Re: simple vector to matrix transformation

[Daniel Heiserer]

address@hidden wrote:
> 
> I just needed to add the same vector to every column of a matrix.
> Since octave does not automatically promote a vector to a matrix, I
> did this:
> 
> a =
> 
>   0.095555
>   0.789421
>   0.122409
> 
> octave:12> b=diag(a)*ones(3,4)
> b =
> 
>   0.095555  0.095555  0.095555  0.095555
>   0.789421  0.789421  0.789421  0.789421
>   0.122409  0.122409  0.122409  0.122409
> 
> I could then add b to the original matrix.
> 
> Of course, the multiplication is inefficient.  I seem to remember
> there was a more direct way to do this transformation, but I can't
> find it in the docs.  Is there?

Sure 
b=a*ones(1,4);

you will see the efficiency once you go to bigger
problems:
octave:7> a=rand(1000,1);tic;b=a*ones(1,1000);toc
ans = 0.22280
octave:8> a=rand(1000,1);tic;c=diag(a)*ones(1000,1000);toc
ans = 142.55
octave:9> 142.55/0.22280
ans = 639.81

A speedup of a factor of 639 is OK or?

-- 
Mit freundlichen Gruessen
                                 Daniel Heiserer
-----
--------------------------------------------------------------
Dipl.-Phys. Daniel Heiserer, BMW AG, Knorrstrasse 147, 80788 Muenchen
Abteilung EK-20
Tel.: 089-382-21187, Fax.: 089-382-42820
mailto:address@hidden



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