Kenn Konstabel wrote:
What is "wrong" is, I think, that you can't write a substitute for
x[1,2,] this way. (Or, in general, empty indexes won't work.)
A related question - Is there a way to replace x[1,2,] with "["(1,2,
*something clever*) ?
Kenn
Dear Kenn,
I can:
> 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
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
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