[R] argh .. if/else .. why?

[David Winsemius]

On Feb 15, 2010, at 11:01 AM, hadley wickham wrote:

>> I, personally, utilize the ifelse(test,statement,statement)  
>> function when possible over the methodology outlined.
>
> if + else and ifelse perform quite different tasks, and in general can
> not (and should not) be exchanged.  In particular, note that for
> ifelse, "the class attribute of the result is taken from ‘test’ and
> may be inappropriate for the values selected from ‘yes’ and  'no’".

I have always been puzzled by that bit of advice/knowledge on the help  
page. "test" will of necessity be of class "logical", and yet I  
regularly succeed in producing numeric and character vectors with  
ifelse. In fact ifelse would be rather limited in utility if it only  
returned logical vectors.

 > test1 <- 1:10 > 5
 > yes1 <- letters[1:10]
 > no1 <- LETTERS[1:10]
 > ifelse(test1, yes1, no1)
  [1] "A" "B" "C" "D" "E" "f" "g" "h" "i" "j"
 > class( ifelse(test1, yes1, no1) )
[1] "character"

Further newbie query: I thought I had read there was also a  
performance "gotcha" in that one of those constructs evaluated both  
the affirmative and negative consequents, but it appears from the help  
page that ifelse does not have that effect. And although that question  
is not addressed in the ?Control help page, testing makes me think it  
is also not true of if() {}else{}:

if (1) {print("x")} else{print("y")}
[1] "x"
-- 
David Winsemius, MD
Heritage Laboratories
West Hartford, CT