[Octave-bug-tracker] [bug #61705] Wrong number of columns when removing entry from empty vector

[Rik]

Update of bug #61705 (project octave):

                Severity:              3 - Normal => 2 - Minor              
                Priority:              5 - Normal => 3 - Low                
              Item Group:        Incorrect Result => Matlab Compatibility   

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Follow-up Comment #3:

This is a minor nuisance, and Matlab isn't much better.  To quickly summarize,
you cannot rely on columns() when a matrix is empty.  If your code will
encounter empty matrices you may need to separate them and use different code
for them in a special case.  You can separate them using isempty() or "if
(numel (x) == 0)".

For a slightly longer explanation, both Matlab and Octave allow you to create
arbitrarily-sized N-dimensional matrices.  And you can request the number of
columns for these matrices regardless of whether they actually hold anything. 
Here's some code that shows that:


x = zeros (0, 101, 5)
x = [](0x101x5)
columns (x)
ans = 101


Now if you do indexing the result is the size of the index.  This applies even
to empty indexing.


x = 1:10;
idx = zeros (0,3)
idx = [](0x3)
y = x(idx)
y = [](0x3)


Of course, this is where it gets silly/stupid.  The expression "x != 2" should
be of size 0x0.  It is when it is executed directly, but not when it is
executed as part of an indexing operation.


x = []
x = [](0x0)
x ~= 2
ans = [](0x0)
x(x ~= 2)
ans = [](0x1)


I've lowered the severity since this is a minor issue that can be more
appropriately handled by detecting empty matrices.  I've also changed the Item
Group to Matlab Compatibility since it seems to generate a 0x0 matrix for the
test case.


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