More on Hash Tables

1. More on Hash Tables Andy Wang Data Structures, Algorithms, and Generic Programming

2. Why Hash Tables? • Arguments • Linear search is simple • Hash table does not save that much time • Counter arguments • What if the data volume is high? (internet routers) • What if the data set if large? (yellow page)

3. Steps to Build a Hash Table 1. Build a table to support basic operations • Insert • Lookup • Remove 2. Use a hash function to determine the table entry 3. Add mechanisms to handle collisions

4. Hash(name) Insert() Hash Table

5. Hash(name) Insert() Hash Table

6. Hash(name) Insert() Icarumba! Hash Table

7. Hash(name) Linear Probing Hash Table

8. Hash(name) Linear Probing Hash Table

9. Remove() Hash Table

10. Hash(name) Remove() Hash Table

11. Hash(name) Remove() Hash Table

12. Lookup() Hash Table

13. Hash(name) Lookup() Hash Table

14. Hash(name) Insert() Hash Table

15. Hash(name) Tricky Case… Oops… Hash Table

16. To Handle Tricky Cases Insert(…) { if (Lookup(…) == false) { // insert } } Remove(…) { if (entry[hash] empty || entry[hash] != key) // linear search for the entry } }

17. To Handle Tricky Cases Lookup(…) { if (entry[hash] empty || entry[hash] != key) // linear search for the entry } }

18. How can we make it more efficient?