Basically dropping by saying what's already been said, but also that
you probably want to do whatever you're trying to achieve using
vectorized operations. A small summary with some tweaks:
## Generate an array with a "random" number of dimensions
dim <- c(4, 5, 6, 7)
dimnames <- lapply(1:4, FUN=function(i) sprintf("%s%d", letters[i], 1:dim[i])
)
x <- array(seq_len(prod(dim)), dim=dim, dimnames=dimnames)
# A single cell
> y0 <-x[1,2,3,4]
> y <- do.call(`[`, c(list(x), alist(1,2,3,4)))
> y
[1] 405
> stopifnot(identical(y, y0))
# A "block" subset of the array
> y0 <- x[1:2,2,3:4,4]
> y <- do.call(`[`, c(list(x), alist(1:2,2,3:4,4)))
> y
c3 c4
a1 405 425
a2 406 426
> stopifnot(identical(y, y0))
# A "block" subset of the array (preserving all 4 dimensions)
> y0 <- x[1:2,2,3:4,4, drop=FALSE]
> y <- do.call(`[`, c(list(x), alist(1:2,2,3:4,4), drop=FALSE))
> str(y)
int [1:2, 1, 1:2, 1] 405 406 425 426
- attr(*, "dimnames")=List of 4
..$ : chr [1:2] "a1" "a2"
..$ : chr "b2"
..$ : chr [1:2] "c3" "c4"
..$ : chr "d4"
> stopifnot(identical(y, y0))
# Why alist() and not list()? Because if you also need to do ...
> y0 <- x[1:2,,1:2,, drop=FALSE]
> y <- do.call(`[`, c(list(x), alist(1:2,,1:2,), drop=FALSE))
> str(y)
int [1:2, 1:5, 1:2, 1:7] 1 2 5 6 9 10 13 14 17 18 ...
- attr(*, "dimnames")=List of 4
..$ : chr [1:2] "a1" "a2"
..$ : chr [1:5] "b1" "b2" "b3" "b4" ...
..$ : chr [1:2] "c1" "c2"
..$ : chr [1:7] "d1" "d2" "d3" "d4" ...
> stopifnot(identical(y, y0))
# Any subset of cells - not a "block" (using what is called "matrix
indexing" in R)
> y0 <- c(x[1,2,2,4], x[3,2,1,3])
> idxs <- cbind(c(1,3), c(2,2), c(2,1), c(4,3))
> y <- x[idxs]
[1] 385 247
> stopifnot(identical(y, y0))
For a (possibly) more readable subsetting by "blocks" than using
do.call(), there's also:
> library(R.utils)
> y <- extract(x, indices=list(1:2,2,3:4,4), drop=TRUE)
> y <- extract(x, indices=list(1:2,2,3:4,4), drop=FALSE)
It also has some other bells and whistles, but it cannot do more than
what already shown above.
/Henrik
(author of R.utils)
On Sat, May 30, 2015 at 10:34 AM, Jeff Newmiller
<jdnew...@dcn.davis.ca.us> wrote:
> Don't.
>
> Arrays in R don't have variable dimensions [1]. To the extent that you
> pretend otherwise, you will degrade performance and complicate your program.
> I would argue that similar effects arise in languages that do let you pretend
> that arrays have variable dimensions.
>
> The main reason variable-sized arrays seem useful is that some algorithms or
> data sources yield variable amounts of data. There are various ways to handle
> these cases, and the use of lists to hold intermediate units of data followed
> by transfer of that data into an array (strategically handled after you know
> how big the array has to be) is one of the more common ones.
>
> Another technique is to use one of the ragged array or sparse matrix data
> structures, but they generally work best when the data actually are sparse.
>
> Being more specific about your immediate problem can elicit more specific
> help.
>
> BTW please post plain text on this list... HTML is not supported by the list
> reflector and leads to misunderstandings.
>
> [1] More precisely, the total number of elements in the underlying vector is
> fixed, though you can redefine how the dimensions use the elements in that
> vector. For example, study the "aperm" function.
> ---------------------------------------------------------------------------
> Jeff Newmiller The ..... ..... Go Live...
> DCN:<jdnew...@dcn.davis.ca.us> Basics: ##.#. ##.#. Live Go...
> Live: OO#.. Dead: OO#.. Playing
> Research Engineer (Solar/Batteries O.O#. #.O#. with
> /Software/Embedded Controllers) .OO#. .OO#. rocks...1k
> ---------------------------------------------------------------------------
> Sent from my phone. Please excuse my brevity.
>
> On May 30, 2015 3:29:09 AM PDT, WRAY NICHOLAS <nicholas.w...@ntlworld.com>
> wrote:
>>Hello folks
>>
>>Supposing I have a multidimensional array in an R prog, say a 4D array.
>>
>>I want the coordinate quantities to be read off from a vector. The
>>values
>>in the vector (vec) are generated by a function.
>>
>>This is easy if the number of dimensions is fixed for both the array
>>and
>>the number of elements in the vector (say 4):
>>
>>X<-array[vec[1],vec[2],vec[3],vec[4]]
>>
>>But if the number of dimensions of the array is not fixed and can
>>change
>>during the course of the prog I am stuck as to how to do this, as I
>>don’t
>>know a way of feeding a vector or list of unspecified beforehand length
>>into the coordinates for an array
>>
>>I can’t find anything useful about this on the net
>>
>>Does anyone have any ideas?
>>Thanks, Nick
>>
>> [[alternative HTML version deleted]]
>>
>>______________________________________________
>>R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>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.
>
> ______________________________________________
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
______________________________________________
R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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