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How to Think Like a Computer Scientist
The PreTeXt Interactive Edition
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Front Matter
Preface to the First and Second Editions
Preface to the Interactive Edition
1
General Introduction
1.1
The Way of the Program
1.2
Algorithms
1.3
The Python Programming Language
1.4
Executing Python in this Book
1.5
More About Programs
1.6
What is Debugging?
1.7
Syntax errors
1.8
Runtime Errors
1.9
Semantic Errors
1.10
Experimental Debugging
1.11
Formal and Natural Languages
1.11
Glossary
1.11
Glossary
1.12
A Typical First Program
1.13
Comments
1.14
Glossary
1.14
Glossary
1.15
Exercises
2
Simple Python Data
2.1
Variables, Expressions and Statements
2.2
Values and Data Types
2.3
Type conversion functions
2.4
Variables
2.5
Variable Names and Keywords
2.6
Statements and Expressions
2.7
Operators and Operands
2.8
Input
2.9
Order of Operations
2.10
Reassignment
2.10.1
Introduction
2.10.2
Developing your mental model of How Python Evaluates
2.11
Updating Variables
2.12
Glossary
2.12
Glossary
2.13
Exercises
3
Debugging Interlude 1
3.1
How to be a Successful Programmer
3.2
How to Avoid Debugging
3.3
Beginning tips for Debugging
3.4
Know Your Error Messages
3.4.1
Introduction
3.4.2
ParseError
3.4.3
TypeError
3.4.4
NameError
3.4.5
ValueError
3.5
Summary
3.6
Exercises
4
Python Turtle Graphics
4.1
Hello Little Turtles!
4.2
Our First Turtle Program
4.3
Instances — A Herd of Turtles
4.4
The
for
Loop
4.5
Flow of Execution of the for Loop
4.6
Iteration Simplifies our Turtle Program
4.7
The range Function
4.8
A Few More turtle Methods and Observations
4.9
Summary of Turtle Methods
4.10
Glossary
4.10
Glossary
4.11
Exercises
5
Python Modules
5.1
Modules and Getting Help
5.2
More About Using Modules
5.3
The math module
5.4
The random module
5.5
Creating Modules
5.6
Glossary
5.6
Glossary
5.7
Exercises
6
Functions
6.1
Functions
6.2
Functions that Return Values
6.3
Unit Testing
6.3.1
Introduction
6.3.2
assert
with
for
loops
6.3.3
Return Value Tests
6.4
Variables and Parameters are Local
6.5
The Accumulator Pattern
6.5.1
Introduction
6.5.2
The General Accumulator Pattern
6.5.3
A Variation on the Accumulator Pattern
6.6
Functions can Call Other Functions
6.7
Flow of Execution Summary
6.8
Using a Main Function
6.9
Program Development
6.10
Composition
6.11
A Turtle Bar Chart
6.12
Glossary
6.12
Glossary
6.13
Exercises
7
Selection
7.1
Boolean Values and Boolean Expressions
7.2
Logical operators
7.2.1
Introduction
7.2.2
Logical Opposites
7.3
Precedence of Operators
7.4
Conditional Execution: Binary Selection
7.5
Omitting the else Clause: Unary Selection
7.6
Nested conditionals
7.7
Chained conditionals
7.8
Boolean Functions
7.8.1
Introduction
7.8.2
More Unit Testing
7.9
Glossary
7.9
Glossary
7.10
Exercises
8
More About Iteration
8.1
Iteration Revisited
8.2
The
for
loop revisited
8.3
The
while
Statement
8.4
Randomly Walking Turtles
8.5
The 3n + 1 Sequence
8.6
Newton’s Method
8.7
The Accumulator Pattern Revisited
8.8
Other uses of
while
8.8.1
Sentinel Values
8.8.2
Validating Input
8.9
Algorithms Revisited
8.10
Simple Tables
8.11
2-Dimensional Iteration: Image Processing
8.11.1
Introduction
8.11.2
The RGB Color Model
8.11.3
Image Objects
8.11.4
Image Processing and Nested Iteration
8.12
Image Processing on Your Own
8.13
Glossary
8.13
Glossary
8.14
Exercises
9
Strings
9.1
Strings Revisited
9.2
A Collection Data Type
9.3
Operations on Strings
9.4
Index Operator: Working with the Characters of a String
