[Rd] ifelse

[Prof Brian Ripley]

Another way of looking at this is that ifelse(TRUE, character(0), "") asks 
for a length-one result containing the first element of yes=character(0), 
and character(0)[1] is NA_character_

I suspect if(TRUE) character(0) else "" was intended.

On Mon, 18 Aug 2008, Marc Schwartz wrote:

> on 08/18/2008 06:35 PM Heikki Kaskelma wrote:
>> I find it slightly surprising, that
>>
>> ifelse(TRUE, character(0), "")
>>
>> returns NA instead of character(0).
>>
>> [WNT 2.6.2 Patched]
>
> Time to upgrade...  :-)
>
>
> The same behavior is in:
>
>  R version 2.7.2 beta (2008-08-16 r46368)
>
>
> The reason for this is that the internal code for ifelse() has:
>
>    if (any(test[!nas]))
>        ans[test & !nas] <- rep(yes, length.out = length(ans))[test &
>            !nas]
>
> where the key code is:
>
>  rep(yes, length.out = length(ans))
>
> In this case, 'yes' is character(0) and 'ans' is TRUE.
>
> Thus, you get:
>
>  > rep(character(0), length.out = length(TRUE))
>  [1] NA
>
>
> This behavior is documented in ?rep, where in the Details section you
> will find:
>
> "If x has length zero and length.out is supplied and is positive, the
> values are filled in using the extraction rules, that is by an NA of the
> appropriate class for an atomic vector (0 for raw vectors) and NULL for
> a list. "
>
> Thus:
>
>> class(rep(character(0), length.out = length(TRUE)))
> [1] "character"
>
>
> which shows that the NA that is returned is of class character, which is
> the class for character(0).
>
> HTH,
>
> Marc Schwartz
>
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>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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