Classes and Objects – digging
deeper
Victor Norman
CS104
Reading Quiz
Vector class
• Suppose we want a 2D Vector “type”.
• What do we need to store? (what attributes?)
– x, y
• What operations do we need to be able to do?
–
–
–
–
–
–
set and get x, y
normalize
get magnitude
scale
add 2 Vectors
cross products, dot products, …
Write constructor for Vector
Write the constructor, which takes params x and
y, with default values 0 and 0.
class Vector:
def __init__(self, x=0, y=0):
self._x = x
self._y = y
constructor
Write getters and setters
Write the getter and setter for x attribute.
class Vector:
…
def getX(self):
return self._x
def setX(self, newX):
self._x = newX
accessor
mutator
Write getMagnitude()
Magnitude is distance of x, y from 0, 0.
class Vector:
…
def getMagnitude(self):
return (self._x * self._x +
self._y * self._y) ** 0.5
Write normalize()
class Vector:
…
def normalize(self):
mag = self.getMagnitude()
self._x /= mag
self._y /= mag
Use it
• Create a Vector pointing to 16, 12.
vect = Vector(16, 12)
• print the vector’s x and y values
print(vect.getX(), vect.getY())
• print the magnitude of the vector
print(vect.getMagnitude())
• normalize the vector
vect.normalize()
Printing a Vector
• Would be nice to be able to tell a vector
object to convert itself to a string
representation. Can be done by defining
__str__ in the class, so that:
– print(vect) calls
– str(vect) calls (which happens automatically)
– vect.__str__() automatically.
• __str__ must return a string representation of
the object.
Code for __str__
• Suppose we want a vector to be printed this
way:
– ---16, 12-->
class Vector:
…
def __str__(self):
return (“---“ + str(self._x) + “, “ +
str(self._y) + “-->”)
Adding two vectors.
• What if we want to add two vectors together?
vect1 = Vector(16, 12)
vect2 = Vector(-12, -16)
res = vect1 + vect2
print(res)
• + is defined for integers, floats, strings, but
python can’t just add two Vector objects
together without being told how.
Defining __add__
• for op1 + op2, intepreter looks at type of op1,
and looks if __add__ is defined for it. If it is, it
calls it, passing op2 as the 2nd parameter. It
returns a new Vector object.
class Vector:
…
def __add__(self, op2):
return Vector(self.getX() + op2.getX(),
self.getY() + op2.getY())
Other built-in operations
•
•
•
•
•
•
__sub__
__mul__
__len__
__int__
__cmp__: for doing ==, <, <=, >, >=, !=, etc.
etc.!