Think Python How to Think Like a Computer Scientist (Allen B. Downey) (Z-Library)

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Allen B. Downey Think Python HOW TO THINK LIKE A COMPUTER SCIENTIST 2nd Edition Updated for Python 3

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Allen B. Downey Boston Think Python SECOND EDITION

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978-1-491-93936-9 [LSI] Think Python by Allen B. Downey Copyright © 2016 Allen Downey. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles (http://safaribooksonline.com). For more information, contact our corporate/ institutional sales department: 800-998-9938 or corporate@oreilly.com. Editor: Meghan Blanchette Production Editor: Kristen Brown Copyeditor: Nan Reinhardt Proofreader: Amanda Kersey Indexer: Allen Downey Interior Designer: David Futato Cover Designer: Karen Montgomery Illustrator: Rebecca Demarest August 2012: First Edition December 2015: Second Edition Revision History for the Second Edition 2015-11-20: First Release See http://oreilly.com/catalog/errata.csp?isbn=9781491939369 for release details. The O’Reilly logo is a registered trademark of O’Reilly Media, Inc. Think Python, the cover image of a Carolina parrot, and related trade dress are trademarks of O’Reilly Media, Inc. While the publisher and the author have used good faith efforts to ensure that the information and instructions contained in this work are accurate, the publisher and the author disclaim all responsibility for errors or omissions, including without limitation responsibility for damages resulting from the use of or reliance on this work. Use of the information and instructions contained in this work is at your own risk. If any code samples or other technology this work contains or describes is subject to open source licenses or the intellectual property rights of others, it is your responsibility to ensure that your use thereof complies with such licenses and/or rights. Think Python is available under the Creative Commons Attribution-NonCommercial 3.0 Unported License. The author maintains an online version at http://greenteapress.com/thinkpython2/.

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Table of Contents Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi 1. The Way of the Program. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 What Is a Program? 1 Running Python 2 The First Program 3 Arithmetic Operators 3 Values and Types 4 Formal and Natural Languages 5 Debugging 7 Glossary 8 Exercises 9 2. Variables, Expressions and Statements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Assignment Statements 11 Variable Names 12 Expressions and Statements 12 Script Mode 13 Order of Operations 14 String Operations 15 Comments 15 Debugging 16 Glossary 17 Exercises 18 3. Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Function Calls 21 Math Functions 22 iii

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Composition 23 Adding New Functions 23 Definitions and Uses 25 Flow of Execution 25 Parameters and Arguments 26 Variables and Parameters Are Local 27 Stack Diagrams 28 Fruitful Functions and Void Functions 29 Why Functions? 30 Debugging 30 Glossary 31 Exercises 32 4. Case Study: Interface Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 The turtle Module 35 Simple Repetition 37 Exercises 38 Encapsulation 38 Generalization 39 Interface Design 40 Refactoring 41 A Development Plan 42 docstring 43 Debugging 43 Glossary 44 Exercises 44 5. Conditionals and Recursion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Floor Division and Modulus 47 Boolean Expressions 48 Logical Operators 49 Conditional Execution 49 Alternative Execution 49 Chained Conditionals 50 Nested Conditionals 50 Recursion 51 Stack Diagrams for Recursive Functions 53 Infinite Recursion 53 Keyboard Input 54 Debugging 55 Glossary 56 Exercises 57 iv | Table of Contents

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6. Fruitful Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Return Values 61 Incremental Development 62 Composition 64 Boolean Functions 65 More Recursion 66 Leap of Faith 68 One More Example 68 Checking Types 69 Debugging 70 Glossary 71 Exercises 72 7. Iteration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Reassignment 75 Updating Variables 76 The while Statement 77 break 78 Square Roots 79 Algorithms 81 Debugging 81 Glossary 82 Exercises 82 8. Strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 A String Is a Sequence 85 len 86 Traversal with a for Loop 86 String Slices 87 Strings Are Immutable 88 Searching 89 Looping and Counting 89 String Methods 90 The in Operator 91 String Comparison 92 Debugging 92 Glossary 94 Exercises 95 9. Case Study: Word Play. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Reading Word Lists 99 Exercises 100 Table of Contents | v

