Table Of ContentThe Practice of Computing Using Python
William F. Punch Richard Enbody
Second Edition
ISBN 10: 1-292-02593-X
ISBN 13: 978-1-292-02593-3
Pearson Education Limited
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ISBN 10: 1-292-02593-X
ISBN 13: 978-1-292-02593-3
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A catalogue record for this book is available from the British Library
Printed in the United States of America
11222334453757258583711175317375539
P E A R S O N C U S T O M L I B R AR Y
Table of Contents
1. The Study of Computer Science
William F. Punch/Richard Enbody 1
2. Beginnings
William F. Punch/Richard Enbody 33
3. Control
William F. Punch/Richard Enbody 79
4. Algorithms and Program Development
William F. Punch/Richard Enbody 151
5. Working with Strings
William F. Punch/Richard Enbody 177
6. Files and Exceptions I
William F. Punch/Richard Enbody 225
7. Functions—QuickStart
William F. Punch/Richard Enbody 253
8. Lists and Tuples
William F. Punch/Richard Enbody 281
9. More on Functions
William F. Punch/Richard Enbody 357
10. Disctionaries and Sets
William F. Punch/Richard Enbody 383
11. More Program Development
William F. Punch/Richard Enbody 437
12. Introduction to Classes
William F. Punch/Richard Enbody 475
13. More on Classes
William F. Punch/Richard Enbody 515
I
556666777758246901239775313591
14. Program Devoplement with Classes
William F. Punch/Richard Enbody 559
15. Files and Exceptions II
William F. Punch/Richard Enbody 587
16. Testing
William F. Punch/Richard Enbody 627
17. Recursion: Another Control Mechanism
William F. Punch/Richard Enbody 645
18. Other Fun Stuff with Python
William F. Punch/Richard Enbody 663
19. Appendix: Getting and Using Python
William F. Punch/Richard Enbody 691
20. Appendix: Simple Drawing with Turtle Graphics
William F. Punch/Richard Enbody 703
21. Appendix: Plotting and Numeric Tools: A Quick Survey
William F. Punch/Richard Enbody 715
22. Appendix: Table of UTF-8 One-Byte Encodings
William F. Punch/Richard Enbody 729
Index 731
II
The Study of Computer
Science
Composingcomputerprogramstosolvescientificproblemsislikewriting
poetry.Youmustchooseeverywordwithcareandlinkitwiththeotherwords
inperfectsyntax.
JamesLovelock
1 W H Y C O M P U T E R S C I E N C E ?
IT IS A FAIR QUESTION TO ASK. WHY SHOULD ANYONE BOTHER TO STUDY COMPUTER
science? Furthermore, what is “computer science”? Isn’t this all just about programming?
Allgoodquestions.Wethinkitisworthdiscussingthembeforeyouforgeaheadwithyour
study.
1.1 Importance of Computer Science
Let’sbehonest.Wewouldn’tbewritingthetextandaskingyoutospendyourvaluabletime
ifwedidn’tthinkthatstudyingcomputerscienceisimportant.Thereareacoupleofways
tolookatwhythisistrue.
First, we all know that computers are everywhere, millions upon millions of them.
What were once rare, expensive items are as commonplace as, well, any commodity you
canimagine.(Weweregoingto saytheproverbialtoaster,buttherearemanytimesmore
computersthantoasters.Infact,thereislikelyasmallcomputerinyourtoaster!)However,
thatisn’tenoughofareason.Therearemillionsandmillionsofcarsanduniversitiesdon’t
requireautomechanicsasanareaofstudy.
A second aspect is that computers are not only common, but they are also more uni-
versally applicable than any other commodity in history. A car is good for transportation,
FromChapter0ofThePracticeofComputingUsingPython,SecondEdition.WilliamF.Punch/RichardEnbody.
Copyright©2013byPearsonEducation,Inc.PublishedbyPearsonAddison-Wesley.Allrightsreserved.
1
THE STUDY OF COMPUTER SCIENCE
butacomputercanbeusedinsomanysituations.Infact,thereisalmostnoareaonecan
imagine where a computer would not be useful. That is a key attribute. No matter what
your area of interest, a computer could be useful there as a tool. The computer’s universal
utilityisunique,andlearninghowtousesuchatoolisimportant.
