Returning to the old question ...
> Is it possible to subset an n-dimensional array by a
> vector of n dimensions?
On Sat, Jul 12, 2008 at 1:34 AM, Wolfgang Huber <[EMAIL PROTECTED]> wrote:
> > do.call("[", list(x,1,2,TRUE))
> [1] 3 9 15 21
>
> See also http://134.148.236.121/R/help/06/01/19403.html
>
This is the Something Clever I thought of, but it's actually not the same as
x[1,2,] : TRUE is a (recycled) logical index ... but what is the "nothing"
after the second comma and before the closing ] ?
as.list(quote(x[]))
is.name(quote(x[])[[3]])
.. it seems to be an empty name (``) but there seems to be no way to
construct it other than something like quote(x[])[[3]] or alist(,)[[1]] --
but this is sufficient to rewrite Richard's subsetArray function without
using parse:
sa2 <- function(x, Subset, Eval=TRUE) {
Subset <- lapply(Subset, function(x) if(length(x)==1 & all(is.na(x)))
alist(,)[[1]] else x)
ex <- quote(x[])
Tail <- 3:(2+length(Subset))
ex[Tail] <- Subset
if(Eval) eval(ex) else ex
}
This is more general than subsetArray because "Subset" can be a list -- to
allow thing like x[1:10,,c(3,9,7)] to be "translated".
... And a replacement function:
`sa2<-` <- function(x, Subset, value){
ex <- sa2(x, Subset, Eval = FALSE)
ex[[1]] <- `[<-`
ex$value <- value
eval(ex)
}
... and some related discoveries:
> is.name(alist(,)[[1]])
[1] TRUE
> X<-alist(,)[[1]]
> is.name(X)
Error: argument "X" is missing, with no default
> dump("X","")
X <-
quote()
> X<-quote()
Error in quote() : 0 arguments passed to 'quote' which requires 1
> as.character(X)
Error: argument "X" is missing, with no default
> as.character(alist(,)[[1]])
[1] ""
Kenn
>
>
> In principle this works also for assignment, but the following benchmark
> shows a big performance hit:
>
> x = array(1:1e6, dim=c(100,100,100))
>
> system.time({
> for(i in 1:100) x = do.call("[<-", list(x, i, 2, TRUE, pi))
> })
>
> system.time({
> for(i in 1:100) x[i, 2, ] <- pi
> })
>
> The first takes ~2sec on my computer, the second 8ms, and I wonder whether
> someone knows how to use the "clever" behaviour of "[<-" also when it is
> called with do.call?
>
> Best wishes
> Wolfgang
>
>
> ----------------------------------------------------
> Wolfgang Huber, EMBL-EBI, http://www.ebi.ac.uk/huber
>
>
>
>>
>> On Sat, Jul 12, 2008 at 12:48 AM, Wolfgang Huber <[EMAIL PROTECTED]<mailto:
>> [EMAIL PROTECTED]>> wrote:
>>
>>
>> Hi Richard,
>>
>> what is wrong with Patrick's suggestion? I get
>>
>>
>> x <- array(1:24, dim=2:4)
>> x[rbind(c(1,1,2))]
>> ## [1] 7
>> x[rbind(c(1,1,2))] <- 13
>> x[rbind(c(1,1,2))]
>> ## [1] 13
>>
>> And you could also do
>>
>> do.call("[", list(x,1,1,2))
>>
>> These should be a bit quicker than the eval/parse constructs - see
>> also library("fortunes")
>> fortune(106)
>>
>> Also note that your "subsetArray<-" function will afaIu each time
>> copy (and then modify) the whole array, which may be quite
>> inefficient. I believe that the "[<-" function has been optimized to
>> avoid that in some cases. Try
>>
>> x=numeric(1e6); system.time({for (i in 1:1000) x[i]=12})
>>
>> versus
>>
>> x=numeric(1e6); system.time({for (i in 1:1000) {y=x; y[i]=12}})
>>
>> Best wishes
>> Wolfgang
>>
>> ----------------------------------------------------
>> Wolfgang Huber, EMBL-EBI, http://www.ebi.ac.uk/huber
>>
>>
>>
>>
>>
>> Richard Pearson wrote:
>>
>> In case anyone's still interested, I now have (I think!) a
>> complete solution (thanks to a quick look at my new favourite
>> document - S Poetry :-)
>>
>> subsetArray <- function(x, subset) {
>> subsetString <- paste(subset, collapse=",")
>> subsetString <- gsub("NA","",subsetString)
>> evalString <- paste(expression(x), "[", subsetString, "]")
>> eval(parse(text=evalString))
>> }
>>
>> "subsetArray<-"<- function(x, subset, value) {
>> subsetString <- paste(subset, collapse=",")
>> subsetString <- gsub("NA","",subsetString)
>> evalString <- paste(expression(x), "[", subsetString, "] <-",
>> expression(value))
>> eval(parse(text=evalString))
>> x
>> }
>>
>> x <- array(1:24, dim=2:4)
>> subsetArray(x, c(1,1,2))
>> subsetArray(x, c(1,1,2)) <- 25
>> x
>>
>> Thanks to Pat!
