[R] NA values in indexing

[Gabor Grothendieck]

Try

> x <- 101:107
> x[c(NA_integer_, NA_integer_)]
[1] NA NA

On Fri, Mar 26, 2010 at 8:09 AM, Barry Rowlingson
<b.rowlingson at lancaster.ac.uk> wrote:
> If you index a vector with a vector that has NA in it, you get NA back:
>
>  > x=101:107
>  > x[c(NA,4,NA)]
>  [1]  NA 104  NA
>  > x[c(4,NA)]
>  [1] 104  NA
>
> All well and good. ?"[" says, under NAs in indexing:
>
>     When extracting, a numerical, logical or character ‘NA’ index
>     picks an unknown element and so returns ‘NA’ in the corresponding
>     element of a logical, integer, numeric, complex or character
>     result, and ‘NULL’ for a list.  (It returns ‘00’ for a raw
>     result.]
>
> But if the indexing vector is all NA, you get back a vector of length
> of your original vector rather than of your index vector:
>
>  > x[c(NA,NA)]
>  [1] NA NA NA NA NA NA NA
>
> Maybe it's just me, but I find this surprising, and I can't see it
> documented. Bug or undocumented feature? Apologies if I've missed
> something obvious.
>
> Barry
>
>  sessionInfo()
> R version 2.11.0 alpha (2010-03-25 r51407)
> i686-pc-linux-gnu
>
> locale:
>  [1] LC_CTYPE=en_GB.UTF-8       LC_NUMERIC=C
>  [3] LC_TIME=en_GB.UTF-8        LC_COLLATE=en_GB.UTF-8
>  [5] LC_MONETARY=C              LC_MESSAGES=en_GB.UTF-8
>  [7] LC_PAPER=en_GB.UTF-8       LC_NAME=C
>  [9] LC_ADDRESS=C               LC_TELEPHONE=C
> [11] LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
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