[Rd] Using 'dimname names' in aperm() and apply()

[ghostwheel]

I'm replying here to quite an old thread started by me.

I think the dimnames facility is underused in R.
I'm currently using a 5-dimensional array. It is quite cumbersome to have to
write
y[,,,,"0"]

Since the array does have dimnames,  I would like to be able to say instead
y[tree="0"]

Below is a function that enables this functionality
--
myslice=function( input.array,...){
  args=list(...)
  if( length(names(args))==0 ) {
    d = args
  } else {
    d=dimnames( input.array )
    for(n in names(args)) {
      i=args[[n]]
      if( is.numeric(i)) {
        if( length(d[[n]]) == 0) d[[n]]=i
        else d[[n]]=d[[n]][i]
      } else d[[n]]=args[[n]]
    }
  }
  dd=dim( input.array )
  for(i in seq(along=dd))
    if( length(d[[i]])==0) d[[i]] = seq( dd[i])
  i=sapply(d,length)>1
  new.dimnames=dimnames(input.array)[i]
  d=c(list( input.array ),d)
  ret = do.call("[",d)
  dimnames(ret) = new.dimnames
  ret
}
---
I'm not sure what is the best way to replace "[" for arrays with this
function. I should also implement [<- in the same manner.

One additional feature of this function is that it keeps dimnames.
So, if we have
A=array(1:24,c(2,3,4),dimnames=list(a=c(),b=c(),c=c()))
(btw, why can't I do A=array(1:24,c(a=2,b=3,c=4))?)

Then A[1,,] will give a matrix without dimnames:
--
> A[1,,]
     [,1] [,2] [,3] [,4]
[1,]    1    7   13   19
[2,]    3    9   15   21
[3,]    5   11   17   23
--
Whereas the new function keeps the dimnames:
--
> myslice(A,a=1)
      c
b      [,1] [,2] [,3] [,4]
  [1,]    1    7   13   19
  [2,]    3    9   15   21
  [3,]    5   11   17   23
--

Another function where using dimnames for MARGIN would be slice.index

Thanks!
Michael

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