Re: Exchanging Indices of a Matrix

[ripley@stats.ox.ac.uk]

On Tue, 25 Jun 2002, Quinn, David wrote:

> Is there an easy way to do the following without the looping?  I'm
guessing not, since I think the internal representation of a matrix
is as a list.   -- Thanks, David

Yes, see ?aperm.

The internal representation of an array (and also of matrices but those
only have two dimensions) is a vector, not a list.  A loop is involved,
but it is done in C.

>
> for (i in 1:x)
> {
>       for (j in 1:y)
>       {
>               for (k in 1:z)
>               {
>                       new.matrix[k,j,i] <- old.matrix[i,j,k]
>               }
>       }
> }
>

-- 
Brian D. Ripley,                  ripley@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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595