by kirupa |
13 October 2006In the
previous page,
I explained how hashtables work and the advantages they have
over an array/list. In this page I will provide some real
world examples that will give you a brief idea of the
various uses dictionaries have.
A real world example that simulates the differences
between arrays/lists and dictionaries/hashtables is finding
information in a reference book. In the linear array/list
based search, you go through each page or sections of pages
in the book to find out where the information may be
located. That works for small books, but for a large
reference book, this approach may not work as quickly as you
would like.
On the other hand, for the
dictionary/hashtable approach, you simply look in the index
of the book to find the page number where your topic is
discussed. With the page number in hand, you immediately go
to the page where the information is stored. You do not
shuffle through many off-topic pages before hitting your
information, and this near-instantaneous speed at returning
information holds for small as well as large reference
books.
The following is a short list of scenarios where a
dictionary can be used:
-
Avoiding Duplicates
If you are only interested in adding values to a list
that are unique, you can check whether the value you are
about to add to the list exists in your dictionary.
If the value exists in the dictionary, you skip the
value. If the value does not exist, you add the value
both to your list as well as your dictionary for future
reference. This method is certainly better than going
through each element in your array and checking whether
it is equal the value you are planning on adding!
-
Retrieving User-Specific Information
If you have a collection of users, you can use a
dictionary to have the key be your user and the value
being a list containing information about the user. This
allows you to, given a user's name, retrieve the
information almost instantaneously without having the
user wait for a long time.
-
Graphs/Nodes/Edges
If you are using a graph structure to represent nodes
and edges, you can use a dictionary to help you quickly
find whether a pair of nodes contain an edge. For
example, when adding an edge between two nodes, your key
could be the string "node1-node2", and the value could
be the edge itself.
When checking whether an edge exists between two nodes,
you simply pass in the two nodes' names as "nodeA-nodeB"
and see if a value is returned. If a value is returned,
then there is an edge between those two nodes. If a
value is not returned, then there is no edge between
them.
-
Calculating Patterns
Pattern detection is something that is quite useful. My
blog post provides code and analyzes a simple example of
this. Click
here to view post.
The above scenarios are only a small subset
of uses for a dictionary. I am sure there are countless
others, but I provided them just as a way for you to think
about dictionaries and what is possible with the key/value
pair they take as their input. Because, if this is your
first introduction to a hashtable data structure, I would be
thrilled if you explored other ways of optimizing your
applications using hashtables. The above examples are merely
a small push in the right direction.
This wraps up this tutorial on how to use the
Dictionary class in .NET. Besides learning about the syntax
on how to use this in your applications, I also hope you
learned a bit about why the Dictionary data structure is
another great choice along with arrays and lists (and
others!) for storing and retrieving data.
I hope the information helped.
If you have any questions or comments, please don't hesitate to post them on the
kirupa.com Forums. Just post your question and I, or our friendly
forum helpers, will help answer it.
The following is a list of related tutorial and help resources that you may find
useful:
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Using a Dictionary (Hashtable) - Page 5
