CSC148H Week 1 >> COURSE INFORMATION, RECAP, OOP. Sadia Rain Sharmin Week of May 7, 2018

Transcription

1 CSC148H Week 1 >> COURSE INFORMATION, RECAP, OOP Sadia Rain Sharmin Week of May 7, 2018

2 Welcome! What is CSC148 about? - Designing programs using OO programming principles - Reasoning about efficiency of code - Writing and using recursive, linked, and other data structures - Thinking recursively and writing recursive functions - Reasoning about sorting algorithms

3 Course website Course website: The website has a link to our discussion board where all important announcements will be posted

4 Evaluation CSC108H LECTURE 4 Ten labs (10%) Labs are marked based on attendance and working hard during the lab. Show your TA your work before leaving the lab to get this mark. Each lab is worth 1% of your final grade. There are NO LABS this week; they start next week please attend the one you are enrolled in (PRA on your timetable). Two assignments (20%) Two tests (24%) The test you score higher on will be worth 14% and the other will be worth 10% Exercises (10%) You will have exercises assigned to you over the course of the semester, based on lecture material. Final exam (36%) You must earn at least 40% on the exam, or else your final mark will automatically be set to a failing grade of 47 or lower.

5 Help! CSC108H LECTURE 4 Office hours If you would like to set up a one-on-one meeting, you are welcome to me to set something up. Labs led by your TA Online discussion boards

6 Academic integrity What are some examples of academic dishonesty?

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8 Academic integrity What are some examples of academic dishonesty? - Asking someone else in the course to do your work for you - Asking someone outside of the course to do your work for you (how can you get caught? someone else in the class might ask that same person) - Showing someone your code - Sending someone your code - Looking at someone else's code - Asking someone else to send you their code - Working together side-by-side with someone else who is not your partner on an assignment

9 Academic integrity Why shouldn't you cheat? - You may fail the assignment - You may fail the course - You have to meet with people at the Dean's office and it probably won't be a very fun meeting - You will have "CHEATER" written on your transcript

10 Let's Get Started CSC108H LECTURE 4

11 review: Python objects Each object in Python has an id, value, and type. Read the page posted on our course website for detailed overview of the Python memory model.

12 review: Python objects Everything in Python is an object. An object has a name, usually a noun (e.g. 'string', 'list') Different instances can be created of an object (e.g. I can create multiple strings, but each string will have similar characteristics and behaviour) An object can have attributes and methods. You can use dir(str) to see all string methods, dir(list) to see list methods, etc.

13 Defining a new class When you define a new class, you are defining all the attributes and methods that any instance of that particular class will have Think of the class definition like a template You can make multiple instances of the class using that template

14 Class Design Recipe 1. Class name and description 2. Example 3. Public methods 4. Public attributes 5. Implement the class See handout on course website for details.

15 Class Design Recipe: Point Let's use the class design recipe to built a Point class based on the following specifications. Somewhere in the real world there is a description of points in two-dimensional space: In two dimensions, a point is two numbers (coordinates) that are treated collectively as a single object. Points are often written in parentheses with a comma separating the coordinates. For example, (0, 0) represents the origin, and (x, y) represents the point x units to the right and y units up from the origin. Some of the typical operations that one associates with points might be calculating the distance of a point from the origin or from another point, or finding a midpoint of two points, or asking if a point falls within a given rectangle or circle. Find the most important noun (good candidate for a class... ), its most important attributes, and operations that this noun should support.

16 Class Design Recipe: Point Let's use the class design recipe to built a Point class based on the following specifications. Somewhere in the real world there is a description of points in two-dimensional space: In two dimensions, a point is two numbers (coordinates) that are treated collectively as a single object. Points are often written in parentheses with a comma separating the coordinates. For example, (0, 0) represents the origin, and (x, y) represents the point x units to the right and y units up from the origin. Some of the typical operations that one associates with points might be calculating the distance of a point from the origin or from another point, or finding a midpoint of two points, or asking if a point falls within a given rectangle or circle. Find the most important noun (good candidate for a class... ), its most important attributes, and operations that this noun should support.

17 Class Design Recipe: Point 1. Class name and description class Point: '''Two dimensional point''' 2. Example p = Point(3, 4) 3. Public methods: What services should your class provide? 4. Public attributes 5. Implement the class

18 Object-oriented Analysis In object-oriented analysis, we find the objects, determine object behaviour (methods), determine object state (instance variables) and determine object interactions. Let's design classes to represent the following: Imagine a Pet Shop. The Pet Shop has a lot of animals. Each animal has a name (like Fido, Fluffy, etc.) and an age (number of years), and they are all able to eat and sleep. There are different kinds of animals. There are cats, dogs and birds. A cat can say meow and purr. A dog can bark and roll over. A bird can fly.

19 Inheritance CSC108H LECTURE 4 Many times, a new class will have much in common with an existing class. Copying-and-pasting from the old class, and then making a few changes, is a bad idea - What if you then discover a bug in the original class? - What if you find that there are many classes you'd like to create from the original one? Inheritance allows a new class to specialize an existing class by specifying only what is different between them The class that inherits is called a subclass, and the class that is inherited from is its superclass

20 Is-a Inheritance models is-a relationships The subclass is-a subset of the superclass e.g. The superclass could be Ship, representing all ships The subclass could be InvisibleShip that is-a Ship but cannot take damage

21 Has-a Be careful not to use inheritance to model has-a relationships For example, consider Segment(line segments) and Point (points) Is a point a line segment? No Is a line segment a point? No Inheritance cannot be used here A line segment has two points, not is two points Has-a relationships are modeled with instance variables, not inheritance

22 Inheritance To inherit from another class, add the superclass name in parentheses: class ChildClass(Parent):... When calling a method on a child class instance: - If the method exists in the child class, it is called - Otherwise, the one in the Parent class is called

23 String representations What happens when I try to print an instance of a custom-made class? To change this, you need to define an str method, or an repr method or both. str should return a readable string representation, something that is convenient and concise for a user to read. repr should return a detailed, unambiguous representation. This is typically used for debugging, and should be more useful for the developer, not as much for the user.

24 String representations If both str and repr are defined then: - str is called for printing and for str() - repr is called when we are looking at the objects representation (e.g. on the console) and for repr() If str is defined but not repr: - str is called when str would be called If repr is defined but not str: - repr is called when str would be called - repr is called when repr would be called

25 Comparing instances Can I compare two instances of a class using <, >, ==? Yes, if you define the following magic methods : lt, gt, le, ge, eq, ne These functions will then get called when we try to do less than, greater than, less than or equal to, greater than or equal to, equal to, and not equal to, respectively. e.g. if p1 and p2 are both instances of the same class, when Python sees p1 < p2, it will do p1. lt (p2)

26 Comparing instances CSC108H LECTURE 4 It can be redundant to define all of these: lt, gt, le, ge, eq, ne What we could do is include these two lines in our code: from functools import class Point:... And now we only need to define lt and eq and Python will fill in the rest for us.

27 Other magic methods CSC108H LECTURE 4 You can also define methods for adding ( add ), multiplying ( mult ), etc. See more on magic methods here:

28 Checklist for this week CSC108H LECTURE 4 - Read the course information sheet - Bookmark the course website and discussion board - Log in to the online discussion board. Make sure your login works! - If you plan on working on your own computer, install software listed on course website - Drop by office hours and introduce yourself