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

[Bill Dunlap]

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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