If you can add it to the magic package, that would be great. It seems like
the appropriate place for it. It would probably be useful to combine that
with the adrop function or apldrop function in order to allow transparent
extraction of subarrays. Perhaps the default could be "drop=FALSE" (please!
:) and if desired one could pass in a drop=TRUE flag at the end of the
invocation of arraytake which would collapse singleton dimensions (using
your existing code).
As for do.call -- I never had occasion to use that before but it seems like
a cool hack! Definitely cleaner than my solution;
I don't like using "eval" in general (esp. in nested function calls) because
sometimes there are issues with environments and namespaces that can be a
pain to work around.
On 10/19/06, Robin Hankin <[EMAIL PROTECTED]> wrote:
>
>
> On 19 Oct 2006, at 14:26, Gabor Grothendieck wrote:
>
> > 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))
> >
>
>
>
> aargggh, you beat me to it. I didn't think to pass TRUE to "[" .
>
> I'll stick it in the package with joint attribution to Gabor and Balaji
> and document it with apltake() and apldrop().
>
>
> best wishes all
>
>
> rksh
>
>
> > On 10/19/06, Balaji S. Srinivasan <[EMAIL PROTECTED]> 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
> >> [EMAIL PROTECTED]
> >> http://jinome.stanford.edu
> >>
> >> [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-devel@r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-devel
> >>
> >
> > ______________________________________________
> > R-devel@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
>
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
> tel 023-8059-7743
>
>
>
>
>
--
Dr. Balaji S. Srinivasan
Stanford University
Depts. of Statistics and Computer Science
318 Campus Drive, Clark Center S251
(650) 380-0695
[EMAIL PROTECTED]
http://jinome.stanford.edu
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