Hi everyone
another documented feature that was a bit unexpected for me:
R> x <- 19
R> f <- function(t){t+x}
R> f(100)
[1] 119
--as expected: x is visible from within f()
..but...
R> g <- function(a){x <- 1e99 ; return(f(a))}
R> g(4)
[1] 23
--the "x" that is visible from within g() is just 19, which is not the
one I expected it to find.
R> rm(x)
R> g(4)
Error in f(a) : Object "x" not found
--g() looks in the first search path place and finds it empty,
returning an error.
QUESTIONS:
Why doesn't g() "keep looking" ?
How do I tell g() where to find x?
Where to look for documentation for this?
--
Robin Hankin, Lecturer,
School of Geography and Environmental Science
Tamaki Campus
Private Bag 92019 Auckland
New Zealand
r.hankin at auckland.ac.nz
tel 0064-9-373-7599 x6820; FAX 0064-9-373-7042
On Tue, Apr 22, 2003 at 04:07:39PM +1200, Robin Hankin wrote:> Hi everyone > > another documented feature that was a bit unexpected for me: > > R> x <- 19 > R> f <- function(t){t+x} > R> f(100) > [1] 119 > > --as expected: x is visible from within f() > ..but... > > > R> g <- function(a){x <- 1e99 ; return(f(a))} > R> g(4) > [1] 23 > > --the "x" that is visible from within g() is just 19, which is not the > one I expected it to find. > > > R> rm(x) > R> g(4) > Error in f(a) : Object "x" not found > > --g() looks in the first search path place and finds it empty, > returning an error. > > > QUESTIONS: > Why doesn't g() "keep looking" ? > How do I tell g() where to find x? > Where to look for documentation for this? >(TENTATIVE) ANSWERS: 1- becase the enclosing frame for f is .GlobalEnv. 2- f <- function(t){t+get("x", envir=parent.frame())} 3- R language definition p.22 is a place to start Hopin' it helps, L.> > > -- > > Robin Hankin, Lecturer, > School of Geography and Environmental Science > Tamaki Campus > Private Bag 92019 Auckland > New Zealand > > r.hankin at auckland.ac.nz > tel 0064-9-373-7599 x6820; FAX 0064-9-373-7042 > > ______________________________________________ > R-help at stat.math.ethz.ch mailing list > https://www.stat.math.ethz.ch/mailman/listinfo/r-help-- -------------------------------------------------------------- currently at the National Yang-Ming University in Taipei, Taiwan -------------------------------------------------------------- Laurent Gautier CBS, Building 208, DTU PhD. Student DK-2800 Lyngby,Denmark tel: +45 45 25 24 89 http://www.cbs.dtu.dk/laurent
Dear Robin, In terms of documentation I have found John Fox's "Frames, environments, and scope in R and S-PLUS" (available at http://socserv.socsci.mcmaster.ca/jfox/Books/companion/appendix-scope.pdf) to be an excellent document. "Lexical scope and statistical computing" by Gentleman & Ihaka (at the J. Comp. Graph. Stat., 2000, 9: 491-508, though I think a pdf of a preprint can be found somewhere in the net) contains more general discussion and more ellaborate examples. Finally, section 10.7 of "An introduction to R" has brief discussion of these issues, but that should answer the specific questions you asked. With regards to your specific situation: x is a "free variable" for f, and f will look for it in (at? I'll never get this right) the environment where it was defined (the global environment in your example). When you call f from g, f still looks for x in the environment where it was defined; first time (before you rm(x)) x is 19 there; the second time, there is no x anymore. This behavior is different from that of S-PLUS. Hope this helps, Ram?n On Tuesday 22 April 2003 06:07, Robin Hankin wrote:> Hi everyone > > another documented feature that was a bit unexpected for me: > > R> x <- 19 > R> f <- function(t){t+x} > R> f(100) > [1] 119 > > --as expected: x is visible from within f() > ..but... > > > R> g <- function(a){x <- 1e99 ; return(f(a))} > R> g(4) > [1] 23 > > --the "x" that is visible from within g() is just 19, which is not the > one I expected it to find. > > > R> rm(x) > R> g(4) > Error in f(a) : Object "x" not found > > --g() looks in the first search path place and finds it empty, > returning an error. > > > QUESTIONS: > Why doesn't g() "keep looking" ? > How do I tell g() where to find x? > Where to look for documentation for this?-- Ram?n D?az-Uriarte Bioinformatics Unit Centro Nacional de Investigaciones Oncol?gicas (CNIO) (Spanish National Cancer Center) Melchor Fern?ndez Almagro, 3 28029 Madrid (Spain) Fax: +-34-91-224-6972 Phone: +-34-91-224-6900 http://bioinfo.cnio.es/~rdiaz
On Tue, 22 Apr 2003, Laurent Gautier wrote:> On Tue, Apr 22, 2003 at 04:07:39PM +1200, Robin Hankin wrote: > > Hi everyone > > > > another documented feature that was a bit unexpected for me: > > > > R> x <- 19 > > R> f <- function(t){t+x} > > R> f(100) > > [1] 119 > > > > --as expected: x is visible from within f() > > ..but... > > > > > > R> g <- function(a){x <- 1e99 ; return(f(a))} > > R> g(4) > > [1] 23 > > > > --the "x" that is visible from within g() is just 19, which is not the > > one I expected it to find. > >Yes. R has lexical, not dynamic, scope. That is, the lookup depends on where a function was defined, not where it was called from. If you need to fake dynamic scope then a) Try not to b) Use eval.parent() -thomas