9.5
String Methods
9.5.1
Introduction
9.5.2
String Format Method
9.6
Length
9.7
The Slice Operator
9.8
String Comparison
9.9
Strings are Immutable
9.10
Traversal and the
for
Loop: By Item
9.11
Traversal and the
for
Loop: By Index
9.12
Traversal and the
while
Loop
9.13
The
in
and
not in
operators
9.14
The Accumulator Pattern with Strings
9.15
Turtles and Strings and L-Systems
9.16
Looping and Counting
9.17
A
find
function
9.18
Optional parameters
9.19
Character classification
9.20
Summary
9.20
Glossary
9.21
Glossary
9.21
Glossary
9.22
Exercises
10
Lists
10.1
Lists
10.2
List Values
10.3
List Length
10.4
Accessing Elements
10.5
List Membership
10.6
Concatenation and Repetition
10.7
List Slices
10.8
Lists are Mutable
10.9
List Deletion
10.10
Objects and References
10.11
Aliasing
10.12
Cloning Lists
10.13
Repetition and References
10.14
List Methods
10.15
The Return of L-Systems
10.16
Append versus Concatenate
10.17
Lists and
for
loops
10.18
The Accumulator Pattern with Lists
10.18.1
Introduction
10.18.2
Accumulating the Max Value
10.18.3
Accumulating a String Result
10.19
Using Lists as Parameters
10.20
Pure Functions
10.21
Which is Better?
10.22
Functions that Produce Lists
10.23
List Comprehensions
10.24
Nested Lists
10.25
Strings and Lists
10.26
List - Type Conversion Function
10.27
Tuples and Mutability
10.28
Tuple Assignment
10.29
Tuples as Return Values
10.30
Glossary
10.30
Glossary
10.31
Exercises
11
Files
11.1
Working with Data Files
11.2
Finding a File on your Disk
11.3
Reading a File
11.4
Iterating over lines in a file
11.5
Alternative File Reading Methods
11.6
Writing Text Files
11.7
With Statements
11.8
Fetching Something From The Web
11.9
Glossary
11.9
Glossary
11.10
Exercises
12
Dictionaries
12.1
Dictionaries
12.2
Dictionary Operations
12.3
Dictionary Methods
12.4
Aliasing and Copying
12.5
Sparse Matrices
12.6
Glossary
12.6
Glossary
12.7
Exercises
13
Exceptions
13.1
What is an exception?
13.2
Runetime Stack and
raise
command
13.3
Standard Exceptions
13.4
Principles for using Exceptions
13.5
Exceptions Syntax
13.5.1
Introduction
13.5.2
Catch All Exceptions
13.5.3
Catch A Specific Exception
13.5.4
Catch Multiple Specific Exceptions
13.5.5
Clean-up After Exceptions
13.5.6
An Example of File I/O
13.6
The
finally
clause of the
try
statement
13.7
Glossary
13.7
Glossary
13.8
Exercises
14
Web Applications
14.1
Web Applications
14.2
How the Web Works
14.3
How Web Applications Work
14.4
Web Applications and HTML Forms
14.5
Writing Web Applications With Flask
14.6
More About Flask
14.7
Input For A Flask Web Application
14.8
Web Applications With a User Interface
14.9
Glossary
14.9
Glossary
15
GUI and Event Driven Programming
15.1
Graphical User Interfaces
15.2
Tkinter Standard Dialog Boxes
15.2.1
Introduction
15.2.2
Messages
15.2.3
Yes/No Questions
15.2.4
Single Value Data Entry
15.2.5
File Chooser
15.2.6
Color Chooser
15.3
GUI Widgets
15.4
Layout Mangers
15.5
Widget Groupings
15.6
Command Events
15.7
Low-Level Event Processing
15.8
The Design of GUI Programs
15.9
Common Widget Properties
15.10
Timer Events
15.10.1
Introduction
15.10.2
Animations and Repeated Tasks
15.10.3
Canceling Timer Events
15.10.4
Multiple Parameters to Timer Callbacks
15.11
A Programming Example
15.11.1
Introduction
15.11.2
A Whack-a-mole Game
15.11.3
Summary
15.12
Managing GUI Program Complexity
15.12.1
Introduction
15.12.2
Creating the
View
15.12.3
Creating the
Model
15.12.4
Creating the
Controller
15.13
Exercises
15.14
Glossary
15.14
Glossary
16
Recursion
16.1
What Is Recursion?
