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

Asked by D. Plotnick on 27 Mar 2019
Latest activity Commented on by D. Plotnick on 28 Mar 2019
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

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2 Answers

Answer by Catalytic
on 27 Mar 2019
Edited by Catalytic
on 27 Mar 2019
 Accepted Answer

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

  1 Comment

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.

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Answer by Sean de Wolski
on 28 Mar 2019
Edited by Sean de Wolski
on 28 Mar 2019

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

  1 Comment

Thanks. That is indeed less invasive than overloading subsref.

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