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

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Chapter 17 Linked lists 17.1 Embedded references We have seen examples of attributes that refer to other objects, which we called embedded references (see Section 12.8). A common data structure, the linked list, takes advantage of this feature. Linked lists are made up of nodes, where each node contains a reference to the next node in the list. In addition, each node contains a unit of data called the cargo. A linked list is considered a recursive data structure because it has a recursive definition. A linked list is either: • the empty list, represented by None, or • a node that contains a cargo object and a reference to a linked list. Recursive data structures lend themselves to recursive methods. 17.2 The Node class As usual when writing a new class, we’ll start with the initialization and str methods so that we can test the basic mechanism of creating and displaying the new type:
182 Linked lists class Node: def __init__(self, cargo=None, next=None): self.cargo = cargo self.next = next def __str__(self): return str(self.cargo) As usual, the parameters for the initialization method are optional. By default, both the cargo and the link, next, aresettoNone. The string representation of a node is just the string representation of the cargo. Since any value can be passed to the str function, we can store any value in a list. To test the implementation so far, we can create a Node and print it: >>> node = Node("test") >>> print node test To make it interesting, we need a list with more than one node: >>> node1 = Node(1) >>> node2 = Node(2) >>> node3 = Node(3) This code creates three nodes, but we don’t have a list yet because the nodes are not linked. The state diagram looks like this: node1 node2 node3 cargo 1 cargo 2 cargo 3 next None next None next None To link the nodes, we have to make the first node refer to the second and the second node refer to the third: >>> node1.next = node2 >>> node2.next = node3 The reference of the third node is None, which indicates that it is the end of the list. Now the state diagram looks like this:
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How to Think Like a Computer Scient
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How to Think Like a Computer Scient
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Foreword By David Beazley As an edu
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Preface By Jeff Elkner This book ow
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make instant changes whenever someo
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call (type) out its name.” Parame
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Contributor List To paraphrase the
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• Paul Sleigh found an error in C
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xvii • Roger Sperberg pointed out
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Contents Foreword v Preface vii Con
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Contents xxi 4.7 Nested conditional
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Contents xxiii 8.15 Matrices . . .
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Contents xxv 14.4 A more complicate
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Contents xxvii 19.4 Improved Linked
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Chapter 1 The way of the program Th
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1.2 What is a program? 3 $ python P
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1.3 What is debugging? 5 1.3.2 Runt
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1.4 Formal and natural languages 7
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1.6 Glossary 9 source code: A progr
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Chapter 2 Variables, expressions an
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2.3 Variable names and keywords 13
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2.4 Statements 15 >>> 76trombones =
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2.6 Operators and operands 17 2.6 O
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2.9 Composition 19 On one hand, thi
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2.11 Glossary 21 state diagram: A g
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Chapter 3 Functions 3.1 Function ca
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3.4 Math functions 25 >>> minute =
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3.6 Adding new functions 27 problem
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3.7 Definitions and use 29 3.7 Defi
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3.10 Variables and parameters are l
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3.12 Functions with results 33 The
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3.13 Glossary 35 local variable: A
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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
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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
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5.8 Checking types 59 >>> factorial
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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
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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
Page 160 and 161: 12.3 Instances as arguments 131 con
Page 162 and 163: 12.5 Rectangles 133 >>> p1 = Point(
Page 164 and 165: 12.8 Copying 135 The variables dwid
Page 166 and 167: 12.9 Glossary 137 12.9 Glossary cla
Page 168 and 169: Chapter 13 Classes and functions 13
Page 170 and 171: 13.3 Modifiers 141 The output of th
Page 172 and 173: 13.5 Prototype development versus p
Page 174 and 175: 13.8 Glossary 145 Similarly, the te
Page 176 and 177: Chapter 14 Classes and methods 14.1
Page 178 and 179: 14.3 Another example 149 class Time
Page 180 and 181: 14.5 Optional arguments 151 if done
Page 182 and 183: 14.7 Points revisited 153 >>> curre
Page 184 and 185: 14.9 Polymorphism 155 def __mul__(s
Page 186 and 187: 14.10 Glossary 157 Of course, we in
Page 188 and 189: Chapter 15 Sets of objects 15.1 Com
Page 190 and 191: 15.3 Class attributes and the str m
Page 192 and 193: 15.5 Decks 163 def __cmp__(self, ot
Page 194 and 195: 15.7 Shuffling the deck 165 >>> dec
Page 196 and 197: 15.9 Glossary 167 class Deck: ... d
Page 198 and 199: Chapter 16 Inheritance 16.1 Inherit
Page 200 and 201: 16.3 Dealing cards 171 16.3 Dealing
Page 202 and 203: 16.6 OldMaidHand class 173 class Ca
Page 204 and 205: 16.7 OldMaidGame class 175 Hand fra
Page 206 and 207: 16.7 OldMaidGame class 177 class Ol
Page 208 and 209: 16.8 Glossary 179 Hand Allen picked
Page 212 and 213: 17.3 Lists as collections 183 node1
Page 214 and 215: 17.5 Infinite lists 185 We invoke t
Page 216 and 217: 17.8 Wrappers and helpers 187 def r
Page 218 and 219: 17.10 Invariants 189 tail = self.ne
Page 220 and 221: Chapter 18 Stacks 18.1 Abstract dat
Page 222 and 223: 18.4 Pushing and popping 193 def po
Page 224 and 225: 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
Page 230 and 231: 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
Page 240 and 241: 20.4 Tree traversal 211 def printTr
Page 242 and 243: 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
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Appendix B Creating a new data type
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B.1 Fraction multiplication 233 >>>
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B.4 Comparing fractions 235 This re
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B.6 Glossary 237 reduce: To change
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Appendix C Recommendations for furt
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C.2 Recommended general computer sc
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Index Make Way for Ducklings, 75 Py
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Index 245 element, 83, 96 embedded
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Index 247 well-formed, 189 list del
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Index 249 redundancy, 7 reference,
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