[R] Generalising to n-dimensions

[Robin Hankin]

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
01223-764877