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]
+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
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