16.2
Calculating the Sum of a List of Numbers
16.3
The Three Laws of Recursion
16.4
Converting an Integer to a String in Any Base
16.5
Visualizing Recursion
16.6
Sierpinski Triangle
16.7
Glossary
16.7
Glossary
16.8
Programming Exercises
16.9
Exercises
17
Classes and Objects - the Basics
17.1
Object-oriented programming
17.2
A change of perspective
17.3
Objects Revisited
17.4
User Defined Classes
17.5
Improving our Constructor
17.6
Adding Other Methods to our Class
17.7
Objects as Arguments and Parameters
17.8
Converting an Object to a String
17.9
Instances as Return Values
17.10
Glossary
17.10
Glossary
17.11
Exercises
18
Classes and Objects - Digging a Little Deeper
18.1
Fractions
18.2
Objects are Mutable
18.3
Sameness
18.4
Arithmetic Methods
18.5
Glossary
18.5
Glossary
18.6
Exercises
19
Inheritance
19.1
Pillars of OOP
19.2
Introduction to Inheritance
19.3
Extending
19.4
Reuse Through Composition
19.5
Class Diagrams
19.6
Composition vs. Inheritance
19.7
Case Study: Structured Postal Addresses
19.7.1
Introduction
19.7.2
Storing Postal Addresses
19.7.3
Storing International Addresses
19.7.4
Inheritance Applied
19.7.5
A List of Addresses
19.7.6
Using isinstance
20
Unit Testing
20.1
Introduction: Unit Testing
20.2
Checking Assumptions With
assert
20.2.1
Introduction
20.2.2
Designing Defensive Functions
20.2.3
The
assert
Statement
20.2.4
More on
assert
and Preconditions
20.3
Testing Functions
20.3.1
Introduction
20.3.2
Automated Unit Tests
20.3.3
Automated Unit Tests with
assert
20.3.4
Unit Tests can have bugs
20.4
Designing Testable Functions
20.4.1
Introduction
20.4.2
Design by Contract
20.5
Writing Unit Tests
20.5.1
Introduction
20.5.2
Specification-Based Testing
20.6
Test-First Development
20.6.1
Introduction
20.6.2
Benefits of Test-First Development
20.7
Testing with pytest
20.7.1
Introduction
20.7.2
Organizing pytest Functions
20.7.3
Using pytest
20.7.4
Understanding pytest Failure Reports
20.7.5
Integrated Unit Testing with pytest
20.8
Glossary
20.8
Glossary
20.9
Exercises
21
Labs
21.1
Astronomy Animation
21.1.1
Object-oriented programming
21.2
Turtle Racing Lab
21.2.1
Random Numbers
21.2.2
Turtle Races
21.3
Drawing a Circle
21.4
Lessons from a Triangle
21.5
Counting Letters
21.5.1
Simple Solution: Counting a Single Letter
21.5.2
General Solution: Counting All Letters
21.6
Letter Count Histogram
21.7
Approximating the Value of Pi
21.7.1
Setup
21.7.2
Throwing Darts
21.7.3
Counting Darts
21.7.4
The Value of Pi
21.8
Python Beyond the Browser
21.8.1
The Python Shell
21.8.2
Running a Python Program
21.8.3
Installing Python on Windows
21.8.4
Glossary
21.8.4
Glossary
21.9
Experimenting With the 3n+1 Sequence
21.10
Plotting a sine Wave
21.10.1
What is the sine function?
