Objec(ves • Defining our own classes Nov 15, 2017 Sprenkle - CSCI111 1 Review: Dic(onaries • What is a dic(onary in Python? • What is the syntax for crea(ng a new dic(onary? • How do we access a key’s value from a dic(onary? Ø What happens if there is no mapping for that key? • How do we create a key à value mapping in a dic(onary? • How can we iterate through a dic(onary? Nov 15, 2017 Sprenkle - CSCI111 2 1
ABSTRACTIONS Nov 15, 2017 Sprenkle - CSCI111 3 Abstrac(ons • Provide ways to think about program and its data Ø Get the jist without the details • Examples we’ve seen encodeFile(filename, key) Ø Func(ons and methods • Used to perform some opera(on but we don’t need to know how they’re implemented Ø Dic(onaries • Know they map keys to values • Don’t need to know how the keys are organized/stored in the computer’s memory Ø Just about everything we do in this class… Nov 15, 2017 Sprenkle - CSCI111 4 2
Classes and Objects • Provide an abstrac(on for how to organize and reason about data • Example: GraphWin class Ø Had a"ributes (i.e., data or state) background color, width, height, and (tle Ø Each GraphWin object had these a`ributes • Each GraphWin object had its own values for these a`ributes Ø Used methods (API) to modify the object’s state, get informa(on about a`ributes Nov 15, 2017 Sprenkle - CSCI111 5 Defining Our Own Classes • Oaen, we want to represent data or informa(on that we do not have a way to represent using built-in types or libraries • Classes provide way to organize and manipulate data Ø Organize: data structures used • E.g., ints, lists, dic(onaries, other objects, etc. Ø Manipulate: methods Nov 15, 2017 Sprenkle - CSCI111 6 3
What is a Class? • Defines a new data type • Defines the class’s a"ributes (i.e., data or state) and methods Ø Methods are like func/ons within a class and are the class’s API Other objects Internal data Object o of manipulate using hidden from type methods Classname others Nov 15, 2017 Sprenkle - CSCI111 7 Defining a Card Class • Create a class that represents a playing card Ø How can we represent a playing card? Ø What informa(on do we need to represent a playing card? Nov 15, 2017 Sprenkle - CSCI111 8 4
Represen(ng a Card object • Every card has two a`ributes: Ø Suite (one of “hearts”, “diamonds”, “clubs”, “spades”) Ø Rank • 2-10: numbered cards • 11: Jack • 12: Queen • 13: King • 14: Ace Nov 15, 2017 Sprenkle - CSCI111 9 Defining a New Class • Syntax: Typically starts with a capital letter Keyword class class ClassName: <method definitions> Nov 15, 2017 Sprenkle - CSCI111 10 5
Card Class (Incomplete) Doc String class class Card: """ A class to represent a standard playing card. The ranks are ints: 2-10 for numbered cards, 11=Jack, 12=Queen, 13=King, 14=Ace. The suits are strings: 'clubs', 'spades', 'hearts', 'diamonds ’ .""" def def __init__(self, rank, suit): """Constructor for class Card takes int rank and string suit.""" self._rank = rank self._suit = suit Methods def def getRank(self): "Returns the card ’ s rank." return return self._rank def def getSuit(self): "Returns the card ’ s suit." return return self._suit card.py Nov 15, 2017 Sprenkle - CSCI111 11 Card Class (Incomplete) Doc String class class Card: """ A class to represent a standard playing card. The ranks are ints: 2-10 for numbered cards, 11=Jack, 12=Queen, 13=King, 14=Ace. The suits are strings: 'clubs', 'spades', 'hearts', 'diamonds ’ .""" def def __init__(self, rank, suit): """Constructor for class Card takes int rank and string suit.""" self._rank = rank Methods are like functions self._suit = suit Methods defined in a class def def getRank(self): "Returns the card ’ s rank." return return self._rank def def getSuit(self): "Returns the card ’ s suit." return return self._suit card.py Nov 15, 2017 Sprenkle - CSCI111 12 6
Defining the Constructor • __ __init init__ __ method is like the constructor • In constructor, define instance variables Ø Data contained in every object Convention: Ø Also called a?ributes or fields named with _ • Constructor never returns anything First parameter of every method is self self - pointer to the object that method acts on def def __init__(self, rank, suit): """Constructor for class Card takes int rank and string suit.""" self._rank = rank Instance self._suit = suit variables Nov 15, 2017 Sprenkle - CSCI111 13 Review • How do we use the constructor for an object? Nov 15, 2017 Sprenkle - CSCI111 14 7