1.2 Computer Science Around You
Computingsurroundsyou,anditiscomputersciencethatputitthere.Thereareamulti-
tudeofexamples,buthereareafewworthnoting.
Social Networking The tools that facilitate social networking sites such as Facebook or
Twitterare,ofcourse,computerprograms.However,thetoolsthathelpstudythein-
teractions within social networks involve important computer science fields such as
graphtheory.Forexample,IraqidictatorSaddamHusseinwaslocatedusinggraphthe-
oreticanalysisofhissocialnetwork.
SmartphonesSmartphones aresmall, very portablecomputers. Apps for smartphones are
simplycomputerprogramswrittenspecificallyforsmartphones.
YourCarYourcarhostsmorethanadozencomputers.Theycontroltheengine,thebrakes,
theaudiosystem,thenavigation,andtheclimatecontrolsystem.Theydetermineifa
crash is occurring and trigger the air bags. Some cars park automatically or apply the
brakesifacrashisimminent.
The Internet The backbone of the Internet is a collection of connected computers called
routersthatdecidethebestwaytosendinformationtoitsdestination.
1.3 Computer “Science”
Anyfieldthathasthewordscienceinitsnameisguaranteedtherebynottobe
ascience.
FrankHarary
A popular view of the term computer science is that it is a glorified way to say “computer
programming.”Itistruethatcomputerprogrammingisoftenthewaythatpeoplearein-
troducedtocomputingingeneral,andthatcomputerprogrammingistheprimaryreason
manytakecomputingcourses.However,thereisindeedmoretocomputingthanprogram-
ming,hencetheterm“computerscience.”Hereareafewexamples.
Theory of Computation
Before there were the vast numbers of computers that are available today, scientists were
thinkingaboutwhatitmeanstodocomputingandwhatthelimitsmightbe.Theywould
ask questions, such as whether there exist problems that we can conceive of but cannot
2
THE STUDY OF COMPUTER SCIENCE
compute. It turns out there are. One of these problems, called the “halting problem,”1
cannot be solved by a program running on any computer. Knowing what you can and
cannotsolveonacomputerisanimportantissueandasubjectofstudyamongcomputer
scientiststhatfocusonthetheoryofcomputation.
Computational Efficiency
The fact that a problem is computable does not mean it is easily computed. Knowing
roughlyhowdifficultaproblemistosolveisalsoveryimportant.Determiningameaningful
measure of difficulty is, in itself, an interesting issue, but imagine we are concerned only
withtime.Considerdesigningasolutiontoaproblemthat,aspartofthesolution,required
youtosort100,000items(say,cancerpatientrecords,orasteroidnames,ormovieepisodes,
etc.). A slow algorithm, such as the sorting algorithm called the Bubble Sort, might take
approximately 800 seconds (about 13 minutes); another sorting algorithm called Quick
Sortmighttakeapproximately0.3second.Thatisadifferenceofaround2400times!That
largeadifferencemightdeterminewhetheritisworthdoing.Ifyouarecreatingasolution,
itwouldbegoodtoknowwhatmakesyoursolutionsloworwhatmakesitfast.
Algorithms and Data Structures
Algorithms and data structures are the currency of the computer scientist. Algorithms are
themethodsusedtosolveproblems,anddatastructuresaretheorganizationsofdatathat
thealgorithmsuse.Thesetwoconceptsaredistinct:ageneralapproachtosolvingaproblem
(such as searching for a particular value, sorting a list of objects, encrypting a message)
differs from the organization of the data that is being processed (as a list of objects, as a
dictionaryofkey-valuepairs,asa“tree”ofrecords).However,theyarealsotightlycoupled.
Furthermore,bothalgorithmsanddatastructurescanbeexaminedindependentlyofhow
they might be programmed. That is, one designs algorithms and data structures and then
actuallyimplementstheminaparticularcomputerprogram.Understandingabstractlyhow
todesignbothalgorithmsanddatastructuresindependentoftheprogramminglanguageis
criticalforwritingcorrectandefficientcode.
