[R] Subsetting a vector using an index with all missing values

Sun Jul 3 01:22:27 CEST 2022

Reread Bill's comment. It referred to logical indices being **longer** than
the vectors they subscript. You have it the other way round.

Bert

On Sun, Jul 3, 2022, 12:40 AM Avi Gross via R-help <r-help using r-project.org>
wrote:

> Actually, Bill, I suspect there is a not uncommon use when you want a
> briefer logical vector to be broadcast or re-used as often as needed.
>
> The example below, if it can be read, uses an abbreviated set of boolean
> vectors to get the odd elements of another vector, then the even ones, and
> then the ones not divisible by three because it gets recycled. I suggest
> there are many variants like this in use, albeit a better design for some
> things might have been to add a flag to allow such expansion and otherwise
> consider it an error.
>
> > test <- 11:20
>
> > test[c(TRUE, FALSE)]
>
> [1] 11 13 15 17 19
>
> > test[c(FALSE, TRUE)]
>
> [1] 12 14 16 18 20
>
> > test[c(FALSE, TRUE, TRUE)]
>
> [1] 12 13 15 16 18 19
>
>
>
> -----Original Message-----
> From: Bill Dunlap <williamwdunlap using gmail.com>
> To: Ebert,Timothy Aaron <tebert using ufl.edu>
> Cc: r-help <r-help using r-project.org>
> Sent: Sat, Jul 2, 2022 11:24 am
> Subject: Re: [R] Subsetting a vector using an index with all missing values
>
> Perhaps it should be an error if the length of a logical subscript is
> bigger than the dimension it is subscripting.  Currently in that case, x is
> extended (with NA or NULL) to the length of the logical subscript.  I doubt
> this is desired very often.
>
> > dput((1:3)[c(FALSE,FALSE,FALSE,TRUE,TRUE)])
> c(NA_integer_, NA_integer_)
> > dput((1:3)[c(FALSE,FALSE,TRUE,FALSE,TRUE)])
> c(3L, NA)
>
> -Bill
>
> On Sat, Jul 2, 2022 at 7:49 AM Ebert,Timothy Aaron <tebert using ufl.edu> wrote:
>
> > That nicely explains the difference in outcome between
> > x[rep(TRUE,3)]
> > x[rep("TRUE",3)]
> >
> >
> > I do not quite get it.
> > x<-1:10
> > x[rep(x<2,3)]
> > [1] 1 NA NA
> > The length is three
> >
> > but
> > x[rep(x>2,3)]
> > [1] 3  4  5  6  7  8  9  10  NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
> > NA
> > The length is 24
> >
> > Tim
> > -----Original Message-----
> > From: R-help <r-help-bounces using r-project.org> On Behalf Of Peter
> Langfelder
> > Sent: Saturday, July 2, 2022 2:19 AM
> > To: Bill Dunlap <williamwdunlap using gmail.com>
> > Cc: r-help <r-help using r-project.org>
> > Subject: Re: [R] Subsetting a vector using an index with all missing
> values
> >
> > [External Email]
> >
> > Ah, thanks, that makes sense.
> >
> > Peter
> >
> > On Fri, Jul 1, 2022 at 10:01 PM Bill Dunlap <williamwdunlap using gmail.com>
> > wrote:
> > >
> > > This has to do with the mode of the subscript - logical subscripts are
> > > repeated to the length of x and integer/numeric ones are not.  NA is
> > > logical, NA_integer_ is integer, so we get
> > >
> > > > x <- 1:10
> > > > x[ rep(NA_integer_, 3) ]
> > > [1] NA NA NA
> > > > x[ rep(NA, 3) ]
> > >  [1] NA NA NA NA NA NA NA NA NA NA
> > >
> > > -Bill
> > >
> > >
> > > On Fri, Jul 1, 2022 at 8:31 PM Peter Langfelder <
> > peter.langfelder using gmail.com> wrote:
> > >>
> > >> Hi all,
> > >>
> > >> I stumbled on subsetting behavior that seems counterintuitive and
> > >> perhaps is a bug. Here's a simple example:
> > >>
> > >> > x = 1:10
> > >> > x[ rep(NA, 3)]
> > >>  [1] NA NA NA NA NA NA NA NA NA NA
> > >>
> > >> I would have expected 3 NAs (the length of the index), not 10 (all
> > >> values in x). Looked at the documentation for the subsetting operator
> > >> `[` but found nothing indicating that if the index contains all
> > >> missing data, the result is the entire vector.
> > >>
> > >> I can work around the issue for a general 'index' using a somewhat
> > >> clunky but straightforward construct along the lines of
> > >>
> > >> > index = rep(NA, 3)
> > >> > x[c(1, index)][-1]
> > >> [1] NA NA NA
> > >>
> > >> but I'm wondering if the behaviour above is intended.
> > >>
> > >> Thanks,
> > >>
> > >> Peter
> > >>
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> ______________________________________________
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