[R] trouble with prcomp()

[Dimitris Rizopoulos]

It is not equal because you have asked from `prcomp' to standardize
your data matrix,

try the following,

pcprim$x==scale(iris)%*%pcprim$rotation

Note also that the argument in `prcomp' is "scale." and not "scale"

I hope this helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Doctoral Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/396887
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm



----- Original Message ----- 
From: "Scott Robbins Loarie" <loarie at stanford.edu>
To: "Prof Brian Ripley" <ripley at stats.ox.ac.uk>
Cc: <r-help at stat.math.ethz.ch>
Sent: Monday, June 14, 2004 9:35 AM
Subject: [R] trouble with prcomp()


> Thanks very much for showing me these functions,
>
> In prcomp(), the documentation says that Value: x is "the data
> multiplied by the 'rotation' matix".  Using sample data:
>
> data(iris)
> iris<-as.matrix(iris[,1:4])
> pcprim<-prcomp(iris, scale=TRUE)
>
> Why does iris%*%pcprim$rotation (the data multiplied by the rotation
> matrix) not equal pcprim$x?
>
> Thanks very much,
>
> Scott
>
>
> On Mon, 14 Jun 2004, Prof Brian Ripley wrote:
>
> > Why are you using the OPRHANED multiv library for a facility
covered by
> > princomp() and prcomp() in base R?  The list might be able to help
you
> > with the latter, but no one is supporting multiv (hence its
status).
> >
> > On Sun, 13 Jun 2004, Scott Robbins Loarie wrote:
> >
> > > I am having trouble using the pca function in the multiv
library.
> > > I am trying to generate the values found in $rproj by using
$evecs to
> > > calculate linear combinations of my input data.  However, I have
been not
> > > been able to correctly calculate the $rproj values.
> > >
> > > Using the following standard sample data, why does
iris%*%pcprim$evecs[,]
> > > not equal pcprim$rproj?
> > >
> > > data(iris)
> > > iris<-as.matrix(iris[,1:4])
> > > pcprim<-pca(iris)
> >
> > --
> > 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)
> > 1 South Parks Road,                     +44 1865 272866 (PA)
> > Oxford OX1 3TG, UK                Fax:  +44 1865 272595
> >
>
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