1. Classes and Objects – digging deeper
Victor Norman
CS104

2. Reading Quiz

3. 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, …

4. 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

5. 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

6. 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

7. Write normalize()
class Vector: … def normalize(self): mag = self.getMagnitude() self._x /= mag self._y /= mag

8. 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()

9. 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.

10. 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) + “-->”)

11. 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.

12. 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())

13. Other built-in operations
__sub__
__mul__
__len__
__int__
__cmp__: for doing ==, <, <=, >, >=, !=, etc.
etc.!