21.10.2
Making the Plot
21.10.3
Making the Plot Better
Back Matter
1
Operator precedence table
Index
Section
21.1
Astronomy Animation
Subsection
21.1.1
Object-oriented programming
import turtle import math class SolarSystem: def __init__(self, width, height): self.thesun = None self.planets = [] self.ssturtle = turtle.Turtle() self.ssturtle.hideturtle() self.ssscreen = turtle.Screen() self.ssscreen.setworldcoordinates(-width/2.0,-height/2.0,width/2.0,height/2.0) # self.ssscreen.tracer(50) def addPlanet(self, aplanet): self.planets.append(aplanet) def addSun(self, asun): self.thesun = asun def showPlanets(self): for aplanet in self.planets: print(aplanet) def freeze(self): self.ssscreen.exitonclick() def movePlanets(self): G = .1 dt = .001 for p in self.planets: p.moveTo(p.getXPos() + dt * p.getXVel(), p.getYPos() + dt * p.getYVel()) rx = self.thesun.getXPos() - p.getXPos() ry = self.thesun.getYPos() - p.getYPos() r = math.sqrt(rx**2 + ry**2) accx = G * self.thesun.getMass()*rx/r**3 accy = G * self.thesun.getMass()*ry/r**3 p.setXVel(p.getXVel() + dt * accx) p.setYVel(p.getYVel() + dt * accy) class Sun: def __init__(self, iname, irad, im, itemp): self.name = iname self.radius = irad self.mass = im self.temp = itemp self.x = 0 self.y = 0 self.sturtle = turtle.Turtle() self.sturtle.shape("circle") self.sturtle.color("yellow") def getName(self): return self.name def getRadius(self): return self.radius def getMass(self): return self.mass def getTemperature(self): return self.temp def getVolume(self): v = 4.0/3 * math.pi * self.radius**3 return v def getSurfaceArea(self): sa = 4.0 * math.pi * self.radius**2 return sa def getDensity(self): d = self.mass / self.getVolume() return d def setName(self, newname): self.name = newname def __str__(self): return self.name def getXPos(self): return self.x def getYPos(self): return self.y class Planet: def __init__(self, iname, irad, im, idist, ivx, ivy, ic): self.name = iname self.radius = irad self.mass = im self.distance = idist self.x = idist self.y = 0 self.velx = ivx self.vely = ivy self.color = ic self.pturtle = turtle.Turtle() #self.pturtle.speed('fast') self.pturtle.up() self.pturtle.color(self.color) self.pturtle.shape("circle") self.pturtle.goto(self.x,self.y) self.pturtle.down() def getName(self): return self.name def getRadius(self): return self.radius def getMass(self): return self.mass def getDistance(self): return self.distance def getVolume(self): v = 4.0/3 * math.pi * self.radius**3 return v def getSurfaceArea(self): sa = 4.0 * math.pi * self.radius**2 return sa def getDensity(self): d = self.mass / self.getVolume() return d def setName(self, newname): self.name = newname def show(self): print(self.name) def __str__(self): return self.name def moveTo(self, newx, newy): self.x = newx self.y = newy self.pturtle.goto(newx, newy) def getXPos(self): return self.x def getYPos(self): return self.y def getXVel(self): return self.velx def getYVel(self): return self.vely def setXVel(self, newvx): self.velx = newvx def setYVel(self, newvy): self.vely = newvy def createSSandAnimate(): ss = SolarSystem(2,2) sun = Sun("SUN", 5000, 10, 5800) ss.addSun(sun) m = Planet("MERCURY", 19.5, 1000, .25, 0, 2, "blue") ss.addPlanet(m) m = Planet("EARTH", 47.5, 5000, 0.3, 0, 2.0, "green") ss.addPlanet(m) m = Planet("MARS", 50, 9000, 0.5, 0, 1.63, "red") ss.addPlanet(m) m = Planet("JUPITER", 100, 49000, 0.7, 0, 1, "black") ss.addPlanet(m) m = Planet("Pluto", 1, 500, 0.9, 0, .5, "orange") ss.addPlanet(m) m = Planet("Asteroid", 1, 500, 1.0, 0, .75, "cyan") ss.addPlanet(m) numTimePeriods = 10000 for amove in range(numTimePeriods): ss.movePlanets() ss.freeze() createSSandAnimate()
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