Re: Is A\b using a sparse solver if A is sparse?

[David Bateman]

Dmitri A. Sergatskov wrote:

> LUK ShunTim wrote:
> ...
> > octave2.9:11> tic;A\r;toc
> > ans = 12.117
> > octave2.9:12> tic;fullA\r;toc
> > ans = 6.6568
>
> I just like to note a couple things:
>
> -- The result of A\r is a full matrix
> this remains the case even if
>   b=speye (2500,1); r=A*b;
> and hence r becomes also a sparse.
> I am not sure if this conversion to a full
> matrix is a bug or a feature.
Its a feature as "sparse * full" is generally full, octave assumes it is 
full so that it doesn't have to a full matrix at the end with the 
required memory overhead that implies..


> -- On my computer the difference is about 20%
>    not a factor of 2. Do you have and use ATLAS?
>
> octave:1> A=sprand(2500,2500,0.03);
> octave:2> b=speye (2500,1);
> octave:3> r=A*b;
> octave:4> tic;C=A\r;toc
> ans = 5.0834
> octave:5> fullA=full(A);
> octave:6> tic;D=fullA\r;toc
> ans = 4.0818
See my previous message about LU factorizations of random sparse matrices...

D.

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David Bateman                                address@hidden
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