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Search 101 Looping with Indices 103 Debugging 104 Glossary 105 Exercises 105 10. Lists. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 A List Is a Sequence 107 Lists Are Mutable 108 Traversing a List 109 List Operations 110 List Slices 110 List Methods 111 Map, Filter and Reduce 111 Deleting Elements 113 Lists and Strings 113 Objects and Values 114 Aliasing 115 List Arguments 116 Debugging 118 Glossary 119 Exercises 120 11. Dictionaries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 A Dictionary Is a Mapping 125 Dictionary as a Collection of Counters 127 Looping and Dictionaries 128 Reverse Lookup 129 Dictionaries and Lists 130 Memos 131 Global Variables 133 Debugging 134 Glossary 135 Exercises 137 12. Tuples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Tuples Are Immutable 139 Tuple Assignment 141 Tuples as Return Values 141 Variable-Length Argument Tuples 142 Lists and Tuples 143 Dictionaries and Tuples 144 vi | Table of Contents

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Sequences of Sequences 146 Debugging 147 Glossary 148 Exercises 148 13. Case Study: Data Structure Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Word Frequency Analysis 151 Random Numbers 152 Word Histogram 153 Most Common Words 155 Optional Parameters 155 Dictionary Subtraction 156 Random Words 157 Markov Analysis 158 Data Structures 159 Debugging 161 Glossary 162 Exercises 163 14. Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Persistence 165 Reading and Writing 166 Format Operator 166 Filenames and Paths 167 Catching Exceptions 169 Databases 169 Pickling 170 Pipes 171 Writing Modules 172 Debugging 173 Glossary 174 Exercises 175 15. Classes and Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Programmer-Defined Types 177 Attributes 178 Rectangles 179 Instances as Return Values 181 Objects Are Mutable 181 Copying 182 Debugging 183 Glossary 184 Table of Contents | vii

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Exercises 185 16. Classes and Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Time 187 Pure Functions 188 Modifiers 189 Prototyping versus Planning 190 Debugging 192 Glossary 192 Exercises 193 17. Classes and Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Object-Oriented Features 195 Printing Objects 196 Another Example 198 A More Complicated Example 198 The init Method 199 The __str__ Method 200 Operator Overloading 200 Type-Based Dispatch 201 Polymorphism 202 Interface and Implementation 203 Debugging 204 Glossary 204 Exercises 205 18. Inheritance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Card Objects 207 Class Attributes 208 Comparing Cards 210 Decks 211 Printing the Deck 211 Add, Remove, Shuffle and Sort 212 Inheritance 213 Class Diagrams 214 Data Encapsulation 215 Debugging 217 Glossary 218 Exercises 219 19. The Goodies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Conditional Expressions 223 viii | Table of Contents

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List Comprehensions 224 Generator Expressions 225 any and all 226 Sets 226 Counters 228 defaultdict 229 Named Tuples 230 Gathering Keyword Args 232 Glossary 233 Exercises 233 20. Debugging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 Syntax Errors 235 I keep making changes and it makes no difference. 237 Runtime Errors 237 My program does absolutely nothing. 237 My program hangs. 238 When I run the program I get an exception. 239 I added so many print statements I get inundated with output. 240 Semantic Errors 241 My program doesn’t work. 241 I’ve got a big hairy expression and it doesn’t do what I expect. 242 I’ve got a function that doesn’t return what I expect. 243 I’m really, really stuck and I need help. 243 No, I really need help. 243 21. Analysis of Algorithms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Order of Growth 246 Analysis of Basic Python Operations 248 Analysis of Search Algorithms 250 Hashtables 251 Glossary 255 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Table of Contents | ix

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Preface The Strange History of This Book In January 1999 I was preparing to teach an introductory programming class in Java. I had taught it three times and I was getting frustrated. The failure rate in the class was too high and, even for students who succeeded, the overall level of achievement was too low. One of the problems I saw was the books. They were too big, with too much unneces‐ sary detail about Java, and not enough high-level guidance about how to program. And they all suffered from the trapdoor effect: they would start out easy, proceed gradually, and then somewhere around Chapter 5 the bottom would fall out. The stu‐ dents would get too much new material, too fast, and I would spend the rest of the semester picking up the pieces. Two weeks before the first day of classes, I decided to write my own book. My goals were: • Keep it short. It is better for students to read 10 pages than not read 50 pages. • Be careful with vocabulary. I tried to minimize jargon and define each term at first use. • Build gradually. To avoid trapdoors, I took the most difficult topics and split them into a series of small steps. • Focus on programming, not the programming language. I included the mini‐ mum useful subset of Java and left out the rest. I needed a title, so on a whim I chose How to Think Like a Computer Scientist. My first version was rough, but it worked. Students did the reading, and they under‐ stood enough that I could spend class time on the hard topics, the interesting topics and (most important) letting the students practice. xi

