[R] inefficient ifelse() ?

[ivo welch]

yikes.  you are asking me too much.

thanks everybody for the information.  I learned something new.

my suggestion would be for the much smarter language designers (than
I) to offer us more or less blissfully ignorant users another
vector-related construct in R.  It could perhaps be named %if% %else%,
analogous to if else (with naming inspired by %in%, and with
evaluation only of relevant parts [just as if else for scalars]), with
different outcomes in some cases, but with the advantage of typically
evaluating only half as many conditions as the ifelse() vector
construct.  %if% %else% may work only in a subset of cases, but when
it does work, it would be nice to have.  it would probably be my first
"goto" function, with ifelse() use only as a fallback.

of course, I now know how to fix my specific issue.  I was just
surprised that my first choice, ifelse(), was not as optimized as I
had thought.

best,

/iaw


On Tue, Mar 1, 2011 at 5:13 PM, William Dunlap <wdunlap at tibco.com> wrote:
> 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
>> >
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
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>> PLEASE do read the posting guide
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>> and provide commented, minimal, self-contained, reproducible code.
>>
>