[R] scoping issues (was help with eval)

[Prof Brian Ripley]

You intended quote() rather than expression(), I believe.  If I do

> show.a <- function(a) {a}
> eval(quote(show.a(a)),envir=my.env)

this works.

R has lexical scoping, so with

> show.a <- function() {a}

'a' is looked for in the frame of the function (not defined there) and 
then in the environment of the function.

> environment(show.a)
<environment: R_GlobalEnv>

since show.a was defined in the workspace.  The envir arg of eval() is 
used to find the object(s), here `show.a', not to sets its environment.
So my first version works because 'a' is part of the expression.

Perhaps you intended something like

init.env <- function() {
  a <- 200
  show.a <- function() {a}
  environment()
}
my.env <- init.env()
eval(quote(show.a()),envir=my.env)

which works, and is a common R idiom (although via local() or new.env()).

However, you could just do

> eval(quote(a), envir=my.env)
[1] 200
> eval(expression(a), envir=my.env)
[1] 200

BTW, use typeof() to see the difference here.

On Fri, 13 May 2005, Whit Armstrong wrote:

> I've been looking at the help page for eval for a while, but I can't
> make sense of why this example does not work.
>
> show.a <- function() {
>  a
> }
>
> init.env <- function() {
>  a <- 200
>  environment()
> }
>
> my.env <- init.env()
>
> ls(envir=my.env)
>
> # returns this:
> # > ls(envir=my.env)
> # [1] "a"
>
>
> # but this does not work:
> eval(expression(show.a()),envir=my.env)
>
> # > eval(expression(show.a()),envir=my.env)
> # Error in show.a() : Object "a" not found
> # >
>
>
> The help page gives the following:
>
>   'eval' evaluates the expression 'expr' argument in the environment
>     specified by 'envir' and returns the computed value. If 'envir' is
>     not specified, then 'sys.frame(sys.parent())', the environment
>     where the call to 'eval' was made is used.
>
> I would be grateful for any help.
>
> Thanks,
> Whit
>
>
> 	[[alternative HTML version deleted]]
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595