[R] Reversing one dimension of an array, in a generalized case

[Jeff Newmiller]

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 )

On Thu, 1 Jun 2017, Roy Mendelssohn - NOAA Federal wrote:

> 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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