What does the acronym DFS stand for? Explain the DFS algorithm in your own words.

Transcription

1 QUIZ: Searching for paths in a graph What does the acronym DFS stand for? Explain the DFS algorithm in your own words. 1

2 QUIZ: Searching for paths in a graph What does the acronym DFS stand for? Explain the DFS algorithm in your own words. We go to the deepest branch, examining all the paths beginning at the child node, before we come back to examine the alternate paths that begin at the sibling. 2

3 QUIZ: Searching for paths in a graph Find a path from Chicago to Dallas using the DFS algorithm. Show the stack and node markings at every step! (Nodes are pushed on the stack in alphabetical order) 3

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5 9.1 OO Methodology OO Design A problem-solving methodology that produces a solution to a problem in terms of self-contained entities called objects Object A thing or entity that makes sense within the context of the problem E.g., a student, a car, the time, the date 5

6 Object-Oriented Design World View of OOD Problems are solved by isolating the objects in a problem, determining their properties and actions (responsibilities), and letting the objects collaborate to solve a problem What? Say again! 6

7 Object-Oriented Example You and your friend are making dinner Objects: you, friend, dinner, steaks, potatoes, lettuce, butter, grill, frying pan, pot, Classes: you and friend are People People have name, eye color, People can shop, cook, steaks, potatoes, and butter are Ingredients Ingredients have weights, costs, quantities, Ingredients can be bought at grocery store, carried home, cooked, 7

8 Object-Oriented Example You and your friend are making dinner Instance of a class: you and friend are instances of class People, you each have your own name and eye color you each can shop and cook steaks and potatoes are instances of class Ingredients they each have their own weight, cost and quantity they each can be bought, carried, cooked Objects collaborate to make dinner! 8

9 Object-Oriented Lingo Class = description of a group of similar objects Object = instance of a class = concrete example of an object in the class Classes contain: Properties (attributes) (name, eye color) Behaviors (actions, responsibilities) (shop, cook) Method = A named algorithm that defines behavior (shop, cook) Python: my_list.append(42) 9

10 Two types of design Top-Down Design decomposes problems into tasks Object-Oriented Design decomposes problems into collaborating objects 10 Yes, but how?

11 QUIZ What is the strategic difference between procedural design and OOD? 11

12 Bottom-up example Not in textbook Gathering wood-chips! How would we do it top-down? (i.e. procedural) Bottom-up: ask termites for help! 12

13 Bottom-up example Not in textbook What termites do: Search for wood-chip, pick it up Search for another Drop first next to second Repeat! 13

14 Evolution of clusters Not in textbook Initial random distribution of chips (yellow) and termites (red) 14

15 Evolution of clusters Not in textbook Follow the centers of mass of these last two piles! 15

16 Evolution of clusters Not in textbook A dynamical equilibrium is reached; periodically, small islands and peninsulas form, only to be reabsorbed in the central pile. 16

17 Implementation: MIT StarLogo (free to download!) Not in textbook 17

18 Recommended reading Not in textbook 18

19 OOD Steps isolate the real-world objects in the problem abstract the objects with like properties into groups (classes) determine the responsibilities of the group in interacting with other groups 19

20 OOD Example Think of design as a mapping from real world objects to classes of objects birth date marriage date Date class dog's birth date 20

21 After the design phase comes the implementation phase: dogbirthdate class Date mybirthdate marriagedate Description Instances We instantiate one or more objects of each class 21

22 22 Going forth and then back!

23 OOD Example Date's Actions in real world? We call an object's interactions with other objects its responsibilities Create itself Know the state of its fields Compare itself to another date Return a date a number of days since another date 23

24 OOD Example Responsibilities become methods associated with the classes and objects class Date getmonth getday getyear dayssince dogbirthdate.getmonth() dogbirthdate.getday() dogbirthdate.getyear() mybirthdate.getday() Etc. marriagedate.getyear() Etc. crtdate.dayssince(mybirthdate) Etc.

25 25 This is how Python refers to the current object in a class, since the name(s) of the object(s) is(are) not know at this time

26 26 Aren t we putting the carriage in front of the horses?

27 27 How do we use the class in a program?

