[Octave-bug-tracker] [bug #33221] subsasgn without LHS should do in-place assignment for performance

[Nicholas Jankowski]

Update of bug #33221 (project octave):

                Severity:               2 - Minor => 1 - Wish               
                Priority:              5 - Normal => 3 - Low                
              Item Group:    Matlab Compatibility => Feature Request        

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Follow-up Comment #5:

regarding the primary focus of this report, the comment #0 code no longer runs
in Matlab:

matlab 2022a:

>> r = randn(4,4);
>> subsasgn(r, struct('type', '()', 'subs', {{2, 2}}), 0)
Error using indexing
SUBSASGN must be called with an output.


Not interested in digging through a decade worth or release notes to see if
it's mentioned, but it appears that sometime over the past decade this
undocumented behavior changed. 

Now Octave still allows a subsasgn without an output, and simply puts the
result in ans, leaving r unchanged:


>> r = randn(4,4);
>> subsasgn(r, struct('type', '()', 'subs', {{2, 2}}), 0)
ans =

   1.8659   0.2015  -1.9407   0.1915
   0.3151        0  -0.1796  -0.8894
  -1.1896  -0.5194   1.1625   1.0586
   0.2087  -1.3228  -0.3853  -0.2635

>> who
Variables visible from the current scope:

ans  r

>> ans
ans =

   1.8659   0.2015  -1.9407   0.1915
   0.3151        0  -0.1796  -0.8894
  -1.1896  -0.5194   1.1625   1.0586
   0.2087  -1.3228  -0.3853  -0.2635

>> r
r =

   1.8659   0.2015  -1.9407   0.1915
   0.3151   0.1767  -0.1796  -0.8894
  -1.1896  -0.5194   1.1625   1.0586
   0.2087  -1.3228  -0.3853  -0.2635


of the two speed examples given in comment #0, Matlab no longer allows the
comparison, but both 'slow' versions run almost instantaneously in both Matlab
and Octave. Don't know if this is because both programs have made these
optimizations already, or if things are just faster 11 years later.  

In any case, I'm going to change the category to Wish/Feature request.  It
should probably be closed as Wont Fix unless someone  can verify the code
optimization still needs to be addressed.


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