Applying a function to the edges of a multidimensional array in R - r

Question

How best can I apply a function to the edges of a multidimensional array in R, without hard-coding the number of dimensions in advance. In a two dimensional array, I could, for instance:
myarray[1,] = f(myarray[1,])
myarray[M,] = f(myarray[M,])
myarray[,1] = f(myarray[,1])
myarray[,N] = f(myarray[,N])
But what if I want to have a function do this for an array of any dimension? In particular, how can I handle the indexing in a relatively painless way? (Assume that have multiple applications of the function taking place at corners is not a problem.)
If I flatten the array, I can do this, but I'd prefer a vectorized approach. Alternatively, I could just hard code this for arrays of every dimension up to, some dimension and fail on higher, but I'd prefer something prettier, if possible.

Answer 1

Here's a solution that should be able to handle an arbitrary number of dimensions. The basic idea is that
For each dimension, apply() is called with the function of choice
Each result of that function is turned into a list of length one
This should make apply() return a list of results for each dimension
The first and last list items for each dimension are stored in the results vector
This would be very time consuming for arrays with large dimensions and/or time consuming functions of choice, since the function is applied to a potentially large number of values that are not used. But it should allow for arbitrary functions and arbitrary results of those functions. Here it goes:
## Set up array
xx<-array(1:24,dim=c(1,2,3,4))
## Determine number of dimensions in array
ndim<-length(dim(xx))
## Set up results vector (a list)
myAns<-vector("list",ndim)
## Iterating over the number of dimensions, apply a function
for(ii in seq_len(ndim)){
tempAns<-apply(xx,ii,function(x)list(mean(x)))
## Store first and last results in myAns vector
## If result is length 1, only store the single result
if(length(tempAns)==1){
myAns[[ii]]<-tempAns
} else {
myAns[[ii]]<-c(head(tempAns,1),tail(tempAns,1))
}
}

Answer 2

Did you have something like this in mind?
> ary <- array(1:27, c(3,3,3))
> apply(ary, MARGIN = 3, function(x) {
+ lastColMean <- mean(x[, ncol(x)])
+ lastRowMean <- mean(x[nrow(x), ])
+ data.frame(lastColMean, lastRowMean)
+ })
[[1]]
lastColMean lastRowMean
1 8 6
[[2]]
lastColMean lastRowMean
1 17 15
[[3]]
lastColMean lastRowMean
1 26 24