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I released the book under the GNU Free Documentation License, which allows users to copy, modify, and distribute the book. What happened next is the cool part. Jeff Elkner, a high school teacher in Virginia, adopted my book and translated it into Python. He sent me a copy of his translation, and I had the unusual experience of learning Python by reading my own book. As Green Tea Press, I published the first Python version in 2001. In 2003 I started teaching at Olin College and I got to teach Python for the first time. The contrast with Java was striking. Students struggled less, learned more, worked on more interesting projects, and generally had a lot more fun. Since then I’ve continued to develop the book, correcting errors, improving some of the examples and adding material, especially exercises. The result is this book, now with the less grandiose title Think Python. Some of the changes are: • I added a section about debugging at the end of each chapter. These sections present general techniques for finding and avoiding bugs, and warnings about Python pitfalls. • I added more exercises, ranging from short tests of understanding to a few sub‐ stantial projects. Most exercises include a link to my solution. • I added a series of case studies—longer examples with exercises, solutions, and discussion. • I expanded the discussion of program development plans and basic design patterns. • I added appendices about debugging and analysis of algorithms. The second edition of Think Python has these new features: • The book and all supporting code have been updated to Python 3. • I added a few sections, and more details on the Web, to help beginners get started running Python in a browser, so you don’t have to deal with installing Python until you want to. • For “The turtle Module” on page 35 I switched from my own turtle graphics package, called Swampy, to a more standard Python module, turtle, which is easier to install and more powerful. • I added a new chapter called “The Goodies”, which introduces some additional Python features that are not strictly necessary, but sometimes handy. xii | Preface

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I hope you enjoy working with this book, and that it helps you learn to program and think like a computer scientist, at least a little bit. —Allen B. Downey Olin College Conventions Used in This Book The following typographical conventions are used in this book: Italic Indicates new terms, URLs, email addresses, filenames, and file extensions. Bold Indicates terms defined in the Glossary. Constant width Used for program listings, as well as within paragraphs to refer to program ele‐ ments such as variable or function names, databases, data types, environment variables, statements, and keywords. Constant width bold Shows commands or other text that should be typed literally by the user. Constant width italic Shows text that should be replaced with user-supplied values or by values deter‐ mined by context. Using Code Examples Supplemental material (code examples, exercises, etc.) is available for download at http://www.greenteapress.com/thinkpython2/code. This book is here to help you get your job done. In general, if example code is offered with this book, you may use it in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a signifi‐ cant amount of example code from this book into your product’s documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “Think Python, 2nd Edition, by Allen B. Downey (O’Reilly). Copyright 2016 Allen Downey, 978-1-4919-3936-9.” Preface | xiii

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If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at permissions@oreilly.com. Safari® Books Online Safari Books Online (www.safaribooksonline.com) is an on- demand digital library that delivers expert content in both book and video form from the world’s leading authors in tech‐ nology and business. Technology professionals, software developers, web designers, and business and crea‐ tive professionals use Safari Books Online as their primary resource for research, problem solving, learning, and certification training. Safari Books Online offers a range of plans and pricing for enterprise, government, and education, and individuals. Members have access to thousands of books, training videos, and prepublication manuscripts in one fully searchable database from publishers like O’Reilly Media, Prentice Hall Professional, Addison-Wesley Professional, Microsoft Press, Sams, Que, Peachpit Press, Focal Press, Cisco Press, John Wiley & Sons, Syngress, Morgan Kauf‐ mann, IBM Redbooks, Packt, Adobe Press, FT Press, Apress, Manning, New Riders, McGraw-Hill, Jones & Bartlett, Course Technology, and hundreds more. For more information about Safari Books Online, please visit us online. How to Contact Us Please address comments and questions concerning this book to the publisher: O’Reilly Media, Inc. 1005 Gravenstein Highway North Sebastopol, CA 95472 800-998-9938 (in the United States or Canada) 707-829-0515 (international or local) 707-829-0104 (fax) We have a web page for this book, where we list errata, examples, and any additional information. You can access this page at http://bit.ly/think-python_2E. To comment or ask technical questions about this book, send email to bookques‐ tions@oreilly.com. For more information about our books, courses, conferences, and news, see our web‐ site at http://www.oreilly.com. xiv | Preface

