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Charles C. Berry wrote:
> On Wed, 7 Jan 2009, Nathan S. Watson-Haigh wrote:
>
> I'm trying to understand some C code in an R package I'm using. I'm address
> this question here as
> it's matrix algebra...and I'm no pro at that!
>
> the C command reads:
>
> double alpha = 1.0, beta = 0.0;
> dsyrk_("L", "N", nGenes, nGenes, & alpha, mat1, nGenes,
> & beta, mat2, nGenes);
>
>> That's a Fortran subroutine. The code is in blas.f (as 'DSYRK') and is
>> commented. So, you should be able to work thru it.
>
>> If you want to see how it is used, grep the sources for 'dsyrk' (lower
>> case). I think array.c uses it for symmetric crossproducts along the lines
>> of the alpha=1.0 and beta = 0.0 example you cite.
>
>> It is often a good idea to search the R sources for routines whose
>> function is puzzling and find examples of their use to help one understand
>> what they do.
Thanks! I'll have a look through that tomorrow.
Cheers,
Nath
>
>> HTH,
>
>> Chuck
>
> - From google, I've found out that dsyrk is for performing one of the
> symmetric rank k operations -
> whatever that means!? From here:
> http://linux.die.net/man/l/dsyrk
>
> I've found that the calculation being performed is:
> alpha*A*A' + beta*C
>
> However, since alpha is 1 and beta is 0, this reduces to:
> => 1*A*A' + 0*C
> => A*A'
>
> Which is simply the cross product....am I correct?
>
> Cheers,
> Nath
>
>>
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>>
> Charles C. Berry (858) 534-2098
> Dept of Family/Preventive
> Medicine
> E mailto:cbe...@tajo.ucsd.edu UC San Diego
> http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
- --
- --------------------------------------------------------
Dr. Nathan S. Watson-Haigh
OCE Post Doctoral Fellow
CSIRO Livestock Industries
Queensland Bioscience Precinct
St Lucia, QLD 4067
Australia
Tel: +61 (0)7 3214 2922
Fax: +61 (0)7 3214 2900
Web: http://www.csiro.au/people/Nathan.Watson-Haigh.html
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______________________________________________
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