IS12 - Introduction to Programming Lecture 10: Classes and Objects

Peter Brusilovsky http://www2.sis.pitt.edu/~peterb/0012-161/

Outline

■  Objects as heterogeneous data aggregates

■  Methods ■  Encapsulation ■  Data processing with classes and

objects

Why Objects and OOP?

■  Heterogeneous data aggregates ■  Encapsulation of actions on data

objects ■  Inheritance and more efficient

programming ■  More efficient GUI programming

From Program to Data Structures

machine level data storage

primitive data types

basic data aggregates

high-level data structures

0100110001101001010001

28! 3.1415 'A'

stack dictionary tree

array structure

list

Object and Classes

■  Objects are – data aggregates –  that can hold heterogeneous data fields –  referring to these elements by name

■  A Class is a blueprint for objects with the same structure (same collection of fields)

Objects: Keeping data together

class Student: # name – student name

# ybirth – year of birth # height – sts' height

john.name = "John Doe" john.ybirth = 1989

john.height = 172.3

name

ybirth

height

172.3

1989

name

ybirth

height

john

John Doe

Objects: keeping data together

Class Point:

def __init__(self, x, y): self.x = x

self.y = y

pt1 = Point(0,0) pt2 = Point(1,3)

x

y

0

0

x

y

pt1

1

3

x

y

pt2

Objects: What we can do?

■  Create object of a class –  Constructors required p1 = Point(20, 0)

■  Assign field p1.y = 1

■  Use field print(p1.y) if p1.x < 30: p1.x = 30

■  Assign object –  Another reference is created p2 = p1

x

y

p1 20 0

x

y

p1 20 1

p2

Structures: What else we can do?

■  Pass the structure as a parameter to a function –  Reference is really passed myfunc(p1)

■  Return the whole structure from a function def mypoint(x1, x2, y):

pt3 = Point((x1 + x2)/ 2, y)

return pt3

Example: Above or Under

class Point:def __init__(self, x, y): self.x = x self.y = y

def readpoint():x = float(input("Enter x:"))y = float(input("Enter y:"))p = Point(x, y)return p

def above(pt1, pt2):if pt1.y > pt2.y: return Trueelse: return False

Example : Above or Under

def main():print("Enter base point")base = readpoint()for i in range(5): print("Enter point") p = readpoint() if above(p, base): print("This is above base point") else: print("This is not above base point")

Objects vs. Lists / Tuples

■  Objects –  Good for

heterogeneous data –  Each element has a

unique name within object: pt.x

–  Elements are accessed by name: top.x, mypt.y

–  Support OOP

■  Lists / Tuples –  Good for

homogeneous data –  Each element has a

position –  Elements are

accessed by index: pt[7], line[23]

–  Index can be computed: ar[i-1]

Classes and Methods

■  Once you have objects, you probably need various functions to work with them

■  Functions that a specifically designed to work with objects of a class are called methods

■  Technically, a class is a collection of methods that work on objects of this class

■  Methods are special kinds of functions –  Defined within a class –  Have a special treatment of its first parameter (self) –  Have a special "object-oriented" way of calling

Calling Functions vs Methods

■  Functions –  Not defined in a class –  No special treatment

of any parameters –  All parameters

passed as an ordered tupple

above(p1, p2)

distance(p1, p2)

■  Methods –  Defined in a class –  First parameter has

special meaning –  First parameter

passed in a special way, others are usual parameters

–  p1.above(p2) –  p1.distance(p2)

Object Creation: __init__

class Student: def __init__(self, name, ybirth, height): self.name = name self.ybirth = y self.height = h john = Student("John Doe", 1989, 172.3)

Class Point: def __init__(self, x, y): self.x = x self.y = y p1 = Point (20, 0)

172.3

1989

name

ybirth

height

john

John Doe

x

y

p1 20 0

Example: Above or Under

class Point:def __init__(self, x, y): self.x = x self.y = y def above(self, pt): if self.y > pt.y: return True else: return False

def readpoint():x = float(input("Enter x:"))y = float(input("Enter y:"))p = Point(x, y)return p

Example : Above or Under

def main():print("Enter base point")base = readpoint()for i in range(5): print("Enter point") p = readpoint() if p.above(base): print("This is above base point") else: print("This is not above base point")

Classes and Modules

■  Classes could be aggregated in modules

■  Modules could be used by other programs with import command

■  There are many exiting modules that could be used to write powerful programs with little own code

import randomimport math

Example: Distance (Class)

import mathclass Point:

def __init__(self, x, y): self.x = x self.y = y def above(self, pt): if self.y > pt.y: return True else: return Falsedef distance(self, p): return math.sqrt((self.x - p.x)**2 + (self.y - p.y)**2) def midpoint(self, p): return Point((self.x + p.x)/2, (self.y + p.y)/2)

Example: Distance (Main)

def main():print("Enter point A")a = readpoint()print("Enter point B")b = readpoint()c = a.midpoint(b)print("Distance between A and B", a.distance(b))print("Distance between A and midpoint", a.distance(c))print("Distance between B and midpoint", b.distance(c))

Nested Objects

class Rectangle:

r.pt1 = point(1,1)

r.pt2 = point(4,4)

pt1

pt2

Exercise

■  Define class Segment where each object is defined by two end points (x1,y1,x2,y2) –  Easy way: no nesting, use 4 numbers –  Harder way: nesting, use 2 objects of class point

■  Create method length to return length ■  Create method above to check whether one

segment if fully above the other ■  Create function to input a segment ■  Create main program that asks for data on 2

segments, return length of each, and checks whether one is above another

Before Next Lecture:

■  Do reading assignment (links online) ■  Run and explore Classroom Examples ■  Exercises:

– Finish classroom exercise ■  Check yourself by working with MG

system ■  Do mandatory exercises ■  Homework programming assignment

due Friday next week