[PPT] - Objects [Andersen, Gries, Lee, Marschner, Van Loan, White] Lecture PowerPoint Presentation

SLIDE 1

Objects

Lecture 7

CS 1110:

Introduction to Computing Using Python

[Andersen, Gries, Lee, Marschner, Van Loan, White]

SLIDE 2

Lecture 7 Announcements

Please check the end of the Lecture 6 slides (slides 25-

29) for many announcements:

http://www.cs.cornell.edu/courses/cs1110/2017sp/lectures/02-14-17/presentation-06.pdf

Incorrect link for how to break up long lines in Section

10 of Assignment 1. Watch course website for announcements about A1:

http://www.cs.cornell.edu/courses/cs1110/2017sp/announcements.php

2/16/17 Objects 2

SLIDE 3

Review: Types

Type int:
Values: integers
Ops: +, –, *, /, %, **
Type float:
Values: real numbers
Ops: +, –, *, /, **
Type bool:
Values: True and False
Ops: not, and, or
Type str:
Values: string literals
Double quotes: "abc"
Single quotes: 'abc'
Ops: + (concatenation)

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SLIDE 4

Built-in Types are not “Enough”

Want a point in 3D space
We need three variables
x, y, z coordinates
What if have a lot of points?
Vars x0, y0, z0 for first point
Vars x1, y1, z1 for next point
…
This can get really messy
How about a single variable

that represents a point?

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x 2.0 y 3.0 z 5.0

SLIDE 5

Can we stick them

together in a “folder”?

Motivation for objects

Built-in Types are not “Enough”

Want a point in 3D space
We need three variables
x, y, z coordinates
What if have a lot of points?
Vars x0, y0, z0 for first point
Vars x1, y1, z1 for next point
…
This can get really messy
How about a single variable

that represents a point?

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x 2.0 y 3.0 z 5.0

SLIDE 6

Objects: Organizing Data in Folders

An object is like a manila folder
It contains other variables
Variables are called attributes
These values can change
It has an ID that identifies it
Unique number assigned by Python

(just like a NetID for a Cornellian)

Cannot ever change
Has no meaning; only identifies

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id1 x 2.0 y 3.0 z 5.0 Unique tab identifier

SLIDE 7

Classes: Types for Objects

Values must have a type
An object is a value
Object type is a class
Modules provide classes
Will show how later
Example: geom
Classes: Point2, Point3

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id1 x 2.0 y 3.0 z 5.0

Point3

class name

SLIDE 8

Classes: Types for Objects

Classes

Point3
Point2

Types

int
float
bool
str
Classes are how we add new types to Python
Sort of like a template

Class

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Point3s have an x, y, and z Point3

SLIDE 9

Constructor: Function to make Objects

How do we create objects?
Constructor Function:
Format: ⟨class name⟩(⟨arguments⟩)
Example: Point3(0.0,0.0,0.0)
Makes a new object (manila folder)

with a new id

Called an instantiated object
Returns folder id as value

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instantiated

bject

id2 x 0.0 y 0.0 z 0.0

Point3

Point3s have an x, y, and z Point3 new id (in this case id2)

SLIDE 10

Constructor: Function to make Objects

How do we create objects?
Constructor Function:
Format: ⟨class name⟩(⟨arguments⟩)
Example: Point3(0.0,0.0,0.0)
Makes a new object (manila folder)

with a new id

Called an instantiated object
Returns folder id as value
Example: p = Point3(0.0, 0.0, 0.0)
Creates a Point object
Stores object’s id in p

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id2 p Variable stores ID not object instantiated

bject

id2 x 0.0 y 0.0 z 0.0

Point3

Like a Greek god!

SLIDE 11

Constructors and Modules

>>> import geom >>> p = geom.Point3(0.0,0.0,0.0) >>> id(p)

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id2 p id2 x 0.0 y 0.0 z 0.0

Point3

Need to import module that has Point class. Constructor is function. Prefix w/ module name. Shows the id of p.