Parallel Processing
Itmayseemoddtoincludewhatmanyconsideranadvancedtopic,butparallelprocessing,
usingmultiplecomputerstosolveaproblem,isanissueforeveryonethesedays.Why?As
it turns out, most computers come with at least two processors or CPUs (see Section 6),
andmanycomewithfourormore.ThePlayStation3TM gameconsoleusesaspecialIBM
chipthathas8processors,andIntelhasrecentlyannouncedprototypesofchipsthathave
80processors!Whatdoesthismeantous,asbothconsumersandnewcomputerscientists?
1http://en.wikipedia.org/wiki/Haltingproblem
3
THE STUDY OF COMPUTER SCIENCE
Theansweristhatnewalgorithms,datastructures, andprogrammingparadigmswill
beneededtotakeadvantageofthisnewprocessingenvironment.Orchestratingmanypro-
cessorstosolveaproblemisanexcitingandchallengingtask.
Software Engineering
Even the process of writing programs itself has developed its own sub-discipline within
computerscience.Dubbed“softwareengineering,”itconcernstheprocessofcreatingpro-
grams: from designing the algorithms they use to supporting testing and maintenance of
the program once created. There is even a discipline interested in representing a devel-
opedprogramasamathematicalentitysothatonecanprove whataprogramwilldoonce
written.
Many Others
We have provided but a taste of the many fields that make computer science such a won-
derfullyrichareatoexplore.Everyareathatusescomputationbringsitsownproblemsto
beexplored.
1.4 Computer Science Through Computer Programming
Wehavetriedtomakethepointthatcomputerscienceisnotjustprogramming.However,it
isalsotruethatwefocusonjustthataspectofcomputerscience:programming.Beginning
with“problemsolvingthroughprogramming”allowsonetoexplorepiecesofthecomputer
sciencelandscapeastheynaturallyarise.
2 T H E D I F F I C U LT Y A N D P R O M I S E
O F P R O G R A M M I N G
Ifcomputerscience,particularlycomputerprogramming,issointeresting,whydoesn’tev-
erybodydoit?Thetruthisthatitcanbehard.Weareoftenaskedbybeginningstudents,
“Whyisprogrammingsohard?”Evengrizzledprogrammingveterans,whenhonestlylook-
ing back at their first experience, remember how difficult that first programming course
was.Why?Understandingwhyitmightbehardgivesyouanedgeonwhatyoucandoto
controlthedifficulty.
2.1 Difficulty 1: Two Things at Once
Let’sconsideranexample.Letussaythat,whenyouwalkintothatfirstdayofProgramming
101,youdiscoverthecourseisnotaboutprogrammingbutFrenchpoetry.Frenchpoetry?
4
THE STUDY OF COMPUTER SCIENCE
Yes,Frenchpoetry.Imaginethatyoucomeinandtheprofessorpoststhefollowingexcerpt
fromapoemontheboard.
AuneDamoysellemalade
Mamignonne,
Jevousdonne
Lebonjour;
Lese´jjour
C’estprison.
Cle´mentMarot
Your assigned task is to translate this poetry into English (or German, or Russian, or
whateverlanguageisyournativetongue).Letusalsoassume,forthemoment,that:
(a) YoudonotknowFrench.
(b) Youhaveneverstudiedpoetry.
Youhavetwoproblemsonyourhands.First,youhavetogainabetterunderstanding
ofthesyntaxandsemantics(theformandsubstance)oftheFrenchlanguage.Second,you
needtolearnmoreaboutthe“rules”ofpoetryandwhatconstitutesagoodpoem.
Lestyouthinkthatthisisatrivialmatter,anentirebookhasbeenwrittenbyDouglas
Hofstadterontheverysubjectofthedifficultyoftranslatingthisonepoem(“LeTonbeau
deMarot”).
Sowhat’syourfirstmove?Mostpeoplewouldbreakoutadictionaryand,linebyline,
try to translate the poem. Hofstadter, in his book, does exactly that, producing the crude
translationinFigure1.
MySweet/Cute
[One](Feminine)
My sweet/cute [one]
(feminine)
AuneDamoysellemalade
I[to]you(respectful)
give/bid/convey
The good day (i.e., a Mamignonne,
hello,i.e.,greetings). Jevousdonne
Thestay/sojourn/ Lebonjour;
visit(i.e.,quarantine) Lese´jjour
{It}isprison. C’estprison.
FIGURE1 Crudetranslationofexcerpt.
5
