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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 )

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Chapter 17 Q and A

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Chapter 17 q and a

Chapter 17 Q and A

Victor Norman, et al.

CS104


What is object oriented programming

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

Example: Set class (type)

  • Old style:

    • set = newSet(<optional values>)

    • set = setAdd(set, newItem)

    • set = setDel(set, delItem)

    • tOrF = setIn(set, value)

    • set = setAnd(set, otherSet)

    • set = setOr(set, otherSet)

  • New style:

    • set = Set(<optional values>)

    • set.add(newItem)

    • set.del(delItem)

    • tOrF = set.in(value)

    • set = set.and(otherSet)

    • set = set.or(otherSet)


Binding together data and ops

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

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

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

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

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

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

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

__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

self parameter

  • if we want to do scrib.getAngle(), we implement:

    …. in Scribbler class ….

    defgetAngle(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

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

Other operators to overload

  • Common operators to overload:

    • __cmp__(self, other): compares two objects. Return -1 if self < other, 0 if self == other, 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.


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