How to Think Like a Computer Scientist - Learning with Python, 2008a

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How to Think Like a Computer Scientist Learning with Python Allen Downey Jeffrey Elkner Chris Meyers Green Tea Press Wellesley, Massachusetts
Copyright c○ 2002 Allen Downey, Jeffrey Elkner, and Chris Meyers. Edited by Shannon Turlington and Lisa Cutler. Cover design by Rebecca Gimenez. Printing history: April 2002: First edition. August 2008: Second printing. Green Tea Press 1GroveSt. P.O. Box 812901 Wellesley, MA 02482 Permission is granted to copy, distribute, and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being “Foreword,” “Preface,” and “Contributor List,” with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the appendix entitled “GNU Free Documentation License.” The GNU Free Documentation License is available from www.gnu.org or by writing to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA. The original form of this book is L A TEX source code. Compiling this L A TEX source has the effect of generating a device-independent representation of a textbook, which can be converted to other formats and printed. The L A TEX source for this book is available from http://www.thinkpython.com Publisher’s Cataloging-in-Publication (provided by Quality Books, Inc.) Downey, Allen How to think like a computer scientist : learning with Python / Allen Downey, Jeffrey Elkner, Chris Meyers. – 1st ed. p. cm. Includes index. ISBN 0-9716775-0-6 LCCN 2002100618 1. Python (Computer program language) I. Elkner, Jeffrey. II. Meyers, Chris. III. Title QA76.73.P98D69 2002 005.13’3 QBI02-200031
Page 2 and 3: How to Think Like a Computer Scient
Page 6 and 7: Foreword By David Beazley As an edu
Page 8 and 9: Preface By Jeff Elkner This book ow
Page 10 and 11: make instant changes whenever someo
Page 12 and 13: call (type) out its name.” Parame
Page 14 and 15: Contributor List To paraphrase the
Page 16 and 17: • Paul Sleigh found an error in C
Page 18 and 19: xvii • Roger Sperberg pointed out
Page 20 and 21: Contents Foreword v Preface vii Con
Page 22 and 23: Contents xxi 4.7 Nested conditional
Page 24 and 25: Contents xxiii 8.15 Matrices . . .
Page 26 and 27: Contents xxv 14.4 A more complicate
Page 28 and 29: Contents xxvii 19.4 Improved Linked
Page 30 and 31: Chapter 1 The way of the program Th
Page 32 and 33: 1.2 What is a program? 3 $ python P
Page 34 and 35: 1.3 What is debugging? 5 1.3.2 Runt
Page 36 and 37: 1.4 Formal and natural languages 7
Page 38 and 39: 1.6 Glossary 9 source code: A progr
Page 40 and 41: Chapter 2 Variables, expressions an
Page 42 and 43: 2.3 Variable names and keywords 13
Page 44 and 45: 2.4 Statements 15 >>> 76trombones =
Page 46 and 47: 2.6 Operators and operands 17 2.6 O
Page 48 and 49: 2.9 Composition 19 On one hand, thi
Page 50 and 51: 2.11 Glossary 21 state diagram: A g
Page 52 and 53: Chapter 3 Functions 3.1 Function ca
Page 54 and 55:
3.4 Math functions 25 >>> minute =
Page 56 and 57:
3.6 Adding new functions 27 problem
Page 58 and 59:
3.7 Definitions and use 29 3.7 Defi
Page 60 and 61:
3.10 Variables and parameters are l
Page 62 and 63:
3.12 Functions with results 33 The
Page 64 and 65:
3.13 Glossary 35 local variable: A
Page 66 and 67:
Chapter 4 Conditionals and recursio
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4.4 Conditional execution 39 In gen
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4.7 Nested conditionals 41 Each con
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4.9 Recursion 43 countdown expects
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4.11 Infinite recursion 45 4.11 Inf
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4.13 Glossary 47 block: A group of
Page 78 and 79:
Chapter 5 Fruitful functions 5.1 Re
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5.2 Program development 51 avoid lo
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5.3 Composition 53 3. Once the prog
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5.5 More recursion 55 But the extra
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5.6 Leap of faith 57 __main__ facto
Page 88 and 89:
5.8 Checking types 59 >>> factorial
Page 90 and 91:
Chapter 6 Iteration 6.1 Multiple as
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6.2 The while statement 63 1. Evalu
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6.3 Tables 65 1.0 0.0 2.0 0.6931471
Page 96 and 97:
6.5 Encapsulation and generalizatio
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6.7 Local variables 69 6.7 Local va
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6.9 Functions 71 the function is ca
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Chapter 7 Strings 7.1 A compound da
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7.3 Traversal and the for loop 75 T
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7.6 Strings are immutable 77 Other
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7.9 The string module 79 As an exer
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7.11 Glossary 81 There are other us
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Chapter 8 Lists A list is an ordere
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8.3 List length 85 If you try to re
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8.6 List operations 87 for VARIABLE
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8.9 List deletion 89 >>> list = [
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8.10 Objects and values 91 8.10 Obj
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8.13 List parameters 93 >>> b[0] =
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8.16 Strings and lists 95 might be
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Chapter 9 Tuples 9.1 Mutability and
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9.3 Tuples as return values 99 9.3
