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]]
The scope of variables within show.a is not affected by the environment
that show.a is called in. The scope of the variables in show.a
is determined by the lexical scope of show.a although you can change
this via:
environment(show.a) <- my.env
show.a() # 200
If you want to create a function that has dynamic (i.e. scope is the caller),
rather than lexical scope, do this:
show2.a <