Chapter 17 Q and A Victor Norman, et al. CS104

What is Object-oriented Programming? Q: What is object-oriented programming? A: It means defining classes/objects, which hold data, but also can do stuff to themselves or for the caller. Functional style: set_add(set, newItem) – Pattern: function(object, values) OO style: set.add(newItem) – Pattern: object.method(values)

Example: Set class (type) Old style: – set = newSet( ) – set = setAdd(set, newItem) – set = setDel(set, delItem) – tOrF = setIn(set, value) – set = setAnd(set, otherSet) – set = setOr(set, otherSet) New style: – set = Set( ) – set.add(newItem) – set.del(delItem) – tOrF = set.in(value) – set = set.and(otherSet) – set = set.or(otherSet)

Binding together data and ops A class definition binds together – the data an instance stores (its attributes) and – the operations on the data (called “methods”) Forces programmer to keep related code together. Allows you to create a new type.

Advantages of OO You can control how an object can be created and used. – because you write the code for how an object is constructed. You can hide how an object implements its operations (data hiding). – because you define what a user can do with objects, so the user doesn’t have to know how it works.

Example Suppose we want to define a type Scribbler, which will represent a programmable robot. We want it to be able to move around, keep track of where it is and what direction it is pointing, etc. – it might even keep track of where it saw obstacles, etc. Its attributes would be: x, y, angle, etc.

Example (2) To create one, we’d do this code: scrib = Scribbler() – calls the constructor code we’ve defined: class Scribbler: def __init__(self): self._x = 0 self._y = 0 self._angle = 0 # east Now, we control how a Scribbler is made and ensure its state is correct.

Example (3) What do we want to do a Scribbler? scrib.move(16, 12) – moves the robot there and object remembers where it is and what angle it is facing. scrib.getX() scrib.getY() scrib.getAngle() – scrib user can ask the scribbler where it is and what angle it is facing.

Example (4) scrib.seeObstacle() – ask the scribbler if it sees an obstacle. scrib.forwardUntilSeeObstacle() scrib.setAngle(newAngle) – turns the scribbler to that angle scrib.setY(newY) – moves the scribbler to new position

String representation (Review: when constructor is called, __init__() is run in the class.) When you do str(object), __str__ is run in the object’s class, if defined. – it must return a string, which should be a readable representation of that object. Very convenient to be able to do: print scrib # which calls str(scrib) before printing

__str__ code class Scribbler:... init method … def __str__(self): return “Scribbler at (%g, %g), facing %g degrees” % (self._x, self._y, self._angle) print scrib output: Scribbler at (3.0, 4.0), facing 45.0 degrees

self parameter if we want to do scrib.getAngle(), we implement: …. in Scribbler class …. def getAngle(self): return self._x Notice that scrib  self. Object before the. is the first parameter in the method definition. self is the name of the object in code in the class.

Operator Overloading if I do 3 + 4, we are adding integers. if I do “hi” + “there” we are concatenating strings. – This is overloading of the operator +. for a Set type, we might want to do this: s = Set(…) s = s + 3# add 3 to the set. We’d have to implement __add__ in Set class.

Other operators to overload Common operators to overload: – __cmp__(self, other): compares two objects. Return -1 if self other. (If implemented, you can sort objects.) – __len__(self): can use len(obj). Useful, e.g., for Set type to ask how many items are in the set. – __contains__(self, item): if implemented, can ask if something is in the object.