Presentation Transcript

1. Section 6.1

   CS 106, Fall 2013
2. The Big Q What do we get by being able to define a class?! Or Do we really need this?!
3. What do you think?
4. How to represent a playing card? A card is a tuple with 2 parts, a suit (one of “s”, “d”, “c”, “h”) and a denom (2 – 14). We create a card by making a tuple. We access the suit via card[0] and denom via card[1]. What is good and what is bad about this implementation?
5. What types of variables can we make? Is this good enough?
6. Second big Q What defines a type? Data + operations what you can store. what you can do to or with it.
7. Lecture... a class is like a recipe (or template). you don't eat the recipe, right? an object is an instantiation of that class that's what you eat. Or, a class is a new type.
8. Class (or Type) Marriage of data and methods. Boundary between caller and implementer perspective on the world. self is a pointer to the object, from within the code. Each object has its own namespace, accessible via self. (In fact, isn't an object just a namespace? Could be...)
9. Creating class instances Q: One creates a new instance of a class by calling the class ______________. A: constructor
10. What does constructor code do? Q: When code instantiates a class, the __init__ method is called. This method's "primary purpose is to establish initial values for the ____________ of the newly created object." A: attributes
11. Code for Car constructor Q: I have a class Car that I instantiate this way: car = Car() Write the method definition for Car’s constructor. A: def __init__(self): # code here to initialize attributes to # default values … self._color = “black”
12. Signature Q: After creating a Car instance (i.e., object), my main code wants to set the Car's color to a new color. Write the signature of the Car member function, setColor(), to set the car's color. A: def setColor(self, color): ‘’’set the color of this car to the given color’’’ self._color = color
13. Signature Q: Then, my main code wants to get the color from my car instance. Write the signature for the member function getColor(). A: def getColor(self): ‘’’return the color of this Car to the caller.’’’ return self._color # my color
14. Designing a Class When you design a class, you decide: what are the important properties (or attributes, characteristics, or state) of an instance. what information a caller should be able to get from an object what state a caller should be able to change in an object what a caller should be able to ask an object to do to itself.
15. Naming Conventions A class name always starts with a capital letter. Each word in the class starts with a capital letter. class MyFathersCar: class Cs106Lab: A class attribute always starts with an underscore: _ self._x_loc, self._y_loc, self._color
16. Naming Conventions local variables must start with a lowercase letter. global variables must also start with a lowercase letter, unless they are CONSTANTS, which must be all uppercase. function names start with a lowercase letter. Either use camelCase for function names and variables or use _'s between words in a name. myXLocation, or, my_x_location getLocation(), or, get_location()
17. Order of methods in a class Q: Does the order the methods are defined in a class matter? A: No. By convention the constructor method is always first, however.
18. Attribute names Q: Why would the coder choose _x and _y instead of x and y? A: It is a convention to name your attributes with names starting with _. The reader of the code can then know what something is when it sees the _ in front of the name.