[Rd] arraytake for extracting subarrays from multidimensional arrays

[Gabor Grothendieck]

Note that it can also be done like with do.call:

a <- array(1:24, 2:4)
L <- list(TRUE, 1:3, c(4, 2))
do.call("[", c(list(a), L))

On 10/19/06, Balaji S. Srinivasan <balajis at stanford.edu> wrote:
> Hi,
>
> I recently encountered a problem with array subsetting and came up with a
> fix. Given an array of arbitrary dimensions, in which the number of
> dimensions is only known at runtime, I wanted to extract a subarray. The
> main issue with doing this is that in order to extract a subarray from an
> array of (say) 4 dimensions you usually specify something like this
>
> a.subarray <- a[,c(4,2),1:5,]
>
> However, if your code needs to handle an array with an arbitrary number of
> dimensions then you can't hard code the number of commas while writing the
> code. (Regarding motivation, the reason this came up is because I wanted to
> do some toy problems involving conditioning on multiple variables in a
> multidimensional joint pmf.)
>
> I looked through commands like slice.index and so on, but they seemed to
> require reshaping and big logical matrix intermediates, which were not
> memory efficient enough for me. apltake in the magic package was the closest
> but it only allowed subsetting of contiguous indices from either the first
> or last element in any given dimension. It was certainly possible to call
> apltake multiple times to extract arbitrary subarrays via combinations of
> index intervals for each dimension, and then combine them with abind as
> necessary, but this did not seem elegant.
>
> Anyway, I then decided to simply generate code with parse and eval. I found
> this post by Henrik Bengtsson which had the same idea:
>
> http://tolstoy.newcastle.edu.au/R/devel/05/11/3266.html
>
> I just took that code one step further and put together a utility function
> that I think might be fairly useful. I haven't completely robustified it
> against all kinds of pathological inputs, but if there is any interest from
> the development team it would be nice to add an error-checked version of
> this to R (or I guess I could keep it in a package).
>
>
> Simple usage example:
> ------
> > source("arraytake.R")
> > a <- array(1:24,c(2,3,4))
>
> > a[,1:3,c(4,2)] ##This invocation requires hard coding the number of
> dimensions of a
> , , 1
>
>     [,1] [,2] [,3]
> [1,]   19   21   23
> [2,]   20   22   24
>
> , , 2
>
>     [,1] [,2] [,3]
> [1,]    7    9   11
> [2,]    8   10   12
>
>
> > arraytake(a,list(NULL,1:3,c(4,2))) ##This invocation does not, and
> produces the same result
> , , 1
>
>     [,1] [,2] [,3]
> [1,]   19   21   23
> [2,]   20   22   24
>
> , , 2
>
>     [,1] [,2] [,3]
> [1,]    7    9   11
> [2,]    8   10   12
>
>
>
> Code below:
> --------
> arraytake <- function(x,indlist) {
>
>  #Returns subarrays of arbitrary dimensioned arrays
>  #1) Let x be a multidimensional array with an arbitrary number of
> dimensions.
>  #2) Let indlist be a list of vectors. The length of indlist is the same as
> the number of
>  #dimensions in x. Each element of the indlist is a vector which specifies
> which
>  #indexes to extract in the corresponding dimension. If the element of the
> indlist is
>  #NULL, then we return all elements in that dimension.
>
>  #The main way this works is by programmatically building up a comma
> separated argument to "[" as a string
>  #and then simply evaluating that expression. This way one does not need to
> specify the number of
>  #commas.
>
>  if(length(dim(x)) != length(indlist)) {
>    return();  #we would put some error message here in production code
>  }
>
>  #First build up a string w/ indices for each dimension
>  d <- length(indlist);  #number of dims
>  indvecstr <- matrix(0,d,1);
>  for(i in 1:d) {
>    if(is.null(indlist[[i]])) {
>      indvecstr[i] <- "";
>    } else{
>      indvecstr[i] <-
> paste("c(",paste(indlist[[i]],sep="",collapse=","),")",sep="")
>    }
>  }
>
>  #Then build up the argument string to "["
>  argstr <- paste(indvecstr,sep="",collapse=",")
>  argstr <- paste("x[",argstr,"]",sep="")
>
>  #Finally, return the subsetted array
>  return(eval(parse(text=argstr)))
> }
>
>
>
>
>
>
>
> --
> Dr. Balaji S. Srinivasan
> Stanford University
> Depts. of Statistics and Computer Science
> 318 Campus Drive, Clark Center S251
> (650) 380-0695
> balajis at stanford.edu
> http://jinome.stanford.edu
>
>        [[alternative HTML version deleted]]
>
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