[Octave-bug-tracker] [bug #47415] out of memory negating a permutation matrix

[Rik]

Follow-up Comment #32, bug #47415 (project octave):

It appears that one possibility for maintaining Matlab compatibility would be
to have the creation of Permutation matrices depend on the full/sparse quality
of the input matrix.

For lu, the motivating example for this bug report, if the input is Sparse we
output a Permutation matrix.  This permutation matrix can either convert to
sparse when any unsupported operation is asked of it.  Or we could at least
implement the unary minus operator (-) for Permutation matrices since this
would be quite easy.  This matches Matlab which outputs a Sparse matrix which
has no concept of -0.

If lu had a full matrix input, then presumably the problem was not so large to
begin with that the outputs L, U, and P can not also be represented by full
matrices.  In this case, we just waste the memory and represent P as a full
matrix the same as Matlab.

Does that sound right?



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