# [R] Generalising to n-dimensions

Robin Hankin rksh1 at cam.ac.uk
Fri Sep 26 11:26:30 CEST 2008

```Laura Bonnett wrote:
> Sorry to hassle you, but I really need to get my code up and running.
> Please can you therefore explain what a and v are?

Hi Laura.  I've been away (in Norwich).  Sorry not to give an example.

Variable 'a' is an array and variable 'd' is the same as in your
original email.

> a <- array(1:4,c(3,2,2,4))
> d <- c(1,2)
> f(a,d)
[,1] [,2]
[1,]    1    4
[2,]    2    1
[3,]    3    2
>

Thus in this case f(a,d) gives a[,,1,2].   Function f() is necessary because
you specified that the length of 'd' is not known in advance.

You can get 'd' from a single row of expand.grid() [but you will have to
coerce it to a matrix]

HTH

rksh

>
> Thank you,
>
> Laura
>
> On Wed, Sep 24, 2008 at 8:27 PM, Laura Bonnett
> <l.j.bonnett at googlemail.com <mailto:l.j.bonnett at googlemail.com>> wrote:
>
>     Can I ask what a and v are?
>
>     Thanks,
>
>     Laura
>
>
>     On Sat, Aug 23, 2008 at 11:41 AM, Robin Hankin <rksh1 at cam.ac.uk
>     <mailto:rksh1 at cam.ac.uk>> wrote:
>
>         Laura Bonnett wrote:
>
>             crosstable[,,expand[d,1],expand[d,2],expand[d,3],...expand[d,n]]
>              crosstable is just a crosstabulation of an
>             n+2-dimensional dataset and I am trying to pick out those
>             that are in combination 'd' of expand. So for example, for
>             5-dimensional data using your example:
>               Var1 Var2 Var3
>             1     1    1    1
>             2     2    1    1
>             3     3    1    1
>             4     1    2    1
>             5     2    2    1
>             6     3    2    1
>             7     1    1    2
>             8     2    1    2
>             9     3    1    2
>             10    1    2    2
>             11    2    2    2
>             12    3    2    2
>              d refers to the row of the matrix above - d=2 is 2,1,1 so
>             crosstable[,,2,1,1] would retrieve all the data where Var1
>             =2, Var2=1, Var3=1 and the two remaining variables are
>             given in the crosstabulations for all values.
>              Is that any better?
>
>
>         OK  I think I understand.  The magic package uses this type of
>         construction extensively, but not this particular one.
>
>         It's trickier than I'd have expected.
>
>         Try this:
>
>         f <- function(a,v){
>           jj <-
>         sapply(dim(a)[seq_len(length(dim(a))-length(v))],seq_len,simplify=FALSE)
>           jj <- c(jj , as.list(v))
>           do.call("[" , c(list(a) , jj, drop=TRUE))
>         }
>
>
>
>         [you will have to coerce the output from expand.grid() to a
>         matrix in order to extract a row from it]
>
>
>         HTH
>
>         rksh
>
>
>
>
>
>
>
>
>         --
>         Robin K. S. Hankin
>         Senior Research Associate
>         Cambridge Centre for Climate Change Mitigation Research (4CMR)
>         Department of Land Economy
>         University of Cambridge
>         rksh1 at cam.ac.uk <mailto:rksh1 at cam.ac.uk>
>         01223-764877
>
>
>

--
Robin K. S. Hankin
Senior Research Associate
Cambridge Centre for Climate Change Mitigation Research (4CMR)
Department of Land Economy
University of Cambridge
rksh1 at cam.ac.uk
01223-764877

```

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