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Find us on Facebook: http://facebook.com/oreilly Follow us on Twitter: http://twitter.com/oreillymedia Watch us on YouTube: http://www.youtube.com/oreillymedia Acknowledgments Many thanks to Jeff Elkner, who translated my Java book into Python, which got this project started and introduced me to what has turned out to be my favorite language. Thanks also to Chris Meyers, who contributed several sections to How to Think Like a Computer Scientist. Thanks to the Free Software Foundation for developing the GNU Free Documenta‐ tion License, which helped make my collaboration with Jeff and Chris possible, and Creative Commons for the license I am using now. Thanks to the editors at Lulu who worked on How to Think Like a Computer Scientist. Thanks to the editors at O’Reilly Media who worked on Think Python. Thanks to all the students who worked with earlier versions of this book and all the contributors (listed below) who sent in corrections and suggestions. Contributor List More than 100 sharp-eyed and thoughtful readers have sent in suggestions and cor‐ rections over the past few years. Their contributions, and enthusiasm for this project, have been a huge help. If you have a suggestion or correction, please send email to feedback@thinkpy‐ thon.com. If I make a change based on your feedback, I will add you to the contribu‐ tor list (unless you ask to be omitted). If you include at least part of the sentence the error appears in, that makes it easy for me to search. Page and section numbers are fine, too, but not quite as easy to work with. Thanks! • Lloyd Hugh Allen sent in a correction to Section 8.4. • Yvon Boulianne sent in a correction of a semantic error in Chapter 5. • Fred Bremmer submitted a correction in Section 2.1. • Jonah Cohen wrote the Perl scripts to convert the LaTeX source for this book into beauti‐ ful HTML. • Michael Conlon sent in a grammar correction in Chapter 2 and an improvement in style in Chapter 1, and he initiated discussion on the technical aspects of interpreters. • Benoit Girard sent in a correction to a humorous mistake in Section 5.6. Preface | xv

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• Courtney Gleason and Katherine Smith wrote horsebet.py, which was used as a case study in an earlier version of the book. Their program can now be found on the website. • Lee Harr submitted more corrections than we have room to list here, and indeed he should be listed as one of the principal editors of the text. • James Kaylin is a student using the text. He has submitted numerous corrections. • David Kershaw fixed the broken catTwice function in Section 3.10. • Eddie Lam has sent in numerous corrections to Chapters 1, 2, and 3. He also fixed the Makefile so that it creates an index the first time it is run and helped us set up a version‐ ing scheme. • Man-Yong Lee sent in a correction to the example code in Section 2.4. • David Mayo pointed out that the word “unconsciously” in Chapter 1 needed to be changed to “subconsciously”. • Chris McAloon sent in several corrections to Sections 3.9 and 3.10. • Matthew J. Moelter has been a long-time contributor who sent in numerous corrections and suggestions to the book. • Simon Dicon Montford reported a missing function definition and several typos in Chap‐ ter 3. He also found errors in the increment function in Chapter 13. • John Ouzts corrected the definition of “return value” in Chapter 3. • Kevin Parks sent in valuable comments and suggestions as to how to improve the distri‐ bution of the book. • David Pool sent in a typo in the glossary of Chapter 1, as well as kind words of encourage‐ ment. • Michael Schmitt sent in a correction to the chapter on files and exceptions. • Robin Shaw pointed out an error in Section 13.1, where the printTime function was used in an example without being defined. • Paul Sleigh found an error in Chapter 7 and a bug in Jonah Cohen’s Perl script that gener‐ ates HTML from LaTeX. • Craig T. Snydal is testing the text in a course at Drew University. He has contributed sev‐ eral valuable suggestions and corrections. • Ian Thomas and his students are using the text in a programming course. They are the first ones to test the chapters in the latter half of the book, and they have made numerous corrections and suggestions. • Keith Verheyden sent in a correction in Chapter 3. • Peter Winstanley let us know about a longstanding error in our Latin in Chapter 3. • Chris Wrobel made corrections to the code in the chapter on file I/O and exceptions. • Moshe Zadka has made invaluable contributions to this project. In addition to writing the first draft of the chapter on Dictionaries, he provided continual guidance in the early stages of the book. • Christoph Zwerschke sent several corrections and pedagogic suggestions, and explained the difference between gleich and selbe. xvi | Preface

