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

[Avi Gross]

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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