Understanding dsyrk_ in C code

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.

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

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901