CSE 250: Data Structures
Week 12
March 31 – April 4, 2008
Announcements
Project 3 due extended to April 7th
Homework 4 due April 11th
Homework 5 due April 18th
Project 4 due April 25th
Final Exam May 1st 11:45 – 2:45 Cooke
121
Week’s Topics
Hashing
(Key, value) pairs
From the key, retrieve a value using a
hash function to tell you where it will be
stored in your table.
Hash functions should be simple to
compute, ensure that distinct keys hash to
distinct values, and ideally, distribute keys
evenly among the elements in the store.
Week’s Topics
It is possible to have two values hash to
the same location in the table. This is
called a collision.
What do we do in this situation?
Two main strategies:
Separate Chaining
Open Addressing
Week’s Topics
Separate Chaining
Keep a list (think linked list) of all elements
that hash to the same value
What are the potential shortcomings of this
approach?
Week’s Notes
Open Addressing
Keep looking (probing) until you find an
appropriate open space to put the element
you are trying to store. The probing must
be systematic so that you can find the
inserted element at a later time using the
same process.
Week’s Notes
Linear Probing
If the spot you hashed to is already taken,
move to the next one and so on, wrapping
around to the beginning of the table if
necessary until you find an available spot
for insertion.
Problem: primary clustering – what is it –
why does it happen?
Week’s Topics
Quadratic Probing
Probe using a quadratic method, first look
one away, then four away, then nine
away…
Problems involving load factor of the table
and size of table – when can quadratic
probing go wrong?
Week’s Topics
Double hashing
Use a second hash function to control the
probing distance that is not necessarily
linear or quadratic
Week’s Topics
How do you delete from a hash table that
uses probing. If you delete the element
outright, you could be cutting off the
search for elements that come after it –
what is the strategy employed when
deleting?
Week’s Notes
Rehashing
After the table gets too full (or there have
been a significant number of deletes), we
should resize the table.
After resizing, you would need a new hash
function and should re-hash the old values
into their new places in the new, larger
table.