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

**From**: |
Rik |

**Subject**: |
[Octave-bug-tracker] [bug #47415] out of memory negating a permutation matrix |

**Date**: |
Fri, 16 Sep 2016 23:34:56 +0000 (UTC) |

**User-agent**: |
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:43.0) Gecko/20100101 Firefox/43.0 |

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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