lexical scope
Dear Robin,
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.
A couple of people have pointed out that the source of the difficulty here
is lexical scoping. One can sometimes take advantage of lexical scoping by
defining a local function. For your illustration, for example,
> x <- 19
> g <- function(a){
+ f <- function(t){t + x}
+ x <- 1e99
+ f(a)
+ }
> g(4)
[1] 1e+99
>
Whether or not this solves your problem depends upon whether it's
reasonable to make f local to g.
John
-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox