Algorithms
What Is An Algorithm?
Source: Wikipedia
Source:
http://en.wikipedia.org/wiki/
Algorithm
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Simply put, an algorithm is a series of
instructions (a procedure) that solves a
problem by using a finite number of steps.
A Computer Scientist would more
accurately describe an algorithm as an
effective method expressed as a finite
list of well-defined instructions for
calculating a function. Starting from an
initial state and initial input the instructions
describe a computation that, when
executed, will proceed through a finite
number of well-defined successive states,
eventually
producing
output
and
terminating at a final ending state.
(Phew)!
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Algorithms: Example 1
Food Recipe
A food recipe is a step-by-step procedure
– like a list of instructions - to complete a
task.
A food recipe therefore is like an algorithm.
In this analogy it can be compared to the
software in a system. The hardware is the
oven and the other cooking utensils.
The input is the ingredients and the output
is the cake.
Source: Wikipedia
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Algorithms: Example 2
“Get To School”
1. Walk to bus
stop
2. Wait for bus
No.123
3. Get on bus
4. Ride bus to
school
5. Get off bus at
school bus
stop
6. Walk to class
1. Get in car
2. Be driven
along City
Road
3. Turn left into
Town Lane
4. Car stops in
drop off zone
5. Get out of car
6. Walk to class
For more examples visit :
http://computer.howstuffworks.com/ques
tion717.htm
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Designing algorithms is about designing
solutions to solve problems.
If you were to design an algorithm to solve
the problem of ‘getting to school’ you
would be able to solve this problem in
several different ways (see left).
Different algorithms can solve the same
problem – but in different ways.
Algorithms that solve the same problem
may differ in length, efficiency and cost.
In computer programming there are many
different ways - algorithms - of solving a
problem and completing a task.
A computer program can be viewed as an
elaborate algorithm.
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Algorithms: Flowcharts
…making your thinking ‘visible’
Lamp does
not work
Lamp
plugged
in?
No
Plug in
lamp
Yes
Bulb
‘blown’?
Yes
Replace
bulb
No
Repair
lamp
What other steps
could be included in
this example?
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Developing a flowchart helps to formalise
the algorithm – and allows more choices,
complexity and variables - crucial as you
move past the simplest of algorithms.
Simple problems can often be shown best
as a flowchart.
Flowcharts can help you to think logically
about problems by working through each
step.
Flowcharts can help you explain your work
to others – and they can follow your
thinking (very useful when trying to ‘fault
find’ errors /mistakes’ in your algorithms).
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Algorithms: Flowcharts
A Guide to understanding flow charts
Source: xkcd.com/518/
http://www.cs.nyu.edu/~acase/classes/spring11/intropr
og/handouts/notes/class5.pdf stion717.htm
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Algorithms: Pseudocode
….like ‘real’ code
1.. If student's grade is greater than or
equal to 60
Print "passed"
else
Print "failed"
2. Set total to zero
Set grade counter to one
While grade counter is less than or
equal to ten
Input the next grade
Add the grade into the total
Set the class average to the total
divided by ten
Print the class average.
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Computer languages need the correct
syntax or they do not work properly.
Pseudocode has no real syntax rules
because it is not real computer code – it is
a natural language that is close to
‘computer ‘language.
The benefit of pseudocode is that it
enables the programmer to concentrate on
the algorithms without worrying about all
the syntax which is specific to a particular
programming language. You can even
write pseudocode without knowing what
programming language you will use for the
solution – and others could take your code
but use it with a different language!
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Algorithms On The Internet (1)
PageRank
Cartoon representation of PageRank.
Source Wikipedia:
http://en.wikipedia.org/wiki/PageRank
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PageRank is a famous algorithm used by
the Google Internet search engine, that
assigns a numeric value that represents
how important a page is on the web. When
one page links to another page, it is
effectively casting a vote for the other
page. The more votes that are cast for a
page, the more important (in theory) the
page must be. PageRank is Google's way
of deciding a page's importance. It matters
because it is one of the factors that
determines a page's ranking in the search
results. It isn't the only factor that Google
uses to rank pages, but it is an important
one.
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Algorithms On The Internet (2)
EdgeRank / Amazon / Facebook
EdgeRank is an algorithm developed by
Facebook to govern what is displayed—
and how high—on each users News Feed.
Amazon uses a number of algorithms. The
popularity list algorithm is the accumulated
sales of a book’s last 30 days compared to
those in its category–but free books given
away only count for roughly 10% of a paid
sale, and price is factored in as well, in that
the higher your price, the more each sale
counts for on the list. Formula:
(sales + (0.1 x free downloads)) x
(unknown sales factor) / last 30 days.
Youtube uses a number of modified
Amazon algorithms.
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Algorithms In The News (1)
‘Right Place Right Time’
Image: Cutegeek.com
Research: Find out more about this type of
algorithm:
http://www.bbc.co.uk/news/business-19286426
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Picture yourself in an expensive designer
clothes shop.
Your smartphone knows where you are –
(thanks to the GPS) - and it alerts your
bank through an automated system that
you've signed up to.
Knowing you've got a history of buying
from similar stores, your bank also knows
that you're running a bit low on cash.
Your phone beeps. A text message. Buy it
in the next 20 minutes and you can borrow
the money at a good rate. Also you'll get
20% off the clothes.
You make the purchase – thanks in part to
a ‘right place, right time’ algorithm.
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Algorithms In The News (2)
Image source: http://pulse2.com
Google Algorithms Sued
Debate: Can the Google algorithm be manipulated?
What problems may a case like this cause for
Google?
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Bettina Wulff - the wife of a former German
president has filed a lawsuit against
Google
to
remove
autocomplete
suggestions that she is claiming as
libel. Some of the suggestions that pop-up
when searching for Bettina include the
words “prostitute” and “red light”.
Head of PR for Google Germany, Kay
Oberbeck, said the site's search terms
were "algorithmically generated" and
"include the popularity of the entered
search terms".
"All terms that appear have been
previously entered by Google users," he
added in a statement.
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