[R] inefficient ifelse() ?

[William Dunlap]

An ifelse-like function that only evaluated
what was needed would be fine, but it would
have to be different from ifelse itself.  The
trick is to come up with a good parameterization.

E.g., how would it deal with things like
   ifelse(is.na(x), mean(x, na.rm=TRUE), x)
or
   ifelse(x>1, log(x), runif(length(x),-1,0)) 
or
   ifelse(x>1, log(x), -seq_along(x))
Would it reject such things?  Deciding that the
x in mean(x,na.rm=TRUE) should be replaced by
x[is.na(x)] would be wrong.  Deciding that
runif(length(x)) should be replaced by runif(sum(x>1))
seems a bit much to expect.  Replacing seq_along(x) with
seq_len(sum(x>1)) is wrong.  It would be better to
parameterize the new function so it wouldn't have to
think about those cases.

Would you want it to depend only on a logical
vector or perhaps also on a factor (a vectorized
switch/case function)?

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com  

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of ivo welch
> Sent: Tuesday, March 01, 2011 12:36 PM
> To: Henrique Dallazuanna
> Cc: r-help
> Subject: Re: [R] inefficient ifelse() ?
> 
> thanks, Henrique.  did you mean
> 
>     as.vector(t(mapply(function(x, f)f(x), split(t, ((t %% 2)==0)),
> list(f, g))))   ?
> 
> otherwise, you get a matrix.
> 
> its a good solution, but unfortunately I don't think this can be used
> to redefine ifelse(cond,ift,iff) in a way that is transparent.  the
> ift and iff functions will always be evaluated before the function
> call happens, even with lazy evaluation.  :-(
> 
> I still think that it makes sense to have a smarter vectorized %if% in
> a vectorized language like R.  just my 5 cents.
> 
> /iaw
> 
> ----
> Ivo Welch (ivo.welch at brown.edu, ivo.welch at gmail.com)
> 
> 
> 
> 
> 
> On Tue, Mar 1, 2011 at 2:33 PM, Henrique Dallazuanna 
> <wwwhsd at gmail.com> wrote:
> > Try this:
> >
> > mapply(function(x, f)f(x), split(t, t %% 2), list(g, f))
> >
> > On Tue, Mar 1, 2011 at 4:19 PM, ivo welch <ivowel at gmail.com> wrote:
> >>
> >> dear R experts---
> >>
> >>  t <- 1:30
> >>  f <- function(t) { cat("f for", t, "\n"); return(2*t) }
> >>  g <- function(t) { cat("g for", t, "\n"); return(3*t) }
> >>  s <- ifelse( t%%2==0, g(t), f(t))
> >>
> >> shows that the ifelse function actually evaluates both f() 
> and g() for
> >> all values first, and presumably then just picks left or 
> right results
> >> based on t%%2.  uggh... wouldn't it make more sense to 
> evaluate only
> >> the relevant parts of each vector and then reassemble them?
> >>
> >> /iaw
> >> ----
> >> Ivo Welch
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> >> http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >
> >
> >
> > --
> > Henrique Dallazuanna
> > Curitiba-Paraná-Brasil
> > 25° 25' 40" S 49° 16' 22" O
> >
> 
> ______________________________________________
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