Computer Science/Computing Chapman & Hall/CRC TEXTBOOKS IN COMPUTING E EXPLORATIONS IN COMPUTING X EXPLORATIONS An Introduction to Computer Science P L and Python Programming IN COMPUTING O R A Explorations in Computing: An Introduction to Computer Science and Py- An Introduction to Computer Science T thon Programming teaches you how to use programming skills to explore I and Python Programming fundamental concepts and computational approaches to solving problems. It O gives you an introduction to computer science concepts and computer pro- N gramming. S The text uses Python as the lab software so that you can seamlessly transition from introductory projects to more advanced studies in later courses. It also I N introduces Python programming, providing you with sufficient programming skills so you can implement your own programs. C The interactive lab projects in each chapter allow you to examine important O ideas in computer science, particularly how algorithms offer computational M solutions to problems. You can type expressions, view results, and run experi- ments that help you understand the concepts in a hands-on way. The Python P software modules for each lab project are available on the author’s website. U Features T • Emphasizes computational thinking and important concepts in computing I N • Gives you sufficient Python programming skills • Includes tutorial projects that encourage you to explore a particular G problem and solve it computationally • Contains standard exercises that test your understanding of the topics covered • Provides the software and other ancillaries on the author’s website Conery John S. Conery K16600 K16600_cover.indd 1 7/30/14 12:26 PM EXPLORATIONS IN COMPUTING An Introduction to Computer Science and Python Programming K16600_FM.indd 1 8/6/14 11:07 AM CHAPMAN & HALL/CRC TEXTBOOKS IN COMPUTING Series Editors John Impagliazzo Andrew McGettrick Professor Emeritus, Hofstra University Department of Computer and Information Sciences University of Strathclyde Aims and Scope This series covers traditional areas of computing, as well as related technical areas, such as software engineering, artificial intelligence, computer engineering, information systems, and information technology. The series will accommodate textbooks for undergraduate and gradu- ate students, generally adhering to worldwide curriculum standards from professional societ- ies. The editors wish to encourage new and imaginative ideas and proposals, and are keen to help and encourage new authors. The editors welcome proposals that: provide groundbreaking and imaginative perspectives on aspects of computing; present topics in a new and exciting context; open up opportunities for emerging areas, such as multi-media, security, and mobile systems; capture new developments and applications in emerging fields of computing; and address topics that provide support for computing, such as mathematics, statistics, life and physical sciences, and business. Published Titles Paul Anderson, Web 2.0 and Beyond: Principles and Technologies Henrik Bærbak Christensen, Flexible, Reliable Software: Using Patterns and Agile Development John S. Conery, Explorations in Computing: An Introduction to Computer Science John S. Conery, Explorations in Computing: An Introduction to Computer Science and Python Programming Ted Herman, A Functional Start to Computing with Python Pascal Hitzler, Markus Krötzsch, and Sebastian Rudolph, Foundations of Semantic Web Technologies Mark J. Johnson, A Concise Introduction to Data Structures using Java Mark J. Johnson, A Concise Introduction to Programming in Python Lisa C. Kaczmarczyk, Computers and Society: Computing for Good Mark C. Lewis, Introduction to the Art of Programming Using Scala Bill Manaris and Andrew R. Brown, Making Music with Computers: Creative Programming in Python Uvais Qidwai and C.H. Chen, Digital Image Processing: An Algorithmic Approach with MATLAB® David D. Riley and Kenny A. Hunt, Computational Thinking for the Modern Problem Solver Henry M. Walker, The Tao of Computing, Second Edition K16600_FM.indd 2 8/6/14 11:07 AM Chapman & Hall/CRC TEXTBOOKS IN COMPUTING EXPLORATIONS IN COMPUTING An Introduction to Computer Science and Python Programming John S. Conery K16600_FM.indd 3 8/6/14 11:07 AM CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2015 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20140630 International Standard Book Number-13: 978-1-4665-7245-4 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material repro- duced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copy- right.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identifica- tion and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Contents Preface ix 1 Introduction 1 1.1 Computation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 TheLimitsofComputation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4 ALaboratoryforComputationalExperiments . . . . . . . . . . . . . . . . . . 12 1.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2 ThePythonWorkbench 17 IntroducingPythonandanenvironmentforinteractiveexperiments 2.1 InteractingwithPython . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2 Numbers,Functions,andtheMathModule. . . . . . . . . . . . . . . . . . . . 24 2.3 Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4 DefiningNewFunctions: defStatements . . . . . . . . . . . . . . . . . . . . . 31 2.5 ConditionalExecution: ifStatements . . . . . . . . . . . . . . . . . . . . . . 36 2.6 Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 2.7 Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 2.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3 TheSieveofEratosthenes 55 Analgorithmforfindingprimenumbers 3.1 TheAlgorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3.2 Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.3 IteratingOverLists: forStatements . . . . . . . . . . . . . . . . . . . . . . . 