[R] spinning and flipping arrays

[Bill Rising]

On 5/26/2003 17:21, Peter Dalgaard BSA wrote

>Bill Rising <brising at louisville.edu> writes:
>
>> On 5/26/2003 15:05, Spencer Graves wrote
>> 
[snip...]

This seems to work for the flipping (and it relies on textual 
representations, and is completely unreadable, and is inelegant):

fliparr <-
function(arr,dims=length(dim(arr))) {
 #defaults to flipping across last dimension, otherwise takes
 #  a vector of dimensions (which ignores dimesions which are out of 
bounds)
 # NO error checking yet
 # initialize a series of blank strings for the indices
  strarg <- vector("character",arrsize <- length(arrdim<-dim(arr)))
 # find which indices will be flipped (logical vector)
  which <-  apply(outer(1:arrsize,as.vector(dims),"=="),1,any)
 # use 1:dimension size for non-flipped dimensions
  strarg[!which] <- paste(NULL,arrdim[!which],sep="1:")
 # use dimension size:1 for flipped dimensions
  strarg[which] <- paste(arrdim[which],NULL,sep=":1")
 # paste all the text together and hope syntax never changes
  
eval(parse(text=paste("arr[",paste(strarg,collapse=","),"]",collapse="")))
}

>
>Relying on textual representation always looks dodgy to me. 

You've got a good point.

>How about
>
>spin <- function(x,amount=c(1,rep(0,length(dim(x))-1))) {
>    indices <- mapply(function(n,k) (seq(length=n)+k-1)%%n + 1,
>                      dim(x),amount, SIMPLIFY=FALSE)
>    do.call("[", c(list(x), indices))
>}

This works quite well for spinning an equal amount in each dimension. I 
think I can use this to work on the equivalent to APL which either uses a 
single number to spin a single dimension (like spin(foo,c(something,0,0)) 
) or an array with the dimensions of the original array minus the 
dimension around which items are spun, so that the items can be shifted 
differenent amounts.

>
>(Note: mapply() was introduced in 1.7.0). 

I'll check out mapply. thanks,

Bill