Re: [Rd] ifelse

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Tue, 19 Aug 2008 08:57:56 +0200

Prof Brian Ripley wrote:
> Another way of looking at this is that ifelse(TRUE, character(0), "")
> asks for a length-one result
Just to rub it in: The length of an ifelse() computation is the length of the "test" argument, so there is no way you should expect character(0) as the result. (The class of the result is also taken from "test", which is arguably one of the lesser convenient features in R.)

> 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).
>

-- 
   O__  ---- Peter Dalgaard             ุster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard_at_biostat.ku.dk)              FAX: (+45) 35327907

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