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

[Lachlan Andrew]

Follow-up Comment #14, bug #47415 (project octave):

On 8.2.0.701 (R2013b), it produces


testsignbit (1)
testsignbit (-1)
testsignbit (0)
testsignbit (-0)
Hello, World!
Hello, World!
SIGNBIT!
Hello, World!
Hello, World!
SIGNBIT!


It seems that this extends to retaining the sign of zeros in diagonal
matrices:


>> d = diag ([1, 2])
d =
     1     0
     0     2
>> -d
ans =
    -1     0
     0    -2
>> 1./-d
ans =
   -1.0000      -Inf
      -Inf   -0.5000


Comment #5 asked if it is important that -full (P) and full (-P) produce the
same result.  I think it is important that


-full (P) == full (-P)


but don't think it is important the result be identical.

However, if permutation matrices are only automatically created from full
matrices, then it is probably not too bad if they are implicitly converted
back.  If the sparsity is required, it would be easy to type


-sparse(jacQ)


Does Octave ever create a permutation matrix from a sparse matrix?

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