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

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1.2 What is a program? 3 $ python Python 2.4.1 (#1, Apr 29 2005, 00:28:56) Type "help", "copyright", "credits" or "license" for more information. >>> print 1 + 1 2 The first line of this example is the command that starts the Python interpreter. The next two lines are messages from the interpreter. The third line starts with >>>, which is the prompt the interpreter uses to indicate that it is ready. We typed print 1 + 1, and the interpreter replied 2. Alternatively, you can write a program in a file and use the interpreter to execute the contents of the file. Such a file is called a script. For example, we used a text editor to create a file named latoya.py with the following contents: print 1 + 1 By convention, files that contain Python programs have names that end with .py. To execute the program, we have to tell the interpreter the name of the script: $ python latoya.py 2 In other development environments, the details of executing programs may differ. Also, most programs are more interesting than this one. Most of the examples in this book are executed on the command line. Working on the command line is convenient for program development and testing, because you can type programs and execute them immediately. Once you have a working program, you should store it in a script so you can execute or modify it in the future. 1.2 What is a program? A program is a sequence of instructions that specifies how to perform a computation. The computation might be something mathematical, such as solving a system of equations or finding the roots of a polynomial, but it can also be a symbolic computation, such as searching and replacing text in a document or (strangely enough) compiling a program. The details look different in different languages, but a few basic instructions appear in just about every language: input: Get data from the keyboard, a file, or some other device.
4 The way of the program output: Display data on the screen or send data to a file or other device. math: Perform basic mathematical operations like addition and multiplication. conditional execution: Check for certain conditions and execute the appropriate sequence of statements. repetition: Perform some action repeatedly, usually with some variation. Believe it or not, that’s pretty much all there is to it. Every program you’ve ever used, no matter how complicated, is made up of instructions that look more or less like these. Thus, we can describe programming as the process of breaking a large, complex task into smaller and smaller subtasks until the subtasks are simple enough to be performed with one of these basic instructions. That may be a little vague, but we will come back to this topic later when we talk about algorithms. 1.3 What is debugging? Programming is a complex process, and because it is done by human beings, it often leads to errors. For whimsical reasons, programming errors are called bugs and the process of tracking them down and correcting them is called debugging. Three kinds of errors can occur in a program: syntax errors, runtime errors, and semantic errors. It is useful to distinguish between them in order to track them down more quickly. 1.3.1 Syntax errors Python can only execute a program if the program is syntactically correct; otherwise, the process fails and returns an error message. Syntax refers to the structure of a program and the rules about that structure. For example, in English, a sentence must begin with a capital letter and end with a period. this sentence contains a syntax error. Sodoesthisone For most readers, a few syntax errors are not a significant problem, which is why we can read the poetry of e. e. cummings without spewing error messages. Python is not so forgiving. If there is a single syntax error anywhere in your program, Python will print an error message and quit, and you will not be able to run your program. During the first few weeks of your programming career, you will probably spend a lot of time tracking down syntax errors. As you gain experience, though, you will make fewer errors and find them faster.
Page 2 and 3: How to Think Like a Computer Scient
Page 4 and 5: 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 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
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
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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
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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
Page 276 and 277:
Index 247 well-formed, 189 list del
Page 278 and 279:
Index 249 redundancy, 7 reference,
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