Re: [R] odd feature

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Mon 22 May 2006 - 23:57:30 EST

Due to lazy evaluation, I don't think a and b are fully evaluated:

> ifelse(1, a <- 1, b <- 2)

[1] 1
> a

[1] 1
> b

Error: object "b" not found

On 5/22/06, Martin Maechler <maechler@stat.math.ethz.ch> wrote:
> >>>>> "Gabor" == Gabor Grothendieck <ggrothendieck@gmail.com>
> >>>>> on Mon, 22 May 2006 09:31:14 -0400 writes:
>
> Gabor> If you don't like f(if (temp) a else b)
> Gabor> then what about
>
> Gabor> temp <- if (test) a else b
> Gabor> f(temp)
>
> Gabor> or
>
> Gabor> temp <- if (test)
> Gabor> a
> Gabor> else
> Gabor> b
> Gabor> f(temp)
>
> Gabor> I think its easier to understand if you factor the temp<- out since
> Gabor> one immediately then knows the purpose of the statement is
> Gabor> to set temp.
>
> I strongly agree with Gabor on the above.
>
> But, to Duncan's question:
> Yes, indeed, my main point was that people use
> ifelse(test, a, b) also in cases where test is known to be of
> length one.
>
> BTW, the 2nd point about why I don't ``like'' ifelse() so much
> is on its help page:
>
> Both 'a' and 'b' are fully evaluated even though only one of the
> two values of a[i], b[i] are used in the result.
>
> Martin
>
> Gabor> On 5/22/06, Duncan Murdoch <murdoch@stats.uwo.ca> wrote:
> >> On 5/22/2006 3:26 AM, Martin Maechler wrote:
> >> >>>>>> "Gabor" == Gabor Grothendieck <ggrothendieck@gmail.com>
> >> >>>>>> on Sun, 21 May 2006 09:47:07 -0400 writes:
> >> >
> >> > Gabor> If you know that test is a scalar
> >> >
> >> > Gabor> result <- if (test) a else b
> >> >
> >> > Gabor> will do it.
> >> >
> >> > Yes, indeed!
> >> > IMO, ifelse(test, a, b) is much overused where as
> >> > if(test) a else b is much UNDER used.
> >> >
> >> >>From some e-mail postings, and even some documents (even printed
> >> > books?), I get the impression that too many people think that
> >> > ifelse(.,.,.) is to be used as expression / function and
> >> > if(.) . else . only for "program flow control".
> >> > This leads to quite suboptimal code, and I personally use
> >> > if(.) . else . __as expression__ much more frequently than ifelse(.,.,.)
> >>
> >> For overuse of ifelse(), do you mean cases where test is length 1, so
> >> if() would work? Or are you thinking of something else?
> >>
> >> I'd also be interested in what you mean by "quite suboptimal" code. Are
> >> you thinking of things like
> >>
> >> if (test)
> >> temp <- a
> >> else
> >> temp <- b
> >> result <- f(temp)
> >>
> >> versus
> >>
> >> result <- f( if (test) a else b )
> >>
> >> ?
> >>
> >> I would generally use the former, because it's easier to get the
> >> formatting right, and I find it easier to read. It's suboptimal in
> >> speed and memory use because of creating the temp variable, but in most
> >> cases I think that would be such a small difference that the small
> >> increase in readability is worthwhile.
> >>
> >> Duncan Murdoch
> >>
> >> >
> >> > Martin Maechler, ETH Zurich.
> >> >
> >> > Gabor> Here is another approach:
> >> >
> >> > Gabor> as.vector(test * ts(a) + (!test) * ts(b))
> >> >
> >> >
> >> >
> >> > Gabor> On 5/21/06, ivo welch <ivowel@gmail.com> wrote:
> >> > >> Dear R wizards:
> >> > >>
> >> > >> I just got stung by the ifelse() feature.
> >> > >>
> >> > >> > a <- 10:15
> >> > >> > b <- 20:300
> >> > >> > test <- 1
> >> > >> > ifelse(test,a,b)
> >> > >> [1] 10
> >> > >>
> >> > >> I had not realized that this was the default behavior---I had expected
> >> > >> 10:15. mea culpa. however, I wonder whether it would make sense to
> >> > >> replace ifelse with a different semantic, where if test is a single
> >> > >> scalar, it means what a stupid user like me would imagine.
> >> > >>
> >> > >> Aside, I like the flexibility of R, but I am not thrilled by all the
> >> > >> recycling rules. I either mean I want a scalar or a vector of
> >> > >> equal/appropriate dimension. I never want a recycle of a smaller
> >> > >> vector. (I do often use a recycle of a scalar.)
> >> > >>
> >> > >> regards,
> >> > >>
> >> > >> /iaw
> >> > >>
> >> > >> ______________________________________________
> >> > >> R-help@stat.math.ethz.ch mailing list
> >> > >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> > >> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
> >> > >>
> >> >
> >> > Gabor> ______________________________________________
> >> > Gabor> R-help@stat.math.ethz.ch mailing list
> >> > Gabor> https://stat.ethz.ch/mailman/listinfo/r-help
> >> > Gabor> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
> >> >
> >> > ______________________________________________
> >> > R-help@stat.math.ethz.ch mailing list
> >> > https://stat.ethz.ch/mailman/listinfo/r-help
> >> > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
> >>
> >> ______________________________________________
> >> R-help@stat.math.ethz.ch mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
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> >>
>



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