MATLAB: Does the access method of a dependent property get called twice when the property is indexed into

access methodsoop

I have a class with a dependent property z. The method get.z(obj) returns a vector. This:

y = myObj.z(end);

appears to call get.z twice! The profiler shows 2 calls from the parent function and the run time is twice the expected run time. On the other hand:

x = myObj.z;
y = x(end);

calls get.z once and cuts down run time by half. Is this the intended behavior?

Best Answer

Confirmed Walter's comment. Indeed the culprit is the keyword end. Here is minimal working example.

classdef Untitled
    properties (Dependent)
        x;
    end
    methods
        function val = get.x(~)
            fprintf('called\n')
            val = [1,2,3];
        end
    end
end
>> obj = Untitled;
>> obj.x(2);
called
ans =
       2
>> obj.x(end)
called
called
ans =
       3

This behavior is mostly benign but occasionally troublesome, and easy to fix. I think this is a case for an m-lint message at least.

Related Solutions

MATLAB: Is calling a superclass constructor after a use of the object not allowed

I can only speculate as to why it would not be allowed, but I assume there are hazards in using an object that is not finished being constructed...like feeding a baby that is only half way through being born, or driving a car that’s only half assembled.

Below is how I would re-implement. In your original design, it doesn't make sense to me that the classB constructor should have to set x if it has already been set in classA.

classdef superclass
    
    %% PROPERTIES
    properties 
        x;
        y;
    end
    
end

classdef classA < superclass
    
    %% PROPERTIES
    properties
        a
    end
    
    properties (Access=private)
        z
    end
    
    %% METHODS
    methods
        % Constructor


        function obj = classA(a,z,y)
            obj.x = a*z;
            obj.a = a;
            obj.z = z;
            if nargin>2
             obj.y=y;   
            end
        end
    end
    
end

classdef classB < superclass
    
    %% PROPERTIES
    properties
        b
    end
    properties (Access=private)
        z
    end
    
    %% METHODS
    methods
        % Constructor
        function obj = classB(b,z,x)
            obj.y = z+b;
            obj.b = b;
            obj.z = z;
            if nargin>2
               obj.x=x; 
            end
        end
    end
    
end

classdef subclass < classA & classB
    
    %% METHODS
    methods
        % Constructor
        function obj = subclass(a,b,z)
            obj@classA(a,z);  %set x but not y
            obj@classB(b,z);  %set y but not x  
        end
    end
    
end

MATLAB: Property Name

set() cannot be used to change numbers that are of any of the built-in numeric types (e.g., uint8, double, logical), and there is no UserData property for any of those numeric classes.

Also, for future reference, those numeric classes cannot have Properties, and no dynamic properties can be set for them.

Best Answer

Related Solutions

MATLAB: Is calling a superclass constructor after a use of the object not allowed

MATLAB: Property Name

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