[R] Compressing a sequence

[Bert Gunter]

Well, as I predicted, my initial suggestions were, ... ummm, rather dumb.
Also, Rui's suggestions are probably preferable to the below. However, it
*is* a very simple, bare-boned approach to converting a sequence of
increasing integers to a character representation using interval notation.
The code is *very* simple-minded and needs no explanation, I think. You
should also be able to easily tweak it to change the output format from my
paste0(..., collapse) choice to something else.

compr <- function(x, sep ="-", collapse = ", ")
{
   left<- c(TRUE, diff(x) != 1)
   right <- c(rev(diff(rev(x)) != -1 ), TRUE)
   xleft <- x[left]
   xright <- x[right]
   ifelse(xleft == xright,
            xleft,
            paste(xleft, xright, sep = sep)) |>
       paste0(collapse = collapse)
}

which yields the following with your example

> compr(c( 1, 3, 4, 5, 7, 8, 12, 13, 14, 15, 20))
[1] "1, 3-5, 7-8, 12-15, 20"   ##Note: A single character string

(Do let me know if this can fail)

Cheers,
Bert

"An educated person is one who can entertain new ideas, entertain others,
and entertain herself."



On Sat, Feb 22, 2025 at 4:36 AM Rui Barradas <ruipbarradas using sapo.pt> wrote:

> Às 00:46 de 22/02/2025, Dennis Fisher escreveu:
> > R 4.4.0
> > OS X
> >
> > Colleagues
> >
> > I have a sequence like:
> >       1, 3, 4, 5, 7, 8, 12, 13, 14, 15, 20
> >
> > I would like to display it as:
> >       1, 3-5, 7-8, 12-15, 20
> >
> > Any simple ways to accomplish this?
> >
> > Dennis
> >
> >
> > Dennis Fisher MD
> > P < (The "P Less Than" Company)
> > Phone / Fax: 1-866-PLessThan (1-866-753-7784)
> > www.PLessThan.com
> >
> >
> >       [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> https://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> Hello,
>
> Here is a way with package R.utils, function seqToIntervals.
>
>
> x <- scan(text = "1, 3, 4, 5, 7, 8, 12, 13, 14, 15, 20", sep = ",")
>
> mat <- R.utils::seqToIntervals(x)
> apply(mat, 1L, \(m) {
>    ifelse(m[1L] == m[2L], m[1L], paste(m, collapse = "-"))
> })
> #> [1] "1"     "3-5"   "7-8"   "12-15" "20"
>
>
> If you want to be fancy, define a special class that prints like that.
>
>
>
> x <- scan(text = "1, 3, 4, 5, 7, 8, 12, 13, 14, 15, 20", sep = ",")
>
> as_seqInterval <- function(x) {
>    old_class <- class(x)
>    class(x) <- c("seqInterval", old_class)
>    x
> }
> print.seqInterval <- function(x, ...) {
>    mat <- R.utils::seqToIntervals(x)
>    out <- apply(mat, 1L, \(m) {
>      ifelse(m[1L] == m[2L], m[1L], paste(m, collapse = "-"))
>    })
>    print(out)
> }
>
> y <- as_seqInterval(x)
> class(y)
> #> [1] "seqInterval" "numeric"
>
> # autoprinting y
> y
> #> [1] "1"     "3-5"   "7-8"   "12-15" "20"
>
> # explicit printing y
> print(y)
> #> [1] "1"     "3-5"   "7-8"   "12-15" "20"
>
>
> Hope this helps,
>
> Rui Barradas
>
>
> --
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>
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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