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29 Your turn! Create a class Fraction with two integer fields: nume and deno. The default value of any fraction should be 1/1. Write methods for: initializing nume and deno getting nume and deno. 29

30 To do for next time: Read and take notes Sec. 9.1 SKIP Sec. 9.2 and 9.3 Read and take notes Sec. 9.4 Solve in notebook end-of-chapter ex. 60 through

31 Your turn! Instantiate two fractions f1 and f2 in the main program: f1 is 3/4 f2 is 5/4 31

32 Your turn! Print f1 and f2 nicely the main program, using the get methods. The output should look like this: f1 is 3/4 f2 is 5/4 32

33 Do not confuse the responsibilities of reallife people with the responsibilities of the objects representing those people! Example: the zoo-keeper In real-life, (s)he has the responsibility to feed the animals, clean the cages, etc. In an OO program: zookeeper.setssn(), zookeeper.getlistofanimalscaredfor(), etc.

34 OOD Methodology Four stages to the decomposition process Brainstorming to locate possible classes Filtering the classes to find duplicates or remove unnecessary ones Scenarios are tried to be sure we understand collaborations Responsibility algorithms are designed for all actions that classes must perform 34

35 OOD Example Let s examine the problem-solving process for creating an address list Brainstorming and filtering Circling the nouns and underlining the verbs is a good way to begin 35

36 36 OOD Example - Filtering

37 Design tool: CRC Cards Class-Responsibility-Collaboration cards = notational device to record information about a class, what is must do and with whom it must collaborate We ll talk about inheritance next time 37

38 Convention: class names always start with uppercase Other useful responsibilities can be added depending on the application, e.g. sending s, checking out books from library, etc. 38

39 39 Can you think of any other useful responsibilities? Hint: think of Python strings.

40 40 Can you think on other useful responsibilities? Hint: Think about the list algorithms from Ch.8

41 Responsibility Algorithms Person Class Initialize name.initialize() Write "Enter phone number; press return." Get telephone number Write "Enter address; press return." Get address Print Tells name to initialize itself Tells name to print itself name.print() Write "Telephone number: " + telephonenumber Write " address: " + address 41

42 Responsibility Algorithms Name Class Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print Print "First name: " + firstname Print "Last name: " + lastname Is has the same 2 responsibilities as Person, but the algorithms are different! 42

43 Name Class Now let s do this in Python! Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print Print "First name: " + firstname Print "Last name: " + lastname 43

44 Name Class Now let s do this in Python! Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print Print "First name: " + firstname Print "Last name: " + lastname 44

45 Name Class Now let s do this in Python! Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print Print "First name: " + firstname Print "Last name: " + lastname Now add the Print responsibility (method) 45

46 Name Class Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print Print "First name: " + firstname Print "Last name: " + lastname 46

47 Name Class Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print In the main program, we instantiate (create) three objects of this class Print "First name: " + firstname Print "Last name: " + lastname 47

48 Name Class Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Now we can instruct the objects what to do, using their methods Print Print "First name: " + firstname Print "Last name: " + lastname 48

49 Name Class Run this program! Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print Print "First name: " + firstname Print "Last name: " + lastname 49

50 Name Class Initialize "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Write the same code for the friend s name and the dog s name, and run it again! Print Print "First name: " + firstname Print "Last name: " + lastname 50

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52 Can we implement the same functionality w/o OO machinery? Of course we can! 52

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54 So then why bother with objects? 54

55 Name Class Initialize Classes "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print Print "First name: " + firstname Print "Last name: " + lastname 55

56 Name Class Initialize Encapsulation "Enter the first name; press return." Read firstname "Enter the last name; press return." Read lastname Print Print "First name: " + firstname Print "Last name: " + lastname 56

57 57 Inheritance

58 Inheritance 58 ?

59 Conclusion: Inheritance makes it easy to reuse code! 59

60 60 Polymorphism

61 Conclusion: Polymorphism makes it easy to understand, evolve and maintain code! 61

62 Inheritance A construct that fosters reuse by allowing an application to take an already-tested class and derive a class from it that inherits the properties the application needs Polymorphism The ability of a language to have duplicate method names in an inheritance hierarchy and to apply the method that is appropriate for the object to which the method is applied 62

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64 Inheritance and Polymorphism Inheritance and polymorphism work together How? They combine to allow the programmer to build useful hierarchies of classes that can be put into a library to be reused in different applications 64

65 OO nitty-gritty: constructors 65 Source:

66 Ethical Issues Spam, hoaxes and scams 66

67 Who am I? I am: An advocate of structured programming A recipient of the Turing award The inventor of a famous graph algorithm that now has my name. Known for my wit. Can you recall some of my witty sayings? Dutch 67

68 Review questions Answer in notebook! only Python 76 only a. (Python) 68