Reversing one dimension of an array, in a generalized case

Not sure it is "obvious", but this function implements what you describe:

revdim <- function( a, d ) {
    dims <- attr( a, "dim" )
    idxs <- lapply( seq_along( dims )
                  , function( dd ) {
                      if ( d == dd ) seq.int( dims[ dd ], 1, -1 )
                      else seq.int( dims[ dd ] )
                    }
                  )
    do.call( `[`, c( list( a ), idxs ) )
}

revdim( junk1, 2 )

Hi All:

I have been looking for an elegant way to do the following, but haven't 
found it, I have never had a good understanding of any of the "apply" 
functions.

A simplified idea is I have an array, say:

junk(5, 10, 3)

where (5, 10, 3) give the dimension sizes, and I want to reverse the 
second dimension, so I could do:

junk1 <- junk[, rev(seq_len(10), ]

but what I am after is a general function that will do that where the 
array could be two, three or four dimensions, and I pass to the function 
which dimension I want to reverse, that is the function can not assume 
the number of dimensions of the array nor which dimension to reverse.

For example,  if i try:

junk1 <- apply(junk, 2, rev)

junk1 comes out as two-dimensional,  not three-dimensional.

It is probably something obvious but I am not getting it.

Thanks for any help.

-Roy

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