# [Rd] Surprising behavior of letters[c(NA, NA)]

Gabor Grothendieck ggrothendieck at gmail.com
Fri Dec 17 16:18:01 CET 2010

```On Fri, Dec 17, 2010 at 9:58 AM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:
> On 17/12/2010 9:32 AM, Gabor Grothendieck wrote:
>>
>> Consider this:
>>
>> >  letters[c(2, 3)]
>> [1] "b" "c"
>> >  letters[c(2, NA)]
>> [1] "b" NA
>> >  letters[c(NA, 3)]
>> [1] NA  "c"
>> >  letters[c(NA, NA)]
>>  [1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
>> NA NA
>> [26] NA
>>
>> The result is a 2-vector in each case until we get to c(NA, NA) and
>> then it unexpectedly changes from returning a 2-vector to returning a
>> 26-vector.  I think most people would have expected that the answer
>> would be c(NA, NA).
>>
>
> This is because  c(NA, NA) is a logical vector, so it gets recycled to the
> length of letters, whereas c(NA, 3) and the others are numeric vectors, so
> they aren't recycled, they're converted to integer indices.  So the surprise
> is due to not recognizing that NA is logical.  You wouldn't expect a length
> 1 result from letters[TRUE], would you?

One tends not to distinguish between logical NA's and integer NA's.
In fact R represents both of them as NA on output so this does  seem
highly error prone.

> NA # logical
[1] NA
> NA_integer_ # integer
[1] NA

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```