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).
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