Professor Hugh C. Lauer CS-1004 Introduction to Programming for Non-Majors

Transcription

1 Objects (again) Professor Hugh C. Lauer CS-1004 Introduction to Programming for Non-Majors (Slides include materials from Python Programming: An Introduction to Computer Science, 2 nd edition, by John Zelle and copyright notes by Prof. George Heineman of ) 1

2 Objects review Reading assignment: Chapter 4 Needed for HW #3 And everything else we do in this course! Object: computational abstraction that includes Data Methods (a. k. a. Functions) P. 81: Objects know stuff Objects do stuff Objects can refer to other objects Objects can interact with other objects An object may be assigned to a variable Or, equivalently, to an element of a list CS-1004, A-Term 2014 Variables, Lists, and Objects 2

3 Objects review (continued) Objects are organized into Classes All objects in a class have the same kind of information All objects in a class have the same set of methods (a.k.a. functions) Classes are discussed in Chapter 10 Including how to define new classes How to construct objects of a class How to define methods of a class How to define data elements of a class object Probably will not get to this chapter in this course CS-1004, A-Term 2014 Variables, Lists, and Objects 3

4 Object Notation review If X is an object of class C, and if class C has a method/function named f, then the notation X.f() means Apply the function f (and its arguments) to the object X I.e., f may cause the object X to DO something Or f may cause the object X to change what it knows CS-1004, A-Term 2014 Variables, Lists, and Objects 4

5 Creating Objects review If C is a class, then the constructor function for that class is also named C V = C(arg 1, arg 2, ) creates a new object of class C and assigns it to variable V All of the methods associated with class C are now available for object V Examples CS-1004, A-Term 2014 Variables, Lists, and Objects 5

6 Assigning Objects review V = SomeClass(<arguments> ) W = V causes W and V to now refer to the very same object Not to copies of each other, but exactly the same object CS-1004, A-Term 2014 Variables, Lists, and Objects 6

7 Questions? 7

8 How do we create new classes? See Chapter 10 Keyword class followed by class name followed by method functions all indented under class the first parameter representing object 8

9 Textbook example Multi-sided die class MSDie: def init (self, sides): self.sides = sides self.value = 1 def roll(self): self.value = randrange(1, self.sides + 1) def getvalue(self): return self.value def setvalue(self, value): self.value = value 9

10 Textbook example (continued) Multi-sided die class MSDie: def init (self, sides): self.sides = sides self.value = 1 def roll(self): self.value = randrange(1, self.sides + 1) def getvalue(self): return self.value def setvalue(self, value): self.value = value Note colon after class name Note indentation of method definitions Note 2 nd indentation of method bodies 10

11 Textbook example (continued) Multi-sided die class MSDie: def init (self, sides): self.sides = sides self.value = 1 Special function name Name init () is function called to create object of class I.e., when application says MSDie(6) Python sets aside memory for one MSDie object and invokes init with self referring to that memory Arguments of classname assigned to additional parameters of init 11

12 Textbook example (continued) Multi-sided die class MSDie: def init (self, sides): self.sides = sides self.value = 1 self is just a parameter name 'Self' could be any name Refers to the object Remember: Parameters act variables assigned at time function/method is called All class methods must have first parameter referring to object 12

13 Textbook example (continued) Multi-sided die class MSDie: def init (self, sides): self.sides = sides self.value = 1 Other parameters def roll(self): self.value = randrange(1, self.sides + 1) def getvalue(self): return self.value def setvalue(self, value): self.value = value All class methods must have first parameter referring to object 13

14 Textbook example (continued) Multi-sided die class MSDie: def init (self, sides): self.sides = sides self.value = 1 These are instance variables They make it possible of object to know stuff Called fields in Java Called members in C/C++ Part of every object of this class Dot notation to access with methods 14

15 Textbook example (continued) Multi-sided die class MSDie: def init (self, sides): self.sides = sides self.value = 1 def roll(self): self.value = randrange(1, self.sides + 1) def getvalue(self): return self.value def setvalue(self, value): self.value = value init initializes instance variables Other methods use and/or manipulate them 15

16 Questions? 16

17 Example Game of Life 17