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . Searching for a Key Number 701466868 [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  18. Calculate the hash value. Check that location of the array for the key. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . Searching for a Key Number 701466868 My hash value is [2]. Not me. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  19. Keep moving forward until you find the key, or you reach an empty spot. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . Searching for a Key Number 701466868 My hash value is [5]. Not me. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  20. Keep moving forward until you find the key, or you reach an empty spot. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . Searching for a Key Number 701466868 My hash value is [5]. Not me. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  21. Keep moving forward until you find the key, or you reach an empty spot. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . Searching for a Key Number 701466868 My hash value is [5]. Yes! [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  22. When the item is found, the information can be copied to the necessary location. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . Searching for a Key Number 701466868 My hash value is [2]. Yes! [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  23. Entrys may also be deleted from a hash table. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 506643548 Number 155778322 . . . Deleting a Entry Please delete me. [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  24. Entrys may also be deleted from a hash table. But the location must not be left as an ordinary "empty spot" since that could interfere with searches. Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 155778322 . . . Deleting a Entry [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  25. Entrys may also be deleted from a hash table. But the location must not be left as an ordinary "empty spot" since that could interfere with searches. The location must be marked in some special way so that a search can tell that the spot used to have something in it. The mark is called a “tombstone” Number 281942902 Number 233667136 Number 701466868 Number 580625685 Number 155778322 . . . Deleting a Entry [ 0 ] [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 700]

  26. Summary • Hash tables store a collection of Entrys with keys. • The location of a Entry depends on the hash value of the Entry's key. • When a collision occurs, the next available location is used. • Searching for a particular key is generally quick. • When an item is deleted, the location must be marked in a special way, so that the searches know that the spot used to be used.

More Related