SLIDE 12

Accessing Attributes

Attributes are variables

that live inside of objects

Can use in expressions
Can assign values to them
Format: ⟨variable⟩.⟨attribute⟩
Example: p.x
Look like module variables
To evaluate p.x, Python:
1. finds folder with id stored in p
2. returns the value of x in that folder

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id3 x 1.0 y 2.0 z 3.0 id3 p

Point3

SLIDE 13

Accessing Attributes

Example:
p = geom.Point3(1.0, 2.0, 3.0)
p.x = p.y + p.z

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id3 x 1.0 y 2.0 z 3.0 id3 p

Point3

5.0 x

SLIDE 14

Object Variables

Variable stores object id
Reference to the object
Reason for folder analogy
Assignment uses object id
Example: liked = disliked
Takes contents from disliked
Puts contents in liked
Does not make new folder!
This is the cause of many

mistakes in this course

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id2 disliked id2 x 0.0 y 0.0 z 0.0

Point3

id2 liked

SLIDE 15

Exercise: Attribute Assignment

>>> p = geom.Point3(0,0,0) >>> q = p

Execute the assignments:

>>> p.x = 5.6 >>> q.x = 7.4

What is value of p.x?

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id4 p id4 q A: 5.6 B: 7.4 C: id4 D: I don’t know id4 x 0.0 y 0.0 z 0.0

Point3

CORRECT

SLIDE 16

Exercise: Attribute Assignment

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id4 p id4 q A: 5.6 B: 7.4 C: id4 D: I don’t know id4 x 0.0 y 0.0 z 0.0

Point3

5.6 CORRECT

x

>>> p = geom.Point3(0,0,0) >>> q = p

Execute the assignments:

>>> p.x = 5.6 >>> q.x = 7.4

What is value of p.x?

SLIDE 17

Exercise: Attribute Assignment

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id4 p id4 q A: 5.6 B: 7.4 C: id4 D: I don’t know id4 x 0.0 y 0.0 z 0.0

Point3

5.6 7.4 CORRECT

x x

>>> p = geom.Point3(0,0,0) >>> q = p

Execute the assignments:

>>> p.x = 5.6 >>> q.x = 7.4

What is value of p.x?

SLIDE 18

>>> p = 5.0 >>> q = p >>> p = 4.0 >>> q 5.0 >>> from geom import * >>> p = Point3(1.0,2.0,3.0) >>> q = p >>> p.x = 4.0 >>> q.x 4.0

The rules of variables have not changed! However, combining variable assignment with object references can be confusing.

Assignment and Attribute Oddness

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!!

SLIDE 19

Call Frames and Objects

Objects can be altered in a

function call

Object variables hold ids!
Folder can be accessed from

global variable or parameter

Example:

def incr_x(q): q.x = q.x + 1.0

>>> p = geom.Point3(1.0, 2.0, 3.0) >>> incr_x(p)

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1 incr_x 1 id5 q

Global STUFF Call Frame

id5 p id5 1.0 …

Point3

x

SLIDE 20

Call Frames and Objects

Objects can be altered in a

function call

Object variables hold ids!
Folder can be accessed from

global variable or parameter

Example:

def incr_x(q): q.x = q.x + 1.0

>>> p = geom.Point3(1.0, 2.0, 3.0) >>> incr_x(p)

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1 incr_x id5 q

Global STUFF Call Frame

id5 p id5 1.0 2.0 …

Point3

x

SLIDE 21

Call Frames and Objects

Objects can be altered in a

function call

Object variables hold ids!
Folder can be accessed from

global variable or parameter

Example:

def incr_x(q): q.x = q.x + 1.0

>>> p = geom.Point3(1.0, 2.0, 3.0) >>> incr_x(p)

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1 Global STUFF Call Frame

id5 p id5 1.0 2.0 …

Point3

x

SLIDE 22

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) Draw everything that gets created. How many folders get drawn?

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SLIDE 23

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) Draw everything that gets created. How many folders get drawn?

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Point3 x 1.0 y 2.0 z 3.0 id1 Point3 x 3.0 y 4.0 z 5.0 id2

SLIDE 24

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) Draw everything that gets created. How many folders get drawn? What else gets drawn?

2/16/17

Point3 x 1.0 y 2.0 z 3.0 id1 Point3 x 3.0 y 4.0 z 5.0 id2

SLIDE 25

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) Draw everything that gets created. How many folders get drawn? What else gets drawn?

2/16/17

Point3 x 1.0 y 2.0 z 3.0 id1 Point3 x 3.0 y 4.0 z 5.0 id2 id1 p id2 q

SLIDE 26

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap_x(p, q) def swap_x(p, q):

1

t = p.x

2

p.x = q.x

3

q.x = t Execute swap_x on what we just drew. There should be a call frame. What is in p.x at the end? A: 1.0 B: 2.0 C: 3.0 D: I don’t know

SLIDE 27

Exercise: Attribute Assignment

2/16/17 Objects 27

def swap_x(p, q):

