"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.de...@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 <ericjber...@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 <ch...@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@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@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@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@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.