Using the Constructor def __init__(self, rank, suit): def • As defined, constructor is called using Card(<rank>,<suit>) Card(<rank>,<suit>) Ø Do not pass anything for the self self parameter Ø Python handles for us • Passes the parameter Object card card automa3cally of type Card _rank = ? _suit = ? Nov 15, 2017 Sprenkle - CSCI111 15 Using the Constructor def def __init__(self, rank, suit): • As defined, constructor is called using Card(<rank>,<suit>) Card(<rank>,<suit>) Ø Do not pass anything for the self self parameter Ø Python handles, passing the parameter for us automa(cally Object card card • Example: of type Card Ø card card = Card(2, "hearts") = Card(2, "hearts") _rank = 2 Ø Creates a 2 of Hearts card _suit = "hearts" Ø Python passes card card as self self for us Nov 15, 2017 Sprenkle - CSCI111 16 8
Review • How do we call a method on an object? Nov 15, 2017 Sprenkle - CSCI111 17 Accessor Methods • Need to be able to get informa(on about the object def def getRank(self): "Returns the card’s rank." • Have self self return self._rank return parameter • Return data/ def getSuit(self): def informa(on "Returns the card’s suit." return return self._suit card = Card(…, …) card = Card(…, …) • These methods will get called as card.getRank() card.getRank () and card.getSuit card.getSuit() () Ø Python plugs card card in for self self Nov 15, 2017 Sprenkle - CSCI111 18 9
Another Special Method: __str__ • Returns a string def def __str__(self): """Returns a string that describes the describing the card as 'rank of object suit'.""" • Whenever you print print result = "" if if self._rank == 11: an object, Python result += "Jack" checks if the object’s elif elif self._rank == 12: __ __str str__ __ method is result += "Queen" defined elif elif self._rank == 13: result += "King" Ø Prints result of calling elif elif self._rank == 14: __str__ method result += "Ace" • str(<object>) else else: result += str(self._rank) also calls __ __str str__ __ result += " of " + self._suit method return return result Nov 15, 2017 Sprenkle - CSCI111 19 Using the Card Class def def main(): c1 = Card(14, "spades") Invokes the print(c1) __str__ method c2 = Card(2, "hearts") print(c2) Displays: Ace of spades Object c1 c1 of Object c2 c2 of 2 of hearts type Card type Card _rank = 14 _rank = 2 _suit = “ spades ” _suit = “ hearts ” Nov 15, 2017 Sprenkle - CSCI111 20 10
Example: Rummy Value • Problem: Add a method to the Card class called getRummyValue getRummyValue that returns the value of the card in the game of Rummy • Procedure for defining a method (similar to func(ons) Ø What is the input? Ø What is the output? Ø What is the method signature/header? Ø What does the method do? • How do we call the method? card2.py Nov 15, 2017 Sprenkle - CSCI111 21 Card API • Based on what we’ve seen/done so far, what does the Card class’s API look like? Nov 15, 2017 Sprenkle - CSCI111 22 11
Card API API Instance Object o of Variables: type Card _rank, _suit Implementa(on of methods is hidden • Card(<rank>, <suit>) • getRank() • getSuit() • getRummyValue() • __str__() Nov 15, 2017 Sprenkle - CSCI111 23 (not covered in class) Defining a Card Class • Create a class that represents a playing card Ø How can we represent a playing card? Ø What informa(on do we need to represent a playing card? • Do we need a class to represent a card? Ø Does any built-in data type naturally represent a card? Ø What are the tradeoffs to those approaches? Nov 15, 2017 Sprenkle - CSCI111 24 12
Using the Card class • Having the Card class means that we can represent a Card in code Now that we have the Card class, how can we use it? Nov 15, 2017 Sprenkle - CSCI111 25 Using the Card class Now that we have the Card class, how can we use it? • Let’s write a simplified version of the game of War Ø Basically just part of a round • What are the rules of a round of War? war.py Nov 15, 2017 Sprenkle - CSCI111 26 13
Recommend
More recommend
