[R] Subsetting an array by a vector of dimensions

[Richard Pearson]

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
> patrick at burns-stat.com
> +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
>>
>>
> 

-- 
Richard D. Pearson             richard.pearson at postgrad.manchester.ac.uk
School of Computer Science,    http://www.cs.man.ac.uk/~pearsonr
University of Manchester,      Tel: +44 161 275 6178
Oxford Road,                   Mob: +44 7971 221181
Manchester M13 9PL, UK.        Fax: +44 161 275 6204