[R] Lexical scoping question
Robert Gentleman
rgentlem at jimmy.harvard.edu
Fri Feb 28 18:22:02 CET 2003
Hi,
it is sort of a bug (and sort of not a bug). You are getting bitten
by lazy evaluation. The value of y is not getting evaluated until
the second function is created and returned.
f <- function(y) {
y
g <- function(x) x + y
g
}
will give the behavior you want and I think there is a proposal to
have a function force, force evaluation so that we have
f <- function(y) {
force(y)
g <- function(x) x + y
g
}
and no one is tempted to optimize it away....
Best,
Robert
On Fri, Feb 28, 2003 at 11:40:11AM -0500, Jim Rogers wrote:
> Hello,
>
> Could someone please tell me what I am thinking about incorrectly:
>
> f <- function(y) {
> g <- function(x) x + y
> g
> }
>
> In the following, I get what I expect based on my understanding of
> lexical scoping:
>
> (f(1))(3) # 4
> (f(2))(3) # 5
>
> But now,
>
> fs <- lapply(c(1, 2), f)
> fs[[1]](3) # 5 (Why not 4 ?)
> fs[[2]](3) # 5
>
>
> Checking the environments of these functions, I see that "y" is indeed
> bound to the value 2 in both cases:
>
> es <- lapply(fs, environment)
> ys <- lapply(es, function(env) get("y", env)) # list(2, 2)
>
> ?
>
> Thanks for help,
> Jim Rogers
>
> James A. Rogers, Ph.D. <rogers at cantatapharm.com>
> Statistical Scientist
> Cantata Pharmaceuticals
> 3-G Gill St
> Woburn, MA 01801
> 617.225.9009
> Fax 617.225.9010
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> http://www.stat.math.ethz.ch/mailman/listinfo/r-help
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