matlab OOP how to transfer value from properties to methods ?

Question

tomer polsky 2018-6-25

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评论： Adam 2018-6-27

Hello I am trying to learn progrmaing OOP in Matlab . How ever i dont have exprienct with this kind of procdiel working .

if for exmaple i have this code :

classdef OOP_101
 properties
a=5
b=10
   end
   methods
       function c=adding(obj)
           c=a.obj+b.obj
       end
   end
end

but when I look for the value of variable c I get this eror :

Undefined function or variable 'c'.

of course this is just an exmaple ...

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Answer 1

OCDER 2018-6-25

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在 MATLAB Online 中打开

classdef OOP_101
    properties
        a=5
        b=10
    end
    methods
        function c=adding(obj)
            c=obj.a + obj.b;  %<=== you have it backwards. It's obj.a not a.obj.
        end
    end
end

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tomer polsky 2018-6-26

I did what you said how ever I cant still see my varbile c . Not in the workspace and not in command window

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Answer 2

Steven Lord 2018-6-26

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https://ww2.mathworks.cn/matlabcentral/answers/407345-matlab-oop-how-to-transfer-value-from-properties-to-methods#answer_326356

在 MATLAB Online 中打开

>> myObj = OOP_101
myObj = 
    OOP_101 with properties:
      a: 5
      b: 10
>> c = adding(myObj)
c =
      15

Note that I could have used whatever variable name I wanted as the output of the call to the adding method. I'm not required to call it c.

>> tomer = adding(myObj)
tomer =
      15

In fact, I can store that output as an element in an array; I'm not limited to storing it in a scalar variable.

>> clear x
>> x(5) = adding(myObj)
x =
       0     0     0     0    15

Creating your own classes in MATLAB is a bit of an advanced maneuver (it's in the Advanced Software Development section in the documentation) so if (as I suspect) you're new to MATLAB or it's been a while since you last used MATLAB I'd recommend starting with some of the more fundamental material, like the MATLAB Onramp course on the tutorials page.

Once you have a good foundation of the fundamentals of writing scripts and functions, to learn more about creating and working with classes I would read through and follow the examples on this documentation page, starting with the "Create a Simple Class" example.

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Adam 2018-6-26

Classes can be quite easy to learn if you give yourself manageable examples. What is a lot less easy to learn, even with 10+ years of experience, is exactly how to design your classes in terms of how they interact with each other. That is the same in any language, although each has their quirks. Matlab OOP has some irritating features certainly, but I have been happily using it for the last 5 years and even though I still write some code in standalone functions, of course, I certainly would never go back now. I am more of a software engineer by trade, but still didn't really get my head around the general concepts of OOP very well prior to using it in Matlab.

So I certainly would not discourage the use of classes, it depends how much time you have to learn and whether you are able to pick things up quickly or not.

Even after many years of doing it I still find myself having created a class that in no way justifies its existence and is really just a convoluted structure for what could have been one or two standalone functions.

OCDER 2018-6-26

在 MATLAB Online 中打开

I use OOP very sparingly in matlab since there are often faster methods that matlab was designed for.

And why can't you make a folder with those 10 separate functions? I've made a 300-function software with Matlab... all works well. Key is to group your functions according to some sort of theme. See example folder structure:

Folder structure
 -Main
    runMain.m
 ---DataAnalysis
 -----Statistics
        calcValues.m
 -----Simulation
        simulateProtein.m
 ---IO
      formatFile.m
 ---Tables
      ProteinProperties.csv

I've done OOP and it can become messy with improper design... Spaghetti code is fixed by coding practice. The fact you have 10 functions is actualy a good thing since you're segmenting your code , which makes it easy to debug. Try debugging a 300-line object code - it's not pleasant.

And COMMENT your codes!

example:

function A = calcSum(X, Y, Z)
%calcSum will calculate the sum of X, Y, and Z.
%  A = calcSum(X, Y, Z)
%  INPUT: X, Y, Z are scalars
%  OUTPUT: sum of inputs
A = X + Y + Z;

That way, you can do "help calcSum" to figure out how to use that function.

OCDER 2018-6-27

在 MATLAB Online 中打开

Use OOP whenever you have to keep track of stored variables, and any of this applies:

1. Setting a property in the object should trigger updates for all the other properties.
     EX: A = VideoReader('video.avi')
             %Behind the scene, extract full path to this file, encoding format, etc.
2. Invoking a method of the object updates the variable
     EX: A.readFrame;  
             %Behind the scene: get the frame, and UPDATE the current frame number

The one thing that functions do not work well with is tracking a set of internal variables over multiple function calls. Often, people use persistent variables in functions, but it's often confusing to use and could lead to issues.

Take a look at the VideoReader object. It has to keep track of the location of the video file, video encoding format, etc. Whenever you use a method of VideoReader, it then knows exactly what video you are referring to, and if needed, update what the current read frame is.

Adam 2018-6-27

And if you do choose to use OOP, here is the Matlab OOP 'bible':

https://www.mathworks.com/help/pdf_doc/matlab/matlab_oop.pdf

though if you are using an earlier version of Matlab you should probably locate it via links from your Matlab version's documentation as it is updated with every version.

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matlab OOP how to transfer value from properties to methods ?

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