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

[William Dunlap]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Peter Dalgaard
> Sent: Tuesday, February 16, 2010 1:33 AM
> To: Gabor Grothendieck
> Cc: r-help at r-project.org
> Subject: Re: [R] argh .. if/else .. why?
> 
> Gabor Grothendieck wrote:
> > On Mon, Feb 15, 2010 at 11:24 AM, David Winsemius
> > <dwinsemius at comcast.net> wrote:
> >> 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.
> >>
> > 
> > I think it had intended to refer to oldClass rather than class.
> > 
> >> oldClass(TRUE)
> > NULL
> >> oldClass(ifelse(TRUE, 1, 2))
> > NULL
> 
> Well, it does date back to S v.3, but the docs do say class 
> _attribute_
> and "logical" & friends aren't. It happens with all attributes, and I
> suspect that the original intention was for things like this to work
> 
> > ifelse(matrix(c(T,T,F,F),2),1,2)
>      [,1] [,2]
> [1,]    1    2
> [2,]    1    2
> 
> I can't think of a situation where it is actually useful to copy the
> class attribute from the condition to the result. 

The container-related attributes of ifelse's first argument
can be profitably copied to the output.  E.g., for classes
"matrix" and "ts", which can contain a variety of primitive
data types, we get:
  > m<-matrix(1:12,3,4)
  > ifelse(m>5, TRUE, FALSE) # return a matrix the shape of m
        [,1]  [,2] [,3] [,4]
  [1,] FALSE FALSE TRUE TRUE
  [2,] FALSE FALSE TRUE TRUE
  [3,] FALSE  TRUE TRUE TRUE
  > t <- ts(1:5, start=2010, freq=12)
  > ifelse(t>3, TRUE, FALSE) # return a ts like t
         Jan   Feb   Mar   Apr   May
  2010 FALSE FALSE FALSE  TRUE  TRUE

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

> Presumably, the idea
> is that of the three possibilities, only the condition attributes are
> unambiguous (think if(cond, A, B) vs. if(!cond, B, A)), so if 
> any set of
> attributes should be copied, those are the ones.
> 
> -- 
>    O__  ---- Peter Dalgaard             Øster 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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