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

[LUK ShunTim]

David Bateman wrote:
> LUK ShunTim wrote:
> 
>> Hello,
>>
>> If A is sparse, will A\b automatically use the sparse variant of the
>> linear equation solver?
>>
>> Regards,
>> ST
>> -- 
>>
> Yes.. In fact in 2.9.x it uses a polymorphic solver that has a selection
> tree
> 

[snipped]

Thanks very much.

I got this rather strange result on octave 2.1.72 + octave-forge in
debian/sid.

<diary>
octave:2> A=sprand(2500,2500,0.03);
octave:3> b=eye(2500,1);
octave:4> r=A*b;
octave:8> fullA=full(A);
octave:9> tic;A\r;toc
ans = 19.277
octave:10> tic;A\r;toc
ans = 19.073
octave:11> tic;A\r;toc
ans = 19.340
octave:12> tic;fullA\r;toc
ans = 5.2551
octave:13> tic;fullA\r;toc
ans = 5.0916
octave:14> tic;fullA\r;toc
ans = 5.1038
</diary>

The full version appears to be *faster* than the sparse version (!),
which seems to be counter-intuitive. What have I missed?

Regards,
ST
--



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