[Octave-bug-tracker] [bug #50359] clearer documentation for difference between isnull and isempty

[Rik]

Follow-up Comment #15, bug #50359 (project octave):

Here is a thread on Matlab Central about how Matlab deals with null assignment
(https://www.mathworks.com/matlabcentral/newsreader/view_thread/310020).

The quick answer is that 


x(idx) = []


is flagged by the parser and the right-handside means NULL or empty and this
triggers a delete action.

When you do the same assignment without indexing


x = [];


The parser, instead, decides that you want an empty matrix so this is
equivalent to


x = zeros (0, 0);


When you code


rhs = [];
x(idx) = rhs;


the parser knows that the right-handside is not the special pattern "[]", but
a variable so it doesn't trigger any delete action.

The prototype for subsasgn is


subsasgn (VAL, IDX, RHS)


and is equivalent to


VAL(IDX) = RHS


Now consider the case of an object which has overloaded the subsasgn function.
 How can the coder know whether the user has typed


object(IDX) = [];


versus


object(IDX) = zeros (0,0);


In either case, the coder will be delivered a variable RHS.  This variable
will be empty, so isempty() can not distinguish what was intended.  Instead,
if you want to discover whether a delete operation was requested you need to
use isnull().  For example, maybe a particular idx is read-only.  Then you
might write something like


if (isnull (RHS))
  if (idx == 3)
    error ("object 3 is read-only");
  endif
endif


The Matlab documentation for subsasgn is here:
http://www.mathworks.com/help/matlab/ref/subsasgn.html.  Conveniently for
them, they do not address this case and how you might distinguish between
deletion and assignment.

I have a thought here, though, and it might be worth investigating.  If the
user writes the subsasgn function prototype as


function retval = subsasgn (VAL, IDX, varargin)


then it may be the case that the Matlab parser does not assign a value to
varargin for the NULL case.  If that is true then one could write


if (nargin < 3)
  % Delete operation
else
  % Assignment operation
end




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