How to keep a class property constant after it's specified the first time

I have defined a class property that shuffles a given set of inputs when it is called, such that each class instantiation would have a unique, randomized array in that property. So far so good. However, I am also trying to store the array corresponding to each particular instantiation, so that I could retrieve that information later on.

Currently, every time I call the property (through get), I obtain a different shuffled version, which is not what I need. For example:

classdef Stimulus < AuditoryStimulus

properties
    carrierRange  =  1e3 : 1e3 : 1e4;
end
properties (Dependent = true, SetAccess = private)
    stimulus
    frequencySeries
end

methods
    function frequencySeries = get.frequencySeries(obj)
        frequencySeries = obj.carrierRange(randperm(length(obj.carrierRange)));
    end
    function stimulus = get.stimulus(obj)  
        freqs = obj.frequencySeries;
        % build the stimulus...
    end

I have tried with constant properties, but I am not able to initialize it with the output of a function. I have also tried with static methods, but that is not what I want either, because I need each particular instantiation to be unique.

Could anyone please tell me what I am missing and/or point out some relevant references? Thanks a lot!

[jgg]

It seems like the issue is that:

    function frequencySeries = get.frequencySeries(obj)
        frequencySeries = frequencySeries(randperm(length(frequencySeries)));
    end

creates a new permutation each time it is called. I think you could get around this by only calling the randperm(length(frequencySeries)) function once, when the object is created. Then, this would just have to return the private value frequencySeries instead.

Am I understanding this correctly?

[paula]

Where would you call randperm(length(frequencySeries)) exactly?

Answer 1

[the cyclist]

I don't have much experience with using OOP with MATLAB (or really OOP at all to be frank), but I think what you want to do is initialize the random variable as part of the class constructor method, that only gets called when the object is instantiated.

[paula]

Thanks, that indeed was what I needed!

[paula]

So, in case anyone else has a similar question, this is the structure of my implementation:

classdef testStimulus < AuditoryStimulus
   properties
       carrierRange  =  1e3 : 1e3 : 1e4;
   end
   properties (Dependent = true, SetAccess = private)
       stimulus
   end
   properties (Dependent = false, SetAccess = private)
       carriers
   end

   methods
        function obj = testStimulus()
            obj = obj@AuditoryStimulus;
            obj.carriers = randomizefrequencies(obj);
        end

        function stimulus = get.stimulus(obj)
            % do other things... (common method)
        end
    end
end
function frequencySeries = randomizefrequencies(obj)
    frequencySeries = obj.carrierRange(randperm(length(obj.carrierRange)));
end

Answer 2

[Adam]

The accepted answer by the cyclist is basically correct, though in addition I would add that what you are looking for is

properties (SetAccess = immutable)
    carriers
end

You can add the Dependent = false if you like, but I tend to leave that off and only add it when it is true for readability purposes. Even then I just put 'Dependent' rather than 'Dependent = true', but that is just a stylistic aside!

The 'immutable' property access specifier is for exactly the case you are asking about - it defines that the property can only be set from the constructor and then even the class itself cannot set it afterwards.

Your solution works fine in realistic circumstances, but if you were working on, for example, a complex class with a team of people then just putting

properties (SetAccess = private)
...
end

does not limit the property to never being changed after it is set. The class' own methods are still allowed to change it whenever they like so for tight programming practices you should make it immutable because that is a clear statement that the property should never be changed. Then others working on the code would also know this is your clear intention.

[paula]

Adam, thanks for the clarification. You provided good suggestions.