Q and A for Sections 6.2, 6.3 Victor Norman CS106

Parameterized Constructor Q: Can you create multiple constructors for a class, so that you can create new objects in different ways? A: No. You can have only 1 __init__ definition. But you can use optional parameters to allow the caller to create objects in multiple ways. Remember: in general, a constructor should initialize all its member attributes to values.

Example class Card: def __init__(self, num=2, suit=“H”): “””Create new card. If num and suit are not specified, it is a 2 of hearts.””” self._num = num self._suit = suit card = Card() # 2 of hearts card2 = Card(11, “S”) # jack of spades

Exercise Given this definition, write the accessor and mutator methods. class Car: def __init__(self, numWheels=4, color=“black”): self._numWh = numWheels self._color = color

How does __str__ get called? Special methods get called by the python interpreter when they are defined in a class. e.g., __str__(), if defined, is called by python whenever the object must be converted to a string – like when str(obj) is called or print obj is called. in other words: the python interpret sees this: print obj. – It looks up the class of obj, and checks if __str__ has been defined for it. If so, it calls it to get the string representation for it. Otherwise, it calls a built-in way to convert it – usually resulting in something like

What other special methods are there? del.html del.html Especially useful: __cmp__(self, other): compare two objects __contains__(self, item): called when in is used __add__, __sub__, __mul__, etc.: x + y  x.__add__(y), based on x’s type.

Which special methods must we know? __init__ __str__ __cmp__ (others, like __add__, are occasionally useful…) NOTE NOTE NOTE: do *not* define your own methods with __ before and after.

Polymorphism Means: operators/functions (like +, -, abs(), len(), in, and, +=, %) may behave completely differently, based on the types of their operands. Also known as “operator overloading” “Josh ” + “Bulten”  concatenation  addition [‘Go’] + [‘Calvin’]  append

jumpPrevChannel() code Television class has two attributes: _channel, _prevChan. Wrong: self._channel = self._prevChan self._prevChan = self._channel Need to swap values: easiest with tuple assignment: self._channel, self._prevChan = self._prevChan, self._channel

isinstance() returns boolean Very useful for checking types of input parameters to a function/method isinstance(object, (tuple, of, types))  boolean if not isinstance(param, (int, float)): print “Illegal parameter!” (We’ll learn how to handle this better next week, with exceptions.)

Objects that reference other objects Very common to have multiple classes, and objects in one class reference other classes. E.g.: deck object has a list of card objects. – Always want to store objects – not, e.g., string representations of objects. Important to get the methods in the correct classes.

Calling methods from methods Q: Is calling a method within a method definition similar to calling a function from within another function? A: Yes and no. Yes: control is transferred to the method and returned back when the method completes. No: you have to use self.calledMethod() to do the call.

Example class Hand: def __init__(self, ……): … def isStraight(self): … def isFlush(self): … def isStraightFlush(self): ‘’’return True if this hand is a straight flush, False otherwise.’’’ return self.isFlush() and self.isStraight()

Two-dimensional arrays/lists Often in modeling the world, we need to use two-dimensional arrays of “cells”. We can model many different things with 2d arrays: – landscapes – petri dishes – mini-world – matrix store as a list of lists.

2D lists Like putting turtles on the screen, the 2d array has a width and height. Typically x is horiz, y is vertical. Often (0, 0) is upper-left corner. If world is your 2d list, then we access by world[x][y] x is column index, y is row index (or, y is the item in the y-th row in the x-th column) world[x] is a column.

Initializing a 2D list (list of lists) Want MAXCOLS columns and MAXROWS rows, each holding subsequent integers from 0. – for 3x4 array: world = [ [0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11] ] MAXROWS = 3 MAXCOLS = 4 world = [] for x in range(MAXCOLS): # create MAXCOLS lists world.append([]) val = 0 for col in world: # add MAXROWS items to each column for y in range(MAXROWS): col.append(val) val += 1

Exercise: average neighbors Write code to find the sum of 4 neighbors of cell at x, y, assuming cell is not on an edge. sum = world[x-1][y] + world[x+1][y] + world[x][y-1] + world[x][y+1]

Example: Teams, Players Write a class definition for a Player class. Each Player object has these attributes: _name _number _age _position _salary _team Write the constructor definition.

Continued Write a class definition for a Team. Each Team object contains _name _players addPlayer () delPlayer () getTotalSalary () Write the constructor (setting name and players). Write addPlayer ().

Continued Want to be able to print out this for a player: Peyton Manning, #18, QB, Denver Write the code to do this.

Continued Write the code for getTotalSalary () in Team.