[R] understanding lexical scope

[Thomas Lumley]

On Thu, 18 Dec 2008 joseph.g.boyer at gsk.com wrote:

> I am trying to understand the concept of lexical scope in "An Introduction
> to R" by the R Core development team.
>
> I'd appreciate it if someone would explain why the following example does
> not work:
>
> q <- function(y) {x + y}; w <- function(x){q(x)}; w(2);
>
> According to the discussion of Scope on page 46, it seems to me that R
> will interpret the free variable x in q as the parameter x in w, and so
> will
> give w(2) = 2+2.
>

No, not at all.   The function q() is not defined inside w(), it is defined in the global environment. Inside q(), x is first looked up as a local variable, without success, and then looked up in the environment where q() was defined (the global environment), also without success.

There is an x in the calling environment of q(), ie, inside w(), but finding things in the calling environment is dynamic scope rather than lexical scope.

       -thomas



Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle