R help - May 2008 - predict.prcomp: 'newdata' does not have the correct number of columns

Hi,

I'm doing PCA on wide matrices and I don't understand why calling 
predict.prcomp on it throws an error:

 > x1 <- matrix(rnorm(100), 5, 20)
 > x2 <- matrix(rnorm(100), 5, 20)
 > p <- prcomp(x1)
 > predict(p, x2)
Error in predict.prcomp(p, x2) :
   'newdata' does not have the correct number of columns
 > dim(x2)
[1]  5 20
 > dim(p$rotation)
[1] 20  5

All predict.prcomp really does is x2 %*% p$rotation, which is a 
perfectly legal operation that should yield a 5x5 matrix:

 > dim(x2 %*% p$rotation)
[1] 5 5

Thanks,
Gad

-- 
Gad Abraham
Dept. CSSE and NICTA
The University of Melbourne
Parkville 3010, Victoria, Australia
email: gabraham at csse.unimelb.edu.au
web: http://www.csse.unimelb.edu.au/~gabraham

This is not to do with 'wide' matrices' but to do with the lack of
column
names.

There is a bug -- the definition of 'p' should have NROW and not NCOL.

On Sun, 18 May 2008, Gad Abraham wrote:
> Hi,
>
> I'm doing PCA on wide matrices and I don't understand why calling 
> predict.prcomp on it throws an error:
>
>> x1 <- matrix(rnorm(100), 5, 20)
>> x2 <- matrix(rnorm(100), 5, 20)
>> p <- prcomp(x1)
>> predict(p, x2)
> Error in predict.prcomp(p, x2) :
>  'newdata' does not have the correct number of columns
>> dim(x2)
> [1]  5 20
>> dim(p$rotation)
> [1] 20  5
>
> All predict.prcomp really does is x2 %*% p$rotation, which is a perfectly 
> legal operation that should yield a 5x5 matrix:
>
>> dim(x2 %*% p$rotation)
> [1] 5 5
>
> Thanks,
> Gad
>
> -- 
> Gad Abraham
> Dept. CSSE and NICTA
> The University of Melbourne
> Parkville 3010, Victoria, Australia
> email: gabraham at csse.unimelb.edu.au
> web: http://www.csse.unimelb.edu.au/~gabraham
>
> ______________________________________________
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>
-- 
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