[Octave-bug-tracker] [bug #33221] subsasgn without LHS assignment Matlab incompatibility

[anonymous]

URL:
  <http://savannah.gnu.org/bugs/?33221>

                 Summary: subsasgn without LHS assignment Matlab
incompatibility
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Tue 03 May 2011 02:41:31 PM UTC
                Category: None
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Other
                  Status: None
             Assigned to: None
         Originator Name: Jochen Weber
        Originator Email: address@hidden
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 3.4.0
        Operating System: Mac OS

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Details:

**This seems to be rather a Matlab-internal, undocumented features**

In some of my code I've been making use of the undocumented feature that if a
call to subsasgn (only on a built-in type!) does not have a LHS assignment,
the assignment is done "in place" (speed gain):


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


The previous code actually alters 'r', which is, to my knowledge, *NOT*
documented by Mathworks.

The reason to use this syntax is that there is no cost to keep another copy of
the variable in memory (to allow error handling, etc.).

So the following two subsasgn calls in Matlab work (and produce the same
result), but the first is instantaneous, whereas the second one takes quite a
while and might even end with an out-of-memory error:


r = zeros(256, 256, 256, 4); % 0.5GByte of double 0's
subsasgn(r, struct('type', '()', 'subs', {{2, 2}}), 1); % instantaneous
r = subsasgn(r, struct('type', '()', 'subs', {{2, 2}}), 2); % creating a copy
of r, assign in copy, then replace r with copy


While this is not a big deal, it would be nice to also have this "speedup"
available in Octave (particularly when working on large, full matrices, such
as 4D datasets that require a large amount of memory).

In Octave, a "manual" call of subsasgn on a large array *always* requires a
copy, whereas using


r = zeros(256, 256, 256, 4);
r(2, 2) = 1;


is instantaneous, which is a little "inconsistent", I guess...




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