MATLAB: Matlab OOP, dependent class properties, correct coding techiques

dependent propertiesMATLABoop

Hi,

I have a conceptual problem I'm struggling with and was wondering if anyone could give some insight into this.

Essentially, I have a class with three (or more) properties. Watered down Pseudo code below:

classdef obj
   Properties
      A
      B
      C
   end
     Methods
        constructor etc
        function set.A
           C = f(obj.A,obj.B)  % C is dependent on A and B (MLINT warns on this line)
           A = inputValue % Set A's value
        end
     end
  end

Now herein lies the problem as I'm sure you have already noticed. I'm setting one properties value while changing another property's value. MLINT warns immediately that this is bad and correctly states that one should make C a dependent property.

This is fine unless speed is an issue. C is going to be called "lots of times". And having Matlab calculate it every time

      *get.C*

is called is expensive. A and B will be updated through out the life of the object and hence I want C recalc'ed on the fly when they do.

Should I disregard MLINT (and probably good coding practice)?

Is there a better but still fast way to achieve what I want?

Regards, Phil

Best Answer

I generally just ignore this warning - you can turn it off for the line, file or all files (I've done the latter).

In your scenario, there's probably not an issue, but there are situations when there could be.

The best practise would be to use events & listeners - add a listener that waits for A to change. You can even use the built in event PreSet/PostSet. Search for "Listen for Changes to Property Values" in the documentation.

Indeed you do not want to make C dependent. In that scenario, you would not even use the first line

 C = f(obj.A, obj.B)

because C cannot be set (in the normal sense)

Related Solutions

MATLAB: [Warning] Why is it bad practice to set additional class properties when using a set method (prompting a warning)

Be very careful not to get yourself into a recursive situation like the class I posted earlier this month for this Answer does.

I would probably make B a Dependent property with a get.B method that accesses the A property. See this documentation page for some information about dependent properties.

A couple observations about your class:

Your default value for the A property does not satisfy the size validation you've listed in your file. Either the default value should be [5; 1] or you should require A to be of size [1 2].
Your doThing method doesn't make sense to me. As written it would make B out of sync with A. You could have doThing update A then the value of the Dependent property B would also be doubled. [I would not define a set.B method, or if I did I would implement it so it throws a user-friendly error message.]

        function obj = doThing(obj)
            obj.A = 2*obj.A;
        end

MATLAB: Class for A = K * B in MATLAB (non-dependent properties with dependent behavior)

"where all properties are non-dependent" Why? Your properties are obviously dependent, so why not use them.

The warnings are telling you that if an object is saved in a mat file it may not be restored properly due to random initialisation order. In practice, with your design, that shouldn't be a problem as the properties' values should always be consistent, so I guess you could just disable the warnings.

However, I can't help but greatly dislike this design, particularly the i state variable. You could achieve a cleaner design, using dependent variables, with no impact on performance:

classdef AeqKxB
    properties(Dependent)
        A;
        B;
        K;
    end
    properties
        Kstate;
    end
      properties (Access = private)
          A_;
          B_;
          K_;
      end
      methods
           function obj = AeqKxB(a, k, kstate)  %   Constructor
              obj.A_ = a;
              obj.B_ = a*k;
              obj.K_ = k;
              obj.Kstate = kstate;
          end
          function a = get.A(obj)
              a = obj.A_;
          end
          function obj = set.A(obj, a) %   setter for A
             obj.A_ = a;
             obj.B_ = a / obj.K_;
          end
          function b = get.B(obj)
              b = obj.B_;
          end
          function obj = set.B(obj, b) %  setter for B
              obj.B_ = b;
              obj.A_ = b * obj.K_;
          end
          function obj = set.K(obj,k) %  setter for K
              obj.K_ = k;
              if strcmp(obj.Kstate, 'FixB')
                  obj.A_ = obj.B_ * k;
              elseif strcmp(obj.Kstate, 'FixA')
                  obj.B_ = obj.A_ / k;
              end
          end
          function obj = set.Kstate(obj,kstate)
              obj.Kstate = kstate;
          end
      end
  end

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