[R] need help in if else condition

[Richard O'Keefe]

"ifelse is very slow"?  Benchmark time.
> x <- runif(1000000)
> y <- runif(1000000)
> system.time(ifelse(x < y, x, y))
   user  system elapsed
  0.403   0.044   0.448
> system.time(y + (x < y)*(x - y))
   user  system elapsed
  0.026   0.012   0.038

This appears to be a quality-of-implementation bug.


On Thu, 11 Jul 2019 at 04:14, Dénes Tóth <toth.denes using kogentum.hu> wrote:

>
>
> On 7/10/19 5:54 PM, Richard O'Keefe wrote:
> > Expectation: ifelse will use the same "repeat vectors to match the
> longest"
> > rule that other vectorised functions do.  So
> > a <- 1:5
> > b <- c(2,3)
> > ifelse(a < 3, 1, b)
> > => ifelse(T T F F F <<5>>, 1 <<1>>, 2 3 <<2>>)
> > => ifelse(T T F F F <<5>>, 1 1 1 1 1 <<5>>, 2 3 2 3 2 <<5>>)
> > => 1 1 2 3 2
> > and that is indeed the answer you get.  Entirely predictable and
> consistent
> > with
> > other basic operations in R.
> >
> > The only tricky thing I see is that R has
> > a strict vectorised  ifelse(logical.vector, some.vector, another.vector)
> > AND
> > a non-strict non-vectorised if (logical.scalar) some.value else
> > another.value
> > AND
> > a statement form if (logical.scalar) stmt.1; else stmt.2;
>
> Just for the records, there is a further form:
> `if`(logical.scalar, stmt.1, stmt.2)
>
> The main problem with ifelse is that 1) it is very slow, and 2) the mode
> of its return value can be unintuitive or not too predictable (see also
> the Value and Warning sections of ?ifelse). One has to be very careful
> and ensure that 'yes' and 'no' vectors have the same class, because
> ifelse will not warn you at all:
>  > ifelse(c(TRUE, TRUE), 1:2, LETTERS[1:2])
> [1] 1 2
>  > ifelse(c(TRUE, FALSE), 1:2, LETTERS[1:2])
> [1] "1" "B"
>
> For options instead of base::ifelse, you might find this discussion
> helpful:
> https://github.com/Rdatatable/data.table/issues/3657
>
>
> Cheers,
> Denes
>
>
> >
> >
> > On Thu, 11 Jul 2019 at 01:47, Eric Berger <ericjberger using gmail.com> wrote:
> >
> >> For example, can you predict what the following code will do?
> >>> a <- 1:5
> >>> b <- c(2,3)
> >>> ifelse( a < 3, 1, b)
> >>
> >>
> >> On Wed, Jul 10, 2019 at 4:34 PM José María Mateos <chema using rinzewind.org>
> >> wrote:
> >>
> >>> On Wed, Jul 10, 2019, at 04:39, Eric Berger wrote:
> >>>> 1. The ifelse() command is a bit tricky in R. Avoiding it is often a
> >> good
> >>>> policy.
> >>>
> >>> You piqued my curiosity, can you elaborate a bit more on this?
> >>>
> >>> --
> >>> José María (Chema) Mateos || https://rinzewind.org
> >>>
> >>> ______________________________________________
> >>> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> >>> 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.
> >>>
> >>
> >>          [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> >> 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.
> >>
> >
> >       [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > 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.
> >
>

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