[R] group consecutive dates in a row

[Bert Gunter]

Here is another way to obtain the day differences that is the argument
of rle() . It is perhaps more reliable in that it uses methods for
class POSIXct rather than depending on the underlying class structure
and conversion via as.numeric. In theory, the methods won't change or
any changes will be documented, whereas class implementations are
allowed to be fluid and undocumented at the R user level (e.g. the
Help system) afaik. In this case, I don't think that would happen, so
I am merely being pedantic, but I hope I do not offend by making the
point.

But I also wanted to say that the OP could have (imo) looked up the
methodology as I describe below, rather than post here. Or perhaps he
did, but got stymied because he did not know about rle(), a somewhat
esoteric base R function. In which case, the search query "how to find
runs of identical values in an R vector" immediately yielded a hit on
rle(). So my message is: **DO** first try to use R's internal Help
before posting, especially for base R related tasks: it is really a
superb resource (again imo). *

OK, enough sermonizing. Here's what I did. Since the data are POSIXct
class, I went to ?POSIXct and browsed through it until I found
"difftime for time intervals" in the *See Also* section. Following
that link to ?difftime showed me this was what was needed, which is:

difftime(mydf[-1,1], mydf[-nrow(mydf), 1], units = "days")
Time differences in days
[1] 1 1 6 8

Cheers,
Bert

*... and I think it is likely that there are time series related
packages that also could be used, perhaps more immediately, to do what
was requested. But that would require a more diligent search. Though
with new LLM's and generative AI for Help systems becoming available,
the degree of diligence required is rapidly decreasing.


On Mon, Aug 7, 2023 at 9:52 AM Ben Bolker <bbolker using gmail.com> wrote:
>
> rle(as.numeric(diff(mydf$data_POSIX)))  should get you started, I think?
>
> On 2023-08-07 12:41 p.m., Stefano Sofia wrote:
> > Dear R users,
> >
> > I have a data frame with a single column of POSIXct elements, like
> >
> >
> > mydf <- data.frame(data_POSIX=as.POSIXct(c("2012-02-05", "2012-02-06", "2012-02-07", "2012-02-13", "2012-02-21"), format = "%Y-%m-%d", tz="Etc/GMT-1"))
> >
> >
> > I need to transform it in a two-columns data frame where I can get rid of consecutive dates. It should appear like
> >
> >
> > data_POSIX_init data_POSIX_fin
> >
> > 2012-02-05 2012-02-07
> >
> > 2012-02-13 NA
> >
> > 2012-02-21 NA
> >
> >
> > I started with two "while cycles" and so on, but this is not an efficient way to do it.
> >
> > Could you please give me an hint on how to proceed?
> >
> >
> > Thank you for your precious attention and help
> >
> > Stefano
> >
> >
> >           (oo)
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