[Octave-bug-tracker] [bug #35679] median (rand (1, 1, 1, 3), 4) segmentation fault

[Rik]

Follow-up Comment #13, bug #35679 (project octave):

The patch works for me.  Quoting the troublesome part of the code (with JWE's
fix):


octave_idx_type ns = dv(dim);

octave_idx_type nn = n.length (ns);

dv(dim) = std::min (nn, ns);
dv.chop_trailing_singletons ();
dim = std::min (dim, dv.length ());


As I understand it, the code is attempting to limit the number of items to
look up either to the number of elements along the selected dimension OR the
number of elements requested to be returned.  When median is called it needs
either 1 return value if the number of elements is odd or 2 values if the
number of elements is even.

This squares with the observed behavior.


median (rand (1,1,1,3), 4)


fails because std::min (3,1) = 1 and this sets dv(dim) to 1 which then causes
this dimension to be removed by chop_trailing_singletons.  After that, stride
is calculated incorrectly because dim is too large.

The counter-example


median (rand (1,1,1,4), 4)


works because median requests the two values 2:3 in order to average them to
form the median.  In this case std::min (4,2) = 2 and the dimension stays in
place.

A second counter-example also works


median (rand (1,1,1,3,2), 4)


In this case, chop_trailing_singletons can't remove dimension 4 because it is
protected by dimension 5 which is greater than 1.

So I think JWE's patch solves the problem by restricting the stride
calculation to what remains of the dimension vector.

As a last aside, the prototype for the length() function takes void as an
argument.  I found the following code confusing because I thought ns was
somehow involved and it isn't.


octave_idx_type nn = n.length (ns);


I think this should be rewritten to


octave_idx_type nn = n.length ();


which compiles and works for me.


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