>>
>> Richard
>>
>>
>> Richard Pearson wrote:
>>
>> My understanding of matrix subscripting is that this can be
>> used to access arbitrary elements from an array and return
>> them as a vector, but I don't understand how that helps me
>> here. I've now written a function that seems to do what I
>> originally wanted, but I've also realised I want to do
>> assignment too. I need to read up more on writing assignment
>> functions to do this. However, I'm thinking that someone has
>> already solved this, in a far more elegant way than my
>> hacking :-). Here's my function for anyone interested:
>>
>> subsetArray <- function(x, subset) {
>>
>> + subsetString <- paste(subset, collapse=",")
>> + subsetString <- gsub("NA","",subsetString)
>> + evalString <- paste(expression(x), "[", subsetString, "]")
>> + eval(parse(text=evalString))
>> + }
>>
>> x <- array(1:24, dim=2:4)
>> subsetArray(x, c(1,1,2))
>>
>> [1] 7
>>
>> subsetArray(x, c(1,NA,2))
>>
>> [1] 7 9 11
>>
>> subsetArray(x, c(1,NA,NA))
>>
>> [,1] [,2] [,3] [,4]
>> [1,] 1 7 13 19
>> [2,] 3 9 15 21
>> [3,] 5 11 17 23
>>
>>
>> subsetArray(x, c(1,1,2)) <- 25
>>
>> Error in subsetArray(x, c(1, 1, 2)) <- 25 : could not find
>> function "subsetArray<-"
>>
>> Best wishes
>>
>> Richard.
>>
>>
>> Patrick Burns wrote:
>>
>> I think you are looking for subscripting with a
>> matrix:
>>
>> x[cbind(1,1,2)]
>>
>> See, for instance, the subscripting section of chapter 1
>> of S Poetry.
>>
>>
>> Patrick Burns
>> [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
>> +44 (0)20 8525 0696
>> http://www.burns-stat.com
>> (home of S Poetry and "A Guide for the Unwilling S User")
>>
>> Richard Pearson wrote:
>>
>> Hi
>>
>> Is it possible to subset an n-dimensional array by a
>> vector of n dimensions? E.g. assume I have
>>
>> x <- array(1:24, dim=2:4)
>> x[1,1,2]
>>
>> [1] 7
>>
>> dims <- c(1,1,2)
>>
>>
>> I would like a function that I can supply x and dims
>> as parameters to, and have it return 7. Also, I
>> would like to do something like:
>>
>> x[1,1,]
>>
>> [1] 1 7 13 19
>>
>> dims2<- c(1,1,NA)
>>
>>
>> And have a function of x and dims2 that gives me
>> back [1] 1 7 13 19
>>
>> Does such a thing exist?
>>
>> Thanks
>>
>> Richard
>>
>>
>> ______________________________________________
>> R-help@r-project.org <mailto:R-help@r-project.org> 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.
>>
>>
>>
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org 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.
