Classdef get methods: allowing get indexing into an array of length either (1x1) or (Nx1).

Hello all, I am trying to sort out an issue best shown in this MWE, where I have a property that may be a column vector, but is initialized with a single value.

classdef myClass
    properties 
        A (:,1) double = 0;
    end
    methods
        
    end
end

Now, lets say I want to index into myClass.A using B = myClass.A(indx). However, I want it so that if myClass.A is still a (1x1) I want to return the initial value, not an "Index exceeds array bounds" error.

I can picture doing this with a get method of the following form

        function out = get.A(indx)
           if numel(obj.A) == 1
               out = obj.A(1);
           else
               out = obj.A(indx);
           end
        end

However, this will clearly not work since (A) get methods are limitited to one input, and I need to pass "indx", (B) I don't pass an instance of the object to get.A, so it has no access, and (C) this would wind up being infinitely recursive if I was able to do it.

SO:

Is there a way to allow use of a single default value as the output of a property regardless of the requested index, if that property has not yet been overwritten with an Nx1 vector.

Cheers,

DP

Answer 1

[Catalytic]

You would need to overload subsref, e.g.,

methods
 function out=subsref(obj,S)

   if S(1).type=="." && S(1).subs{1}=="A" && numel(S)>1 
       
       out=obj.A(min(S(2).subs{1},1));
   else
       error 'Undefined Indexing'
   
   end

  end
end

[D. Plotnick]

Darn. I was hoping someone had another idea in the last day, because I really don't want to overload subsref or any other low level Matlab functions. Thanks for the input, I hadn't even thought of that, I may just need to bite the bullet and put the check:

if indx > length(obj.A)
    indx = 1;
end
val = obj.A(indx);

into all of the higher level functions, rather than something in the class definition itself.

Answer 2

[Sean de Wolski]

You could GetAccess Protect the A property and then use a method instead of straight property reference.

methods
function a = getA(obj, indx)
    if numel(obj.A) == 1 || nargin == 1
        out = obj.A(1);
    else
        out = obj.A(indx);
    end
end
end

[D. Plotnick]

Thanks. That is indeed less invasive than overloading subsref.