Matlab - object orientated with abstract Interface - How creating object? - matlab

Question

Hello lovely community,
i am quite new here, but still hope someone can help me out. I just worked a bit with Matlab in the past and want to do a new project. Earlier I just stored all in one Matlab file and didn't had the need to use classes. This has changed now, so I hope someone can explain me what I did wrong.
My desire is to create the project object based. Therefore I started creating new folders in the main folder:
C:\Users\Luftfahrt\MAV\
The folder names are:
+Data
+Model
After I read the first chapters of
A guide to MATLAB orientaded programming
I figured out that with the plus symbol I am creating public objects. Exactly as I want. In each folder I inserted an abstract Interface and a child for it.
DataLoaderIF
classdef DataLoaderIF< handle
methods (Abstract=true)
Data = LoadData(Asset,Start,End)
Status = getStatus(Obj)
Info = getInfo(Obj)
end
end
When i run this code above he is telling me that Abstract classes cannot be instantiated, but I did it exactly as in the book. Maybe I thought, it is stupid run code from an abstract interface, so that there is no problem, is that true?
DataLoader
classdef DataLoader < DataManager.DataLoaderIF
%Inheritent from DataLoaderIF. Is setting parameter and Importing
properties (Access=protected)
mStatus =-1; %nothing done
mInfo ='Import'; %
mData; %saving the imported data
mCollector; %to save the prices after structuring
end
methods
%creating the constructor
function Obj = DataManager
function getStatus(Obj)
mStatus=1;
end
function getInfo(Obj)
Info='Import';
mInfo= Info;
end
function LoadData(Asset,Start,End)
connection = yahoo;
mData= fetch(connection, Asset, Start,End, 'd');
close(connection)
%'JPY=X', '01-Jun-2011', datestr(now,'dd-mmm-yyyy'), 'd');
mCollector= [{'date' 'open' 'high' 'low' 'close' 'volume' 'adj-close'}
cellstr(datestr(mData(:,1))) num2cell(mData(:,2:end))];
end
end
What I wanted to do now is to create an Object and then give this Object the variables to perform the fetch. But how can I instantiate now the Object? He says my constructor is false.
Someone has some ideas?
P.S. I already looked here Documentation Matlab, but this didn't help me out.
Thanks a lot!

Answer 1

It is not clear what you want to do exactly and why you need an abstract class then a derived one, but assuming you really need all that for more purpose than you described, take the following into account:
By definition in OOP, an abstract class cannot be instantiated. It is only a basic framework for the different classes which will inherit from it, and you can only instantiate these inheriting classes. abstract classes are only useful if the different inheriting classes represent different objects but need to share a set of (almost) common methods. If not, forget the 'abstract' parent and just design the class that does what you need.
Be careful with the classes which inherit from the handle class. In Matlab they behave differently than the value classes. Read this article and decide which type you want.
If you decide to use a handle class, consider the following very important factor Initialising property value:
Initializing Properties to Unique Values
MATLAB assigns properties to the specified default values only once
when MATLAB loads the class definition. Therefore, if you initialize a
property value with a handle-class constructor, MATLAB calls this
constructor only once and every instance references the same handle
object. If you want a property value to be initialized to a new
instance of a handle object each time you create an object, assign the
property value in the constructor.
Now with all that in mind, below is a version of your class that does run, and that you can instantiate as below:
>> dl = DataLoader
dl =
DataLoader with no properties.
>> dl.getInfo
ans =
Import
>> dl.getStatus
ans =
-1
>> dl.LoadData(1,2,3) %// etc ...
The class definition is as follow
classdef DataLoader < DataLoaderIF
properties (Access=protected)
mStatus %// nothing done
mInfo %//
mData %// saving the imported data
mCollector %// to save the prices after structuring
end
methods
%// Constructor (and initialise default value). For classes which
%// inherit from the "handle" class it is very important to
%// intialise your default values IN THE CONSTRUCTOR, and NOT in
%// the property definition.
function obj = DataLoader
obj.mStatus = -1 ;
obj.mInfo = 'Import' ;
obj.mData = [] ;
obj.mCollector = [] ;
end
function Status = getStatus(obj)
Status = obj.mStatus ;
end
function Info = getInfo(obj)
%// a "get" type of function should not assign value, only
%// return information about the object.
Info = obj.mInfo ;
end
%// the "obj" parameter has to be the first parameter of the
%// function in it's signature.
function LoadData(obj,Asset,Start,End)
%// Put function help here
%// I don't know what this part of the code is supposed to do
%// so I leave it as it is. Be aware that the fucntion "fetch"
%// will have to be accessible in your context or the function
%// will error
connection = 'yahoo' ;
obj.mData = fetch(connection, Asset, Start , End, 'd') ;
close(connection)
obj.mCollector= [{'date' 'open' 'high' 'low' 'close' 'volume' 'adj-close'}
cellstr(datestr(obj.mData(:,1))) num2cell(obj.mData(:,2:end))];
%// Here do your own test with your own conditions to decide
%// what the status of the object should be.
if ~isempty(obj.mData) && ~isempty(obj.mCollector)
obj.Status = 1 ;
end
end
end
end
You seemed a bit confused on some concept of OOP. If your first contact with OOP is through Matlab I totally understand where you come from. I had the same experience and my understanding of OOP was wrong for many years (my OOP code was very poor and inefficient too as a result), until I properly learned my way in C++ and .Net. These last two languages are real OOP languages (I know, not the only ones, or even better one, I don't want to start a debate), in the sense that you need to understand the OOP concepts to get anything out of them. On the other hand, you can spend your life doing wonderful things on Matlab and remain blissfully unaware of what even OOP means.
OOP capability was introduced in Matlab as a "convenience" for programmers who missed this way of structuring their code. The first implementations were very clunky and unpractical (raise your hand if you remember having to code each and every one of your subassign, subsref, display, get, set etc for each single object !!). It has been drastically improved since then, until it is now something worth the extra coding effort if you want to get the benefit of OOP organisation (code reuse, polymorphism, inheritance etc ...). However, to this day the OOP syntax in Matlab remain un-instinctive at first even for OOP veterans (why the hell do we have to declare the object itself as the first parameter of the function, is the compiler so blind it does't see the function is in the class definition?). You just have to get used to it first, then it gets ok.
The Matlab documentation on OOP is mostly oriented to these veterans. It explains to people who knows OOP on another languages how to apply the concepts in the Matlab specific syntax. It is not at all a very good guide for pure beginners.
This (longer than I wanted, sorry) blurb, was to try to explain that:
If you need to learn OOP concepts, train yourself with another language first. (seriously, even with the learning curve of learning another language, you'll loose less time than with the trial and error approach on Matlab).
If you do not have a definite identified need for OOP code in Matlab, do not insist. You can do a lot of things (in fact almost everything) in Matlab without resorting to classes (at least not knowingly, of course you will use the base Matlab classes in the background, but you can use and maintain your car for many years without necessarily having an intimate knowledge of its every component).