[Rd] Thought on crossprod

Jonathan Rougier J.C.Rougier@durham.ac.uk
Fri, 15 Mar 2002 10:01:44 +0000

Hi everyone,

I do a lot of work with large variance matrices, and I like to use
"crossprod" for speed and to keep everything symmetric, i.e. I often
compute "crossprod(Q %*% t(A))" for "A %*% Sigma %*% t(A)", where
"Sigma" decomposes as "t(Q) %*% Q".  I notice in the code that
"crossprod", current definition

> crossprod
function (x, y = x) 
.Internal(crossprod(x, y))

does not recognise the case where y is the same as x as special.  I'd
like to extend crossprod to something like

> crossprod
function(x, y = NULL)
if (is.null(y))
  .Internal(crossprod(x, y))

Before I start mesing up my R-1.4.1 installation, does anyone have any
thoughts on this?

Cheers, Jonathan.
Jonathan Rougier                       Science Laboratories
Department of Mathematical Sciences    South Road
University of Durham                   Durham DH1 3LE
tel: +44 (0)191 374 2361, fax: +44 (0)191 374 7388
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