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• James Mayer sent us a whole slew of spelling and typographical errors, including two in the contributor list. • Hayden McAfee caught a potentially confusing inconsistency between two examples. • Angel Arnal is part of an international team of translators working on the Spanish version of the text. He has also found several errors in the English version. • Tauhidul Hoque and Lex Berezhny created the illustrations in Chapter 1 and improved many of the other illustrations. • Dr. Michele Alzetta caught an error in Chapter 8 and sent some interesting pedagogic comments and suggestions about Fibonacci and Old Maid. • Andy Mitchell caught a typo in Chapter 1 and a broken example in Chapter 2. • Kalin Harvey suggested a clarification in Chapter 7 and caught some typos. • Christopher P. Smith caught several typos and helped us update the book for Python 2.2. • David Hutchins caught a typo in the Foreword. • Gregor Lingl is teaching Python at a high school in Vienna, Austria. He is working on a German translation of the book, and he caught a couple of bad errors in Chapter 5. • Julie Peters caught a typo in the Preface. • Florin Oprina sent in an improvement in makeTime, a correction in printTime, and a nice typo. • D. J. Webre suggested a clarification in Chapter 3. • Ken found a fistful of errors in Chapters 8, 9 and 11. • Ivo Wever caught a typo in Chapter 5 and suggested a clarification in Chapter 3. • Curtis Yanko suggested a clarification in Chapter 2. • Ben Logan sent in a number of typos and problems with translating the book into HTML. • Jason Armstrong saw the missing word in Chapter 2. • Louis Cordier noticed a spot in Chapter 16 where the code didn’t match the text. • Brian Cain suggested several clarifications in Chapters 2 and 3. • Rob Black sent in a passel of corrections, including some changes for Python 2.2. • Jean-Philippe Rey at Ecole Centrale Paris sent a number of patches, including some updates for Python 2.2 and other thoughtful improvements. • Jason Mader at George Washington University made a number of useful suggestions and corrections. • Jan Gundtofte-Bruun reminded us that “a error” is an error. • Abel David and Alexis Dinno reminded us that the plural of “matrix” is “matrices”, not “matrixes”. This error was in the book for years, but two readers with the same initials reported it on the same day. Weird. • Charles Thayer encouraged us to get rid of the semicolons we had put at the ends of some statements and to clean up our use of “argument” and “parameter”. • Roger Sperberg pointed out a twisted piece of logic in Chapter 3. • Sam Bull pointed out a confusing paragraph in Chapter 2. Preface | xvii

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• Andrew Cheung pointed out two instances of “use before def ”. • C. Corey Capel spotted a missing word and a typo in Chapter 4. • Alessandra helped clear up some Turtle confusion. • Wim Champagne found a braino in a dictionary example. • Douglas Wright pointed out a problem with floor division in arc. • Jared Spindor found some jetsam at the end of a sentence. • Lin Peiheng sent a number of very helpful suggestions. • Ray Hagtvedt sent in two errors and a not-quite-error. • Torsten Hübsch pointed out an inconsistency in Swampy. • Inga Petuhhov corrected an example in Chapter 14. • Arne Babenhauserheide sent several helpful corrections. • Mark E. Casida is is good at spotting repeated words. • Scott Tyler filled in a that was missing. And then sent in a heap of corrections. • Gordon Shephard sent in several corrections, all in separate emails. • Andrew Turner spotted an error in Chapter 8. • Adam Hobart fixed a problem with floor division in arc. • Daryl Hammond and Sarah Zimmerman pointed out that I served up math.pi too early. And Zim spotted a typo. • George Sass found a bug in a Debugging section. • Brian Bingham suggested Exercise 11-5. • Leah Engelbert-Fenton pointed out that I used tuple as a variable name, contrary to my own advice. And then found a bunch of typos and a “use before def ”. • Joe Funke spotted a typo. • Chao-chao Chen found an inconsistency in the Fibonacci example. • Jeff Paine knows the difference between space and spam. • Lubos Pintes sent in a typo. • Gregg Lind and Abigail Heithoff suggested Exercise 14-3. • Max Hailperin has sent in a number of corrections and suggestions. Max is one of the authors of the extraordinary Concrete Abstractions (Course Technology, 1998), which you might want to read when you are done with this book. • Chotipat Pornavalai found an error in an error message. • Stanislaw Antol sent a list of very helpful suggestions. • Eric Pashman sent a number of corrections for Chapters 4–11. • Miguel Azevedo found some typos. • Jianhua Liu sent in a long list of corrections. • Nick King found a missing word. • Martin Zuther sent a long list of suggestions. xviii | Preface