R help - Nov 2004 - PCA prcomp problem

I've just starting using the prcomp function, and I want to be able to
extract
individual principal components (e.g. PC1, PC2) in vector format.  I haven't
been able to find any documentation that explains how to do this (or even if it
is possible).  Any help on the subject would be greatly appreciated.

Many thanks
Deirdre Toher
Teagasc National Food Centre

On Tue, 9 Nov 2004, Deirdre Toher wrote:
> I've just starting using the prcomp function, and I want to be able to
extract
> individual principal components (e.g. PC1, PC2) in vector format.  I
haven't
> been able to find any documentation that explains how to do this (or even
if it
> is possible).  Any help on the subject would be greatly appreciated.
Depends of course what you mean: do you want the coefficients or the data 
predicted onto principal components?

The coefficients are component `rotation' of the result (see the help 
page).

Projections onto the PCs is component `x' of the result (see the help 
page).  E.g.

fit <- prcomp(USArrests, scale = TRUE, ret.x = TRUE)
PC1 <- fit$x[, 1]
PC2 <- fit$x[, 2]

OR predict(fit)  gives the projections of old (or new, with newdata) 
data onto the PCs.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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