[R] sorting variable names containing digits

[Gabor Grothendieck]

mixedsort in gtools will give the same result as mysort(s) but
differs in the case of t.

On Sun, Dec 21, 2008 at 8:33 PM, John Fox <jfox at mcmaster.ca> wrote:
> Dear r-helpers,
>
> I'm looking for a way of sorting variable names in a "natural" order, when
> the names are composed of digits and other characters. I know that this is a
> vague idea, and that sorting character strings is a complex topic, but
> perhaps a couple of examples will clarify what I mean:
>
>> s <- c("x1b", "x1a", "x02b", "x02a", "x02", "y1a1", "y10a2",
> +   "y10a10", "y10a1", "y2", "var10a2", "var2", "y10")
>
>> sort(s)
>  [1] "var10a2" "var2"    "x02"     "x02a"    "x02b"    "x1a"
>  [7] "x1b"     "y10"     "y10a1"   "y10a10"  "y10a2"   "y1a1"
> [13] "y2"
>
>> mysort(s)
>  [1] "var2"    "var10a2" "x1a"     "x1b"     "x02"     "x02a"
>  [7] "x02b"    "y1a1"    "y2"      "y10"     "y10a1"   "y10a2"
> [13] "y10a10"
>
>> t <- c("q10.1.1", "q10.2.1", "q2.1.1", "q10.10.2")
>
>> sort(t)
> [1] "q10.1.1"  "q10.10.2" "q10.2.1"  "q2.1.1"
>
>> mysort(t)
> [1] "q2.1.1"   "q10.1.1"  "q10.2.1"  "q10.10.2"
>
> Here, sort() is the standard R function and mysort() is a replacement, which
> sorts the names into the order that seems natural to me, at least in the
> cases that I've tried:
>
> mysort <- function(x){
>  sort.helper <- function(x){
>    prefix <- strsplit(x, "[0-9]")
>    prefix <- sapply(prefix, "[", 1)
>    prefix[is.na(prefix)] <- ""
>    suffix <- strsplit(x, "[^0-9]")
>    suffix <- as.numeric(sapply(suffix, "[", 2))
>    suffix[is.na(suffix)] <- -Inf
>    remainder <- sub("[^0-9]+", "", x)
>    remainder <- sub("[0-9]+", "", remainder)
>    if (all (remainder == "")) list(prefix, suffix)
>    else c(list(prefix, suffix), Recall(remainder))
>    }
>  ord <- do.call("order", sort.helper(x))
>  x[ord]
>   }
>
> I have a couple of applications in mind, one of which is recognizing
> repeated-measures variables in "wide" longitudinal datasets, which often are
> named in the form x1, x2, ... , xn.
>
> mysort(), which works by recursively slicing off pairs of non-digit and
> digit strings, seems more complicated than it should have to be, and I
> wonder whether anyone has a more elegant solution. I don't think that
> efficiency is a serious issue for the applications I'm considering, but of
> course a more efficient solution would be of interest.
>
> Thanks,
>  John
>
> ------------------------------
> John Fox, Professor
> Department of Sociology
> McMaster University
> Hamilton, Ontario, Canada
> web: socserv.mcmaster.ca/jfox
>
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