More on Hash Tables

Andy WangData Structures, Algorithms, and

Generic Programming

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)

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

Insert()

Name: Bobby HillAddress: Arlen

0123

Hash(name)

Hash Table

Insert()

Name: Bobby HillAddress: Arlen

01 Bobby Hill Arlen23

Hash(name)

Hash Table

Insert()

Name: Bart SimpsonAddress: Springfield

01 Bobby Hill Arlen23

Hash(name)

Hash Table

Icarumba!

Linear Probing

Name: Bart SimpsonAddress: Springfield

01 Bobby Hill Arlen23

Hash(name)

Hash Table

Linear Probing

Name: Bart SimpsonAddress: Springfield

01 Bobby Hill Arlen2 Bart Simpson Springfield3

Hash(name)

Hash Table

Remove()

Name: Bobby Hill

01 Bobby Hill Arlen2 Bart Simpson Springfield3

Hash Table

Remove()

01 Bobby Hill Arlen2 Bart Simpson Springfield3

Hash(name)

Hash Table

Name: Bobby Hill

Remove()

012 Bart Simpson Springfield3

Hash(name)

Hash Table

Name: Bobby Hill

Lookup()

Name: Bart Simpson

012 Bart Simpson Springfield3

Hash Table

Lookup()

012 Bart Simpson Springfield3

Hash(name)

Hash Table

Name: Bart Simpson

Insert()

Name: Bart SimpsonAddress: Springfield

012 Bart Simpson Springfield3

Hash(name)

Hash Table

Tricky Case…

Name: Bart SimpsonAddress: Springfield

01 Bart Simpson Springfield2 Bart Simpson Springfield3

Hash(name)

Hash Table

Oops…

To Handle Tricky Cases

Insert(…) {if (Lookup(…) == false) {// insert

}}Remove(…) {

if (entry[hash] empty || entry[hash] != key) // linear search for the entry

}}

To Handle Tricky Cases

Lookup(…) {if (entry[hash] empty || entry[hash] != key)

// linear search for the entry}

}

How can we make it more efficient?