62 3.4 ListIndexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 3.5 TheAnimatedSieve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.6 AHelperFunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 3.7 The sieve Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 3.8 (cid:169) RunningaProgramfromtheCommandLine . . . . . . . . . . . . . . . . . 81 3.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 v vi 4 AJourneyofaThousandMiles 91 Iterationasastrategyforsolvingcomputationalproblems 4.1 TheLinearSearchAlgorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 4.2 ImplementingLinearSearch . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 4.3 TheInsertionSortAlgorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.4 AHelperFunctionforInsertionSort . . . . . . . . . . . . . . . . . . . . . . . 101 4.5 ImplementingInsertionSort . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 4.6 Scalability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 4.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 5 DivideandConquer 117 Anewstrategy: Breakinglargeproblemsintosmallersubproblems 5.1 TheBinarySearchAlgorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 5.2 ImplementingBinarySearch . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 5.3 BinarySearchExperiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 5.4 TheMergeSortAlgorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 5.5 ImplementingMergeSort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 5.6 MergeSortExperiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 5.7 (cid:169) RecursiveFunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 5.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 6 Spam,Spam,Spam,Mail,andSpam 147 Amachinelearningapproachtofilteringjunkmail 6.1 ACloserLookatStrings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 6.2 ReadingTextfromaFile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 6.3 CountingWordsinaTextFile . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 6.4 Dictionaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 6.5 WordFrequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 6.6 Spamicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 6.7 AnAlgorithmforIdentifyingJunkMail . . . . . . . . . . . . . . . . . . . . . . 176 6.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 7 NowforSomethingCompletelyDifferent 191 Analgorithmforgeneratingrandomnumbers 7.1 PseudorandomNumbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 7.2 NumbersonDemand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 7.3 ModulesandEncapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 7.4 GameswithRandomNumbers . . . . . . . . . . . . . . . . . . . . . . . . . . 208 7.5 RandomShuffles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 7.6 TestsofRandomness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 7.7 DefiningNewObjects: classStatements . . . . . . . . . . . . . . . . . . . . . 224 7.8 (cid:169) AdditionalFeaturesfortheCardClass . . . . . . . . . . . . . . . . . . . . . 234 7.8.1 (cid:169) ClassVariables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 7.8.2 (cid:169) Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 7.8.3 (cid:169) OptionalArguments . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 7.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 vii 8 BitbyBit 249 Binarycodesandalgorithmsfortextcompressionanderrordetection 8.1 BinaryCodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 8.2 CodesforCharacters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 8.3 ErrorDetection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 8.4 ParityBitsandtheExclusiveOR . . . . . . . . . . . . . . . . . . . . . . . . . . 265 8.5 HuffmanTrees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 8.6 HuffmanCodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 8.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 9 TheWaroftheWords 289 Computerarchitectureandmachinelevelprogramming 9.1 Hello,MARS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 9.2 TheTemperatureonMARS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 9.3 Corewar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 9.4 Self-ReferentialCode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 9.5 (cid:169) Clones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 9.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 10 I’dLiketoHaveanArgument,Please 315 AprogramthatunderstandsEnglish(ordoesit?) 10.1 OverviewofELIZA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 10.2 SentencePatterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 10.3 BuildingResponsesfromPartsofSentences . . . . . . . . . . . . . . . . . . . 325 10.4 Substitutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 10.5 AnAlgorithmforHavingaConversation . . . . . . . . . . . . . . . . . . . . . 333 10.6 (cid:169) WritingScriptsforELIZA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 10.7 ELIZAandtheTuringTest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 10.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342 11 TheMusicoftheSpheres 349 ComputersimulationandtheN-bodyproblem 11.1 RunningAroundinCircles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 11.2 TheForceofGravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358 11.3 ForceVectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 11.4 N-BodySimulationoftheSolarSystem . . . . . . . . . . . . . . . . . . . . . . 369 11.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 12 TheTravelingSalesman 379 Ageneticalgorithmforacomputationallydemandingproblem 12.1 MapsandTours . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 12.2 ExhaustiveSearch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 12.3 RandomSearch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 viii 12.4 PointMutations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 12.5 TheGeneticAlgorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 12.6 Crossovers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 12.7 (cid:169)TSPHelperFunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 12.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 Index 417