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

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Chapter 7 Strings 7.1 A compound data type So far we have seen three types: int, float, andstring. Strings are qualitatively different from the other two because they are made up of smaller pieces— characters. Types that comprise smaller pieces are called compound data types. Depending on what we are doing, we may want to treat a compound data type as a single thing, or we may want to access its parts. This ambiguity is useful. The bracket operator selects a single character from a string. >>> fruit = "banana" >>> letter = fruit[1] >>> print letter The expression fruit[1] selects character number 1 from fruit. The variable letter refers to the result. When we display letter, we get a surprise: a The first letter of "banana" is not a. Unless you are a computer scientist. In that case you should think of the expression in brackets as an offset from the beginning of the string, and the offset of the first letter is zero. So b is the 0th letter (“zero-eth”) of "banana", a is the 1th letter (“one-eth”), and n is the 2th (“two-eth”) letter. To get the first letter of a string, you just put 0, or any expression with the value 0, in the brackets:
74 Strings >>> letter = fruit[0] >>> print letter b The expression in brackets is called an index. An index specifies a member of an ordered set, in this case the set of characters in the string. The index indicates which one you want, hence the name. It can be any integer expression. 7.2 Length The len function returns the number of characters in a string: >>> fruit = "banana" >>> len(fruit) 6 To get the last letter of a string, you might be tempted to try something like this: length = len(fruit) last = fruit[length] # ERROR! That won’t work. It causes the runtime error IndexError: string index out of range. The reason is that there is no 6th letter in "banana". Since we started counting at zero, the six letters are numbered 0 to 5. To get the last character, we have to subtract 1 from length: length = len(fruit) last = fruit[length-1] Alternatively, we can use negative indices, which count backward from the end of the string. The expression fruit[-1] yields the last letter, fruit[-2] yields the second to last, and so on. 7.3 Traversal and the for loop A lot of computations involve processing a string one character at a time. Often they start at the beginning, select each character in turn, do something to it, and continue until the end. This pattern of processing is called a traversal. Oneway to encode a traversal is with a while statement: index = 0 while index < len(fruit): letter = fruit[index] print letter index = index + 1
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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
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
Page 68 and 69: 4.4 Conditional execution 39 In gen
Page 70 and 71: 4.7 Nested conditionals 41 Each con
Page 72 and 73: 4.9 Recursion 43 countdown expects
Page 74 and 75: 4.11 Infinite recursion 45 4.11 Inf
Page 76 and 77: 4.13 Glossary 47 block: A group of
Page 78 and 79: Chapter 5 Fruitful functions 5.1 Re
Page 80 and 81: 5.2 Program development 51 avoid lo
Page 82 and 83: 5.3 Composition 53 3. Once the prog
Page 84 and 85: 5.5 More recursion 55 But the extra
Page 86 and 87: 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
Page 92 and 93: 6.2 The while statement 63 1. Evalu
Page 94 and 95: 6.3 Tables 65 1.0 0.0 2.0 0.6931471
Page 96 and 97: 6.5 Encapsulation and generalizatio
Page 98 and 99: 6.7 Local variables 69 6.7 Local va
Page 100 and 101: 6.9 Functions 71 the function is ca
Page 104 and 105: 7.3 Traversal and the for loop 75 T
Page 106 and 107: 7.6 Strings are immutable 77 Other
Page 108 and 109: 7.9 The string module 79 As an exer
Page 110 and 111: 7.11 Glossary 81 There are other us
Page 112 and 113: Chapter 8 Lists A list is an ordere
Page 114 and 115: 8.3 List length 85 If you try to re
Page 116 and 117: 8.6 List operations 87 for VARIABLE
Page 118 and 119: 8.9 List deletion 89 >>> list = [
Page 120 and 121: 8.10 Objects and values 91 8.10 Obj
Page 122 and 123: 8.13 List parameters 93 >>> b[0] =
Page 124 and 125: 8.16 Strings and lists 95 might be
Page 126 and 127: Chapter 9 Tuples 9.1 Mutability and
Page 128 and 129: 9.3 Tuples as return values 99 9.3
Page 130 and 131: 9.6 Counting 101 >>> randomList(8)
Page 132 and 133: 9.7 Many buckets 103 bucketWidth =
Page 134 and 135: 9.9 Glossary 105 9.9 Glossary Assig
Page 136 and 137: Chapter 10 Dictionaries The compoun
Page 138 and 139: 10.2 Dictionary methods 109 10.2 Di
Page 140 and 141: 10.5 Hints 111 An alternative is to
Page 142 and 143: 10.6 Long integers 113 Using this v
Page 144 and 145: 10.8 Glossary 115 overflow: A numer
Page 146 and 147: Chapter 11 Files and exceptions Whi
Page 148 and 149: 11.1 Text files 119 The following f
Page 150 and 151: 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
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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
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18.9 Glossary 197 18.9 Glossary abs
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Chapter 19 Queues This chapter pres
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19.3 Performance characteristics 20
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19.5 Priority queue 203 and it is m
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19.6 The Golfer class 205 19.6 The
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Chapter 20 Trees Like linked lists,
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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
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20.5 Building an expression tree 21
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20.7 The animal tree 217 Are you th
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20.8 Glossary 219 def yes(ques): fr
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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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