1

t = p.x

2

p.x = q.x

3

q.x = t id1 p id2 q p q t swap_x:1 Point3 x 1.0 y 2.0 z 3.0 id1 Point3 x 3.0 y 4.0 z 5.0 id2 import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap_x(p, q) p q swap_x

SLIDE 28

Exercise: Attribute Assignment

2/16/17 Objects 28

def swap_x(p, q):

1

t = p.x

2

p.x = q.x

3

q.x = t id1 p id2 q p q t swap_x:1 Point3 x 1.0 y 2.0 z 3.0 id1 Point3 x 3.0 y 4.0 z 5.0 id2 import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap_x(p, q) p q id1 id2 swap_x 1

SLIDE 29

Exercise: Attribute Assignment

2/16/17 Objects 29

def swap_x(p, q):

1

t = p.x

2

p.x = q.x

3

q.x = t id1 p id2 q p q t swap_x:1 Point3 x 1.0 y 2.0 z 3.0 id1 Point3 x 3.0 y 4.0 z 5.0 id2 import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap_x(p, q) p q t id1 id2 1.0 swap_x 2

SLIDE 30

Exercise: Attribute Assignment

2/16/17 Objects 30

def swap_x(p, q):

1

t = p.x

2

p.x = q.x

3

q.x = t id1 p id2 q p q t swap_x:1 Point3 x 1.0 3.0 y 2.0 z 3.0 id1 Point3 x 3.0 y 4.0 z 5.0 id2 import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap_x(p, q) p q t id1 id2 1.0 swap_x 3

x

SLIDE 31

Exercise: Attribute Assignment

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def swap_x(p, q):

1

t = p.x

2

p.x = q.x

3

q.x = t id1 p id2 q p q t swap_x:1 Point3 x 1.0 3.0 y 2.0 z 3.0 id1 Point3 x 3.0 1.0 y 4.0 z 5.0 id2 import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap_x(p, q) p q t id1 id2 1.0 swap_x

x x

SLIDE 32

Exercise: Attribute Assignment

2/16/17 Objects 32

def swap_x(p, q):

1

t = p.x

2

p.x = q.x

3

q.x = t id1 p id2 q Point3 x 1.0 3.0 y 2.0 z 3.0 id1 Point3 x 3.0 1.0 y 4.0 z 5.0 id2 import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap_x(p, q)

x x

SLIDE 33

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap_x(p, q) def swap_x(p, q):

1

t = p.x

2

p.x = q.x

3

q.x = t Execute swap_x on what we just drew. There should be a call frame. What is in p.x at the end? A: 1.0 B: 2.0 C: 3.0 D: I don’t know CORRECT

SLIDE 34

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap(p, q) def swap(p, q):

1

t = p

2

p = q

3

q = t What is in global p after calling swap? A: id1 B: id2 C: I don’t know id1 p id2 q Before calling swap(p, q):

SLIDE 35

Exercise: Attribute Assignment

2/16/17 Objects 35

import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap(p, q) def swap(p, q):

1

t = p

2

p = q

3

q = t id1 p id2 q p q id1 id2 Point x 1.0 y 2.0 z 3.0 id1 Point x 3.0 y 4.0 z 5.0 id2 swap 1

SLIDE 36

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap(p, q) def swap(p, q):

1

t = p

2

p = q

3

q = t id1 p id2 q p q t id1 id2 id1 Point x 1.0 y 2.0 z 3.0 id1 Point x 3.0 y 4.0 z 5.0 id2 swap 2

SLIDE 37

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap(p, q) def swap(p, q):

1

t = p

2

p = q

3

q = t id1 p id2 q p q t swap id1 id2 id2 id1 Point x 1.0 y 2.0 z 3.0 id1 Point x 3.0 y 4.0 z 5.0 id2

x

3

SLIDE 38

Exercise: Attribute Assignment

2/16/17 Objects 38

import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap(p, q) def swap(p, q):

1

t = p

2

p = q

3

q = t id1 p id2 q p q t id1 id2 id2 id1 id1 Point x 1.0 y 2.0 z 3.0 id1 Point x 3.0 y 4.0 z 5.0 id2

x x

swap

SLIDE 39

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap(p, q) def swap(p, q):

1

t = p

2

p = q

3

q = t id1 p id2 q Point x 1.0 y 2.0 z 3.0 id1 Point x 3.0 y 4.0 z 5.0 id2

SLIDE 40

Exercise: Attribute Assignment

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import geom p = geom.Point3(1.0,2.0,3.0) q = geom.Point3(3.0,4.0,5.0) swap(p, q) def swap(p, q):

1

t = p

2

p = q

3