[Rd] order(x,y, decreasing = c(FALSE, TRUE)) - how / elegantly?

[Martin Maechler]

I've found the need to compute a version of  order(x,y)
where I want the sort order 
for *increasing* x  and *decresing* y ...

something we could imagine could be provided in the future as

  order(x,y, decreasing = c(FALSE, TRUE))

i.e., using a 'vectorized' decreasing argument.
{No, I'm not volunteering right now!}

I've found the following R-level solution and
like to quiz you for more elegant / faster solutions 
{but I am not really interested in replacing  
 order(x)  by  sort.list(x, method="quick") and similar things;
 one thing to consider *is* using an   'na.last = . ' correctly,
 and I haven't had the need for that and so not bothered to "do it"}

## Here's a script with my version and a small example 
## (if you want speed comparisons, use larger examples) :

orderXuYd <- function(x,y)
{
    ## Purpose: order(x,y): x up, y down
    ## ----------------------------------------------------------------------
    ## Arguments: x,y:  vectors of the same length
    ## ----------------------------------------------------------------------
    ## Author: Martin Maechler, Date: 21 Aug 2008

    ix <- order(x) 
    xx <- x[ix]
    iy <- tapply(y[ix], xx, order, decreasing = TRUE)
    ## Note: 'SIMPLIFY', 'USE.NAMES', 'use.names' are just for efficiency:
    unlist(mapply(`[`, split(ix,xx), iy, 
                  SIMPLIFY = FALSE, USE.NAMES = FALSE),
           use.names = FALSE)
}

x <- c(1, 1, 2, 0, 0, 2, 1, 2, 2, 0, 2)
y <- c(27, 21, 45, 11, 13, 58, 35, 74, 95, 16, 122)
ii <- orderXuYd(x,y)
## yes, this is it :
cbind(ii=ii, x=x[ii],y=y[ii])

------------------

Yes, the real reason this goes to R-devel is that it might be
neat to provide this (well, its generalization) via an enhanced
order() function.

Martin Maechler, ETH Zurich

PS: I will be basically offline all day tomorrow, so don't
    expect my reactions to your ideas quickly