Depth First Search

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Depth First Search
CS 146 Lecture 9

• Prof. S M Lee
• Department of Computer Science

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• ''Nothing in life is to be feared. It is only to
  be understood.'' - Marie Curie

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Games

• Problem a position in a Chess/Go/ game
• Solution a strategy to guarantee winning game
• Harder than NP problems
• it is not easy to check that a strategy wins

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Presenting Search Abstractly

• Helps to understand the abstract nature of
  search
• search states, search spaces, search trees
• know what particular search algorithms are trying
  to do
• There are two kinds of search algorithm
• Complete
• guaranteed to find solution or prove there is
  none
• Incomplete
• may not find a solution even when it exists
• often more efficient (or there would be no point)
• e.g. Genetic Algorithms
• For now concerned with complete algorithms

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Search States

• Search states summarises the state of search
• A solution tells us everything we need to know
• e.g. in SAT, whether each letter is UPPER or
  lower case
• in TSP, route taken round nodes of graph
• This is a (special) example of a search state
• it contains complete information
• it solves the problem
• In general a search state may not do either of
  those
• it may not specify everything about a possible
  solution
• it may not solve the problem or extend to a
  solution

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Norbert Wiener 1894-1964
Yuk Wing Lee (left) ,Amar G. Bose and Norbert
Wiener
N. Wiener drove 150 miles to a math conference at
Yale University. When the conference was over, he
forgot he came by car, so he returned home by
bus. The next morning, he went out to his garage
to get his car, discovered it was missing, and
complained the police that while he was away,
someone stole his car.
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"We mathematicians who operate with nothing more
expensive than paper and possibly printers' ink
are quite reconciled to the fact that, if we are
working in an active field, our discoveries will
commence to be obsolete at the moment that they
are written down or even at the moment they are
conceived. We know that for a long time
everything we do will be nothing more than the
jumping off point for those who have the
advantage of already being aware of our ultimate
results. This is the meaning of the famous
apothegm of Newton, when he said, "If I have seen
further than other men, it is because I have
stood on the shoulders of giants".(Nobert
Wiener, I am a Mathematician )
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Edsger W.Dijkstra (1930------6 August 2002),
died in cancer
Sometimes we discover unpleasant truths. Whenever
we do so, we are in difficulties suppressing
them is scientifically dishonest, so we must tell
them, but telling them, however, will fire back
on us. If the truths are sufficiently
impalatable, our audience is psychically
incapable of accepting them and we will be
written off as totally unrealistic, hopelessly
idealistic, dangerously revolutionary, foolishly
gullible or what have you. (Besides that, telling
such truths is a sure way of making oneself
unpopular in many circles, and, as such, it is an
act that, in general, is not without personal
risks. Vide Galileo Galilei.....) ---Edsger
W.Dijkstra 18th June 1975