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

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16.6 OldMaidHand class 173 class CardGame: def __init__(self): self.deck = Deck() self.deck.shuffle() This is the first case we have seen where the initialization method performs a significant computation, beyond initializing attributes. To implement specific games, we can inherit from CardGame and add features for the new game. As an example, we’ll write a simulation of Old Maid. The object of Old Maid is to get rid of cards in your hand. You do this by matching cards by rank and color. For example, the 4 of Clubs matches the 4 of Spades since both suits are black. The Jack of Hearts matches the Jack of Diamonds since both are red. To begin the game, the Queen of Clubs is removed from the deck so that the Queen of Spades has no match. The fifty-one remaining cards are dealt to the players in a round robin. After the deal, all players match and discard as many cards as possible. When no more matches can be made, play begins. In turn, each player picks a card (without looking) from the closest neighbor to the left who still has cards. If the chosen card matches a card in the player’s hand, the pair is removed. Otherwise, the card is added to the player’s hand. Eventually all possible matches are made, leaving only the Queen of Spades in the loser’s hand. In our computer simulation of the game, the computer plays all hands. Unfortunately, some nuances of the real game are lost. In a real game, the player with the Old Maid goes to some effort to get their neighbor to pick that card, by displaying it a little more prominently, or perhaps failing to display it more prominently, or even failing to fail to display that card more prominently. The computer simply picks a neighbor’s card at random. 16.6 OldMaidHand class A hand for playing Old Maid requires some abilities beyond the general abilities of a Hand. We will define a new class, OldMaidHand, that inherits from Hand and provides an additional method called removeMatches: class OldMaidHand(Hand): def removeMatches(self): count = 0 originalCards = self.cards[:]
174 Inheritance for card in originalCards: match = Card(3 - card.suit, card.rank) if match in self.cards: self.cards.remove(card) self.cards.remove(match) print "Hand %s: %s matches %s" % (self.name,card,match) count = count + 1 return count We start by making a copy of the list of cards, so that we can traverse the copy while removing cards from the original. Since self.cards is modified in the loop, we don’t want to use it to control the traversal. Python can get quite confused if it is traversing a list that is changing! For each card in the hand, we figure out what the matching card is and go looking for it. The match card has the same rank and the other suit of the same color. The expression 3 - card.suit turns a Club (suit 0) into a Spade (suit 3) and a Diamond (suit 1) into a Heart (suit 2). You should satisfy yourself that the opposite operations also work. If the match card is also in the hand, both cards are removed. The following example demonstrates how to use removeMatches: >>> game = CardGame() >>> hand = OldMaidHand("frank") >>> game.deck.deal([hand], 13) >>> print hand Hand frank contains Ace of Spades 2 of Diamonds 7 of Spades 8 of Clubs 6 of Hearts 8 of Spades 7 of Clubs Queen of Clubs 7 of Diamonds 5 of Clubs Jack of Diamonds 10 of Diamonds 10 of Hearts >>> hand.removeMatches() Hand frank: 7 of Spades matches 7 of Clubs Hand frank: 8 of Spades matches 8 of Clubs
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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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Page 154 and 155: 11.5 Exceptions 125 Or trying to op
Page 156 and 157: 11.6 Glossary 127 format sequence:
Page 158 and 159: 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
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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 204 and 205: 16.7 OldMaidGame class 175 Hand fra
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Page 208 and 209: 16.8 Glossary 179 Hand Allen picked
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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
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Page 232 and 233: 19.5 Priority queue 203 and it is m
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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
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A.2 Runtime errors 223 A.1.1 I can
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A.2 Runtime errors 225 If there is
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A.3 Semantic errors 227 A.3 Semanti
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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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