[R] Evaluation of defaults in functions

[Peter Dalgaard]

"Gabor Grothendieck" <ggrothendieck at gmail.com> writes:

> foo2 could be written:
> 
>    foo3 <- function(x, y = x) { force(y); x <- 0; y }
> 
> to make it clear that evaluation of y is being forced.  See ?force


And, to answer the specific question: Yes, R has lazy evaluation,
everywhere. Arguments are always evaluated if and when they are
needed. 

 
> On 9/28/06, Ulrich Keller <uhkeller at web.de> wrote:
> > Hello,
> >
> > and sorry if this is already explained somewhere. I couldn't find anything.
> >
> > R (2.3.1, Windows) seems to perform some kind of lazy evaluation when
> > evaluating defaults in function calls that, at least for me, leads to
> > unexpected results. Consider the following, seemingly equivalent functions:
> >
> >  > foo1 <- function(x, y=x) {
> > +   x <- 0
> > +   y
> > + }
> >  > foo1(1)
> > [1] 0
> >  > foo2 <- function(x, y=x) {
> > +   y <- y
> > +   x <- 0
> > +   y
> > + }
> >  > foo2(1)
> > [1] 1
> >
> > Obviously, y is not evaluated until it is used in some way. I would
> > expect it to be evaluated where it is defined. Is this intended behavior?
> > Thanks for clarifying,
> >
> > Uli
> >
> > ______________________________________________
> > R-help at 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
> > and provide commented, minimal, self-contained, reproducible code.
> >
> 
> ______________________________________________
> R-help at 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
> and provide commented, minimal, self-contained, reproducible code.
> 

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