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9.6 Counting 101 >>> randomList(8)
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9.7 Many buckets 103 bucketWidth =
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9.9 Glossary 105 9.9 Glossary Assig
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Chapter 10 Dictionaries The compoun
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10.2 Dictionary methods 109 10.2 Di
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10.5 Hints 111 An alternative is to
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10.6 Long integers 113 Using this v
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10.8 Glossary 115 overflow: A numer
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Chapter 11 Files and exceptions Whi
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11.1 Text files 119 The following f
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11.2 Writing variables 121 the str
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11.3 Directories 123 11.3 Directori
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11.5 Exceptions 125 Or trying to op
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11.6 Glossary 127 format sequence:
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Chapter 12 Classes and objects 12.1
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12.3 Instances as arguments 131 con
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12.5 Rectangles 133 >>> p1 = Point(
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12.8 Copying 135 The variables dwid
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12.9 Glossary 137 12.9 Glossary cla
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Chapter 13 Classes and functions 13
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13.3 Modifiers 141 The output of th
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13.5 Prototype development versus p
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13.8 Glossary 145 Similarly, the te
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Chapter 14 Classes and methods 14.1
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14.3 Another example 149 class Time
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14.5 Optional arguments 151 if done
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14.7 Points revisited 153 >>> curre
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14.9 Polymorphism 155 def __mul__(s
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14.10 Glossary 157 Of course, we in
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Chapter 15 Sets of objects 15.1 Com
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15.3 Class attributes and the str m
Page 192 and 193:
15.5 Decks 163 def __cmp__(self, ot
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15.7 Shuffling the deck 165 >>> dec
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15.9 Glossary 167 class Deck: ... d
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Chapter 16 Inheritance 16.1 Inherit
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16.3 Dealing cards 171 16.3 Dealing
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16.6 OldMaidHand class 173 class Ca
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16.7 OldMaidGame class 175 Hand fra
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16.7 OldMaidGame class 177 class Ol
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16.8 Glossary 179 Hand Allen picked
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Chapter 17 Linked lists 17.1 Embedd
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17.3 Lists as collections 183 node1
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17.5 Infinite lists 185 We invoke t
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17.8 Wrappers and helpers 187 def r
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17.10 Invariants 189 tail = self.ne
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Chapter 18 Stacks 18.1 Abstract dat
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18.4 Pushing and popping 193 def po
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18.7 Evaluating postfix 195 Python
Page 226 and 227:
18.9 Glossary 197 18.9 Glossary abs
Page 228 and 229:
Chapter 19 Queues This chapter pres
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19.3 Performance characteristics 20
Page 232 and 233:
19.5 Priority queue 203 and it is m
Page 234 and 235:
19.6 The Golfer class 205 19.6 The
Page 236 and 237:
Chapter 20 Trees Like linked lists,
Page 238 and 239:
20.2 Traversing trees 209 20.2 Trav
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20.4 Tree traversal 211 def printTr
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20.5 Building an expression tree 21
Page 244 and 245:
20.5 Building an expression tree 21
Page 246 and 247:
20.7 The animal tree 217 Are you th
Page 248 and 249:
20.8 Glossary 219 def yes(ques): fr
Page 250 and 251:
Appendix A Debugging Different kind
Page 252 and 253:
A.2 Runtime errors 223 A.1.1 I can
Page 254 and 255:
A.2 Runtime errors 225 If there is
Page 256 and 257:
A.3 Semantic errors 227 A.3 Semanti
Page 258 and 259:
A.3 Semantic errors 229 return self
Page 260 and 261:
Appendix B Creating a new data type
Page 262 and 263:
B.1 Fraction multiplication 233 >>>
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B.4 Comparing fractions 235 This re
Page 266 and 267:
B.6 Glossary 237 reduce: To change
Page 268 and 269:
Appendix C Recommendations for furt
Page 270 and 271:
C.2 Recommended general computer sc
Page 272 and 273:
Index Make Way for Ducklings, 75 Py
Page 274 and 275:
Index 245 element, 83, 96 embedded
Page 276 and 277:
Index 247 well-formed, 189 list del
Page 278 and 279:
Index 249 redundancy, 7 reference,
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