How do I set a class property?

Good afternoon! I'm bad at object-oriented and I need some advice. The situation is as follows: in one part of the code, I calculate the integer variable var. Then I want this variable to be declared as the default property in the class. For example,

var = a * b;

...

classdef vector
    properties
        v = var;
    end
    methods
        %%
    end
end

How can I implement this?

[Walter Roberson]

If I recall correctly you need to declare var as a constant to be able to use it that way, and there are restrictions on how the constant can be created.

Are you trying to create a class variable, one that belongs to the class itself?

[Igor Arkhandeev]

I didn't fully understand your question, so I'll try to explain my thinking. I create my class objects very often. Each time I need to pass the same number as a variable. At the very beginning of my code, I calculate this variable once. In order not to pass a variable to the input every time, I want to bind it to the properties.

[Mario Malic]

Hey Igor,

See this video on YouTube for the brief introduction on OOP:

Object-Oriented Programming in MATLAB | Master Class with Loren Shure

From my slim knowledge of OOP, you can write the constructor method to create a class with desired value for your variable.

[Igor Arkhandeev]

I understand how to do this through the constructor. The thing is, I don't want to pass a variable to the input of a function every time. My code is complex and consists of a large number of nested functions. I want to avoid using global, so I need to pass my variable every time. This variable is only used in the class.

Answer 1

[per isakson]

This might help as a start

>> vec = vector;
>> vec.v1
ans =
   120
>> 

where

classdef vector
    properties
        v1 = var1_calculation;
        v2 = vector.var2_calculation;
    end
    methods ( Static = true )
        function    var = var2_calculation()
            a = -10;     
            b = 12;
            var = a*b;
        end
    end
end

and

function    var = var1_calculation()
    a = 10;     
    b = 12;
    var = a*b;
end

Then the question is where do a and b come from

[Igor Arkhandeev]

As you can see, such a transfer is very expensive and I would like to set the variable value in the properties once.

function f = f1(var)
    %%
        f = f2(var);
    %%
end
function f = f2(var)
    %%
    f = f3(var);
    %%
end
function f = f3(var)
    %%
    vct = vector();
    vct.v = var;
    %% some operations
end

[Igor Arkhandeev]

In this case, you need to calculate this value each time, which will take a wildly long time.

[per isakson]

I assume that a*b represents some larger costly calculation.

AFAIK: Matlab doesn't offer any better solution than the one in my answer (possibly I misunderstood your question).

The rhs of the assignments in the property block must be possible to evaluate in the base workspace. Both functions and static methods fullfill that requirement.

When vector is called for the first time the default value is calculated and the value is stored together with the class in memory. Doc says: "Evaluation of property default values occurs only when the value is first needed, and only once when MATLAB first initializes the class. MATLAB does not reevaluate the expression each time you create an instance of the class."

[Igor Arkhandeev]

Okay, thank you so much for your response and for your time. I should have asked that. Now I should rebuild the code in a more convenient way.

Answer 2

[Steven Lord]

If you want to initialize the value once and have that be unchangeable for the rest of the lifetime of the object, make it a Constant property.