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

[Carlo de Falco]

Follow-up Comment #11, bug #47415 (project octave):


I was considering whether it would make sense 
to add a 'sign' property to the permutation matrix 
class in order to avoid the conversion altogether.

But after running the following test:


n = 1e4;
a = eye (n);
a = a ([end 2:end-1 1], :);
typeinfo (a)
b = sparse (a);
typeinfo (b)
c = sprandn (n);
tic, a * c; toc
tic, b * c; toc
c = randn (n);
tic, b * c; toc
tic, a * c; toc


I see that matrix-matrix multiplication is 
consistently at least twice as fast with sparse
matrices than permutation matrices, so I 
think lachlan's idea makes more sense ...

c.



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