[R] filling an array, vectorized

[Martin Maechler]

>>>>> "Robin" == Robin Hankin <r.hankin at noc.soton.ac.uk>
>>>>>     on Thu, 16 Nov 2006 11:54:38 +0000 writes:

    Robin> Gabor
    Robin> Tamas

    Robin> yet again I find myself trumped by Gabor because I forget that
    Robin> TRUE is a perfectly acceptable argument to "[".

    Robin> Heh.


    Robin> I'll stick do.index2() in the magic package.

[ calling it 'do.index' - not 'do.index2' hopefully. ]

Note that there are two things I'd further change:

1) expand.grid() had grown a 'KEEP.OUT.ATTRS' argument recently
   which you want to set to FALSE for efficiency
2) jj(TRUE) maybe short and cute, but I find it ugly that it
   really relies on
   seq(<integer-vector>) |--> seq(along = <integer-vector>)
   and would prefer the more efficient and clearer
       seq_len(length(dim(a)))


Note however that for typical cases these two changes don't make
the result (noticeably) faster at all


do.indexM <- function(a,f) {
    jj <- function(i) seq_len(dim(a)[i])
    index <- as.matrix(expand.grid(lapply(seq_len(length(dim(a))), jj),
                                   KEEP.OUT.ATTRS = FALSE) )
    a[index] <- apply(index, 1, f)
    a
}

Martin

    Robin> best wishes


    Robin> rksh


    Robin> On 16 Nov 2006, at 11:27, Gabor Grothendieck wrote:

    >> Here is minor simplification:
    >> 
    >> do.index2 <- function(a,f){
    >> jj <- function(i) seq(dim(a)[i])
    >> index <- as.matrix(expand.grid(lapply(jj(TRUE), jj)))
    >> a[index] <- apply(index, 1, f)
    >> a
    >> }
    >> 
    >> # test
    >> a <- array(0,c(2,3,4))
    >> identical(do.index(a, f), do.index2(a, f))
    >> b <- array(0,c(2,2,2,2))
    >> identical(do.index(b, f), do.index2(b, f))
    >> 
    >> 
    >> On 11/16/06, Robin Hankin <r.hankin at noc.soton.ac.uk> wrote:
    >>> Tamas
    >>> 
    >>> first of all, Thank You for a really well-posed, interesting problem.
    >>> Answer follows.
    >>> 
    >>> 
    >>> 
    >>> do.index <- function(a,f){
    >>> jj <- function(i){seq_len(dim(a)[i])}
    >>> index <- as.matrix(expand.grid(sapply(1:length(dim
    >>> (a)),jj,simplify=FALSE)))
    >>> a[index] <- apply(index,1,f)
    >>> return(a)
    >>> }
    >>> 
    >>> 
    >>> 
    >>> f <- function(l){
    >>> sum(unlist(l))
    >>> }
    >>> 
    >>> a <- array(0,c(2,3,4))
    >>> b <- array(0,c(2,2,2,2))
    >>> 
    >>> do.index(a,f)
    >>> do.index(b,f)
    >>> 
    >>> 
    >>> 
    >>> best wishes
    >>> 
    >>> Robin
    >>> 
    >>> 
    >>> 
    >>> 
    >>> 
    >>> On 15 Nov 2006, at 19:16, Tamas K Papp wrote:
    >>> 
    >>> > Hi,
    >>> >
    >>> > I am sure this has come up before, but my searches of the archive
    >>> > didn't give any results (maybe I didn't use the right keywords,  
    >>> but if
    >>> > I use too many, the search times out).
    >>> >
    >>> > I have a vector of dimensions n, length is not fixed, eg
    >>> >
    >>> > n <- c(4,5,7)
    >>> >
    >>> > or
    >>> >
    >>> > n <- c(19,4,5,7)
    >>> >
    >>> > and a function f that takes a vector of indices, same length of  
    >>> n, and
    >>> > gives a scalar.
    >>> >
    >>> > I would like to fill the array
    >>> >
    >>> > a <- array(dim=n)
    >>> >
    >>> > so that (... is just notation, not R's ...)
    >>> >
    >>> > a[i,j,k,...] <- f(list(i,j,k,...))
    >>> >
    >>> > I would use loops, but since n can have different lengths, I don't
    >>> > know how many loops I would need beforehand.  Is there a way to do
    >>> > this?
    >>> >
    >>> > Thanks,
    >>> >
    >>> > Tamas
    >>> >
    >>> > ______________________________________________
    >>> > R-help at stat.math.ethz.ch mailing list
    >>> > https://stat.ethz.ch/mailman/listinfo/r-help
    >>> > PLEASE do read the posting guide http://www.R-project.org/posting-
    >>> > guide.html
    >>> > and provide commented, minimal, self-contained, reproducible code.
    >>> 
    >>> 

    Robin> --
    Robin> Robin Hankin
    Robin> Uncertainty Analyst
    Robin> National Oceanography Centre, Southampton
    Robin> European Way, Southampton SO14 3ZH, UK
    Robin> tel  023-8059-7743

    Robin> ______________________________________________
    Robin> R-help at stat.math.ethz.ch mailing list
    Robin> https://stat.ethz.ch/mailman/listinfo/r-help
    Robin> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
    Robin> and provide commented, minimal, self-contained, reproducible code.