Message-ID: <5.1.0.14.2.20030422102545.01e2cb18@mcmail.cis.mcmaster.ca>
Date: 2003-04-22T14:32:35Z
From: John Fox
Subject: lexical scope
In-Reply-To: <Pine.A41.4.44.0304220643010.95464-100000@homer06.u.washing ton.edu>
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