Day 3 – OOPs, I wrote it again… - Scott Gorlin

OOPs, I wrote it again...
IAP 2009
Scott Gorlin
gorlins@mit.edu
http://stellar.mit.edu/S/project/advanced-matlab/

Excuse the title…
• OOP - Object Oriented Programming
• Object is a marriage of:
• Methods – functions
• Fields/Properties – data

OOP
• In the beginning there was
• Hole code
• Hex code
• Spaghetti code
• Function code
• Object code

• Spaghetti Code
OOP
• Before functions, programs were lists of
commands that sometimes jumped from line to
line
• Debugging was awful, because you had to
unravel the spaghetti trace!

• Function Code
OOP
• Functions, with limited scope, are the ideal
basis for reusable subunits of code
• Many programs/engines exist as a collection of
functions, making design/debugging easier

• Function Code
OOP
• BUT, with larger/cooperative projects, functions can
intertwine just like spaghetti!
• Simple example – loadData(), loadData() , but now what kind of
data did you want to load?
• loadDataForScott()
• loadMyData()
• loadMyData2()
• cortinaLoadData3()
• loadDataForScottsExperimentOnNov12_1987_noDisp()

OOP
• Object-Oriented Programming
• Organizes functions by context, relation, and data type
• Goals
• Create larger, more significant, reusable subunits of code
• Isolate and protect all methods related to a kind of data
• Easily reusable collections for doing a given thing
• Concise, related documentation

OOP
• So, What Is An Object?
Terminology
• Terminology
• Class – definition of a kind of object, along
with what that object can do
• Object – instance of a class, containing unique
data

OOP
• Real world examples
• Class Car (type ( type of variable) variable
• Methods
• drive, turn, park, cruise, fillUp, crash
• Fields
• Owner, Color, Speed, Gas, BatteryLevel
• Objects (variables ( variables)
• myCar = [];
• timsCar.Owner = ‘Tim’, timsCar.Gas = 0.9;

OOP
• Real world examples
• Car method definitions (pseudocode):
function drive(car, driver):
if driver == car.Owner
car.Speed = 10
else
car.soundAlarm()
if car.hasOnStar()
car.callPolice()

• An Object is:
OOP
• A set of intimately related data and functions
• A Class is:
• The definition of those relations

OOP
• Basic Usable Example
• Class for a given data/file type
• Methods
• read(myData), write(myData), plot(myData)
• Instead of
• readMyDataType(d), writeMyDataType(d),
plotMyDataType(d)

OOP
• What is an object in Matlab?
• Actually just a structure with a different name
• Contains all properties of a structure (has any
set of fields/sizes) with the additional benefits:
• Calling a function on that structure calls a class
method instead of the general function on the path,
if it exists
• Access to fields strictly controlled by methods
• Dual syntax:
method(obj, args1, arg2, ...)
===
obj.method(arg1, arg2, ...)

• One problem...
OOP
• Syntax has completely changed in 7.6
• Much better, but 7.6 code will not run in prior
versions
• Old classes will run in 7.6+ (for now)
• Old syntax not covered here, but see my slides
from last year

OOP
• How to create a class in Matlab
• Create an mfile named MyClass.m or a folder
named @MyClass
• This folder should be in a path directory, but you do
NOT need to add the folder itself to the path!
• Create MyClass.m in the @MyClass folder
• Allows defining methods in new m-files in this
folder, otherwise two methods are identical

OOP Example
• Let’s create a class for a polynomial function
N
∑
i=1
• f(x) =
Ai x
• Finished class is online
B i
• Create m-file Polynomial.m

OOP Example
• MyClass.m must define:
• Class name, class attributes, class inheritance
• Properties of the class
• And property attributes
• Methods for the class
• Alternatively, each method can be defined in
a new m-file in the @class directory

OOP Example

Encapsulation
• Encapsulation
OOP
• Object fields are (usually) PROTECTED
• Cannot read/write them from parent function,
only object methods can!
• Works like scope for functions
• We must write methods to allow user to modify
object fields (like argument passing and local
variables)

Encapsulation
• Encapsulation
OOP
• ‘Private’ Private’ or 'protected' items only visible to the
object itself
• ‘Public’ Public’ items visible and/or modifiable from
outside

Encapsulation
• Encapsulation
• Why?
OOP
• Defensive: All property controls go through object
• Can’t write ‘3 rd party’ functions that modify data
without object knowing it!
• Provides only one access path to critical data

Encapsulation
• Encapsulation
OOP Example
• Here, A and B are protected
• They must always be synced, ie same shape,
and in the same order
• Now they can never change unless the object
does it itself

OOP Example
• Class constructor
• Must build an object!
• Method with same name
as class
• W/o constructor,
properties get default
values
• Here, we ensure A and B
are properly handled

• Overloading
OOP
• method(myObject) calls class method instead
of general function method if it exists
• Define how basic functions handle class (plot, ( plot,
…)
display, plus, get, set, subsref…)
subsref
• Simpler function names!

OOP Example
• Overloading
• Changing built-in
functions to handle
our class

OOP Example

OOP Example
• Inheritance – building upon existing classes
• Gets all fields and methods from parent
• Now we can overload parent functions!
Passes arguments to
parent constructor
Could add props or change defaults if desired
Calls eval in parent class, passing in
the LogPoly. object lp
Declares that L.P. inherits from P.

Inheritance
• Inheritance
OOP
• Write hierarchy of classes
• Define methods/properties in root, all subclasses
get them
• Modify subclasses as needed
• All subclasses now benefit from same set of
functions
• Much better than writing similar functions for
many types of data!!!

• Inheritance – Example
PsychExp class
OOP
• PsychExp
• Methods to set up computer for experiment, load subject data,
create datafile, calibrate equipment
• VisualExp inherits PsychExp
• Methods to create visual stimuli, saves slightly different subject
data, convert pixels to degrees of visual angle...
• Now create a VisualExp object and you have all the general
experiment methods and the specific ones
• AudioExp inherits PsychExp
• Now, modifying PsychExp will update all classes
which inherit it!

• The Big Advantages
• Encapsulation
• Overloading
• Inheritance
OOP
• Aesthetics/Deployment/Collaboration

• Class Tricks
OOP
• Object fields can contain child objects!
• At command prompt, we get the following:
lists all the defined methods
• >> methods myClass lists all the defined methods
• >> help @myClass lists defined methods and the first
line of their documentation
• >> edit myClass/myMethod
Pass by value: obj = method(obj);
• Pass by value:

• Class Tricks
• struct(myObject)
OOP
struct(myObject) returns the structure inside the
object, allows you to view data without encapsulation
(great for debugging)
• To avoid encapsulation
• function p = get(myObj, property)
p = myObj.(property);
• Similar with set
• Also can overload subsref to customize myObject.x,
myObject(x), myObject{x} usage

• Wrap up OOP
Monday
• Dynamic Programming
• An introduction to ducks