[R] sorting variable names containing digits
John Fox
jfox at mcmaster.ca
Mon Dec 22 02:33:59 CET 2008
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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