Setting object properties from a class method

Hi,

I'm exploring OOP with matlab and have the following code:

    % method declaration
function Update(obj, Time)
   obj.DeltaTime = Time;  % saves time parameter to a class-wide property
   obj.Height = obj.Height + obj.DeltaHeight;      % new tank height
end

DeltaTime, Height are public properties of the class this sits inside. I call the method using the following syntax:

    % method call
Update(TankA, 7)

There are other methods that use DeltaTime to calculate DeltaHeight. These all work perfectly fine. However, calling this method doesn't change the DeltaTime property, nor set Height to its new value. I don't get any errors thrown when the code runs, so have no idea why its not working as I expect it to. I don't know how to pass the values to the properties as method outputs either:

    % using method outputs
function [DeltaTime, Height] = Update(obj,Time)
   DeltaTime = Time;
   Height = obj.Height + DeltaHeight;
end

Ideally I want DeltaTime, DeltaHeight to be private properties, and not manipulated directly from a separate file. Can someone please give me an idea what could be causing this?

Answer 1

[per isakson]

1. Are you aware of the difference between handle and value classes? From your code fragments, I conclude that you anticipate the behavior of a handle class. Thus, the class must inherit from the handle class.
2. One must use some convention to distinguish between class properties and local variables. It is confusing to use the same name for a property and a local variable - to say the least. A property the shall always be referred to with the full name, obj.PropertyName, e.g. obj.Height.

Here is an example, which illustrates what I believe you try to do.

>> clear all, clear classes
>> tank_a = MyTankClass;
>> display( tank_a )
Height: 0.000000
DeltaHeight: 
DeltaTime: 
>> Update( tank_a, 17, 5 )
>> display( tank_a )
Height: 5.000000
DeltaHeight: 5.000000
DeltaTime: 17.000000

where

    classdef    MyTankClass  < handle

        properties ( Access = private )
            Height          = 0;
            DeltaHeight     
            DeltaTime
        end

        methods 
            function    Update( obj, time, height )
                obj.DeltaTime   = time;  
                obj.DeltaHeight = height;  
                obj.Height      = obj.Height + obj.DeltaHeight;      
            end
            function    display( obj )
                fprintf( 'Height: %f\nDeltaHeight: %f\nDeltaTime: %f\n' ...
                    ,   obj.Height, obj.DeltaHeight, obj.DeltaTime      )
            end
        end
    end

[Manish Makwana]

Thank you so much! Simply turning my value class into a handle class was enough to have it work as I wanted it to.

    % code
classdef CSTR < handle

You're right, I wasn't aware of the difference between value and handle classes until i saw your answer.

Absolutely brilliant.

[Manish Makwana]

Oops, meant to select your answer as the correct one but clicked in the wrong place... o_o

[emre parlak]

i was looking for the same thing. simply adding a <handle solved my problem. i believe i dont get it though. i am just "memorizing" it i guess. but oh well. also why cant i define default arguments. even python lets me do it. thanks for the help

[Steven Lord]

See this documentation page for a description of the difference between handle classes and value classes.

As for default arguments, perhaps function argument validation does what you want.

[emre parlak]

i used varargin and nargins and some if else statements to do it. it is not that of a big deal i guess. thanks for the help

Answer 2

[Image Analyst]

What is obj? Is it some other object? It's not the same object is it? If it is, why aren't you simply doing:

function Height = Update(Time)
   Height = Height + Time;
end

You don't even need DeltaTime since it's the same as Time that you passed in.

[Manish Makwana]

Yes, obj refers to the object the method is inside. I need to use the reference else matlab creates a new variable 'Height', independent of the class property 'Height'.

Note that i have about 4 other variables that use DeltaTime, and that DeltaTime does not equal DeltaHeight. Hence why i want to save it as a property, which can be privately referenced (instead of having to expose lots of dependent properties).

I'll post further code when i get to a desktop.

[Robert Jack]

I don't understand why this is the accepted answer. In my view the answer by per isakson is better. I'm troubled why an mvp is asking what 'obj' is when the context is clearly that of class properties.