Since when has it been possible to dot-index the output of a class method?

Since when has it been possible to directly dot-index the output of a class method call, like this -

classdef myClass

properties

p

end

methods

function obj=myClass(val)

obj.p=val;

end

function obj=increment(obj)

obj.p=obj.p+1;

end

end

end

obj=myClass(2)

obj = 
  myClass with properties:

    p: 2

obj.increment.p

ans = 3

And why then, it is still not possible to do something similar with function calls -

subfunc.a

Error: File: test.m Line: 1 Column: 1

Using the dot operator to index into the output of function 'subfunc' is not

supported.

function S=subfunc()

  S.a=1;
  S.b=2;

end

[Matt J]

Quite intriguing. It works with brace indexing, too:

classdef myClass

properties

p

end

methods

function obj=myClass(val)

obj.p=val;

end

function out=num2cell(obj)

out=num2cell(obj.p);

end

end

end

obj=myClass([30,10,70]);

obj.num2cell{1}

ans = 30

obj.num2cell{3}

ans = 70

Answer 1

[Walter Roberson]

Since R2019b.

https://www.mathworks.com/help/matlab/release-notes.html?category=language-programming&rntext=&startrelease=R2019b&endrelease=R2019b&groupby=release&sortby=descending&searchHighlight=

Indexing: Use dot indexing into function calls

You can now use dot indexing to index into the result of a function call. MATLAB evaluates the function and then applies the dot indexing operation to the result.

For example, this function creates a structure:

function out = createStruct(in)

out = struct("aField", in);

end

You can call this function and immediately access the structure field it creates:

createStruct(3).aField

For more information, see Indexing into Function Call Results.

[Catalytic]

So it's supposed to work for all functions, not just class methods? Then why does my posted example fail?

subfunc.a

Using the dot operator to index into the output of function 'subfunc' is not supported.

Error in connector.internal.fevalMatlab

Error in connector.internal.fevalJSON

function S=subfunc()

  S.a=1;
  S.b=2;

end

[Walter Roberson]

https://www.mathworks.com/help/releases/R2019b/matlab/matlab_prog/indexing-into-function-call-results.html#mw_bad6a5ee-8de3-4a0d-9dd5-bfcdf42089b1

The () are required part of the syntax.

[Matt J]

Interestingl, though, none of those restrictions seem to apply for class methods, including Static methods,

classdef myclass

methods (Static)

function s=method1

s.a=1;

end

function s=method2

s={10,20,30};

end

end

end

myclass.method1.a

ans = 1

myclass.method2{3}

ans = 30

[Matt J]

Apparently this feature does not allow assignment operations. Too bad. It could have been useful for things like,

gcf().Name='name'

[Walter Roberson]

@Matt J

Oddly...

>> figure(gcf()).Name = 'hello'

Unable to use a value of type matlab.ui.Figure as an index.

Kind of understandable, it looks like it is trying to use the Figure object to index into a variable named figure and failing on that... we can see the reasoning.

But

>> struct('F', gcf()).F.Name = 'hello'

Unable to use a value of type matlab.ui.Figure as an index.

Ummm -- where is a Figure being used as an index there? Indices are fieldname F and property name Name

[Walter Roberson]

Oh, of course, that second example thinks it is indexing into a variable named 'struct', makes more sense now.

[Bruno Luong]

It does not do what you want but it runs without error

figure(gcf().Number).Name = 'hello'

figure = struct with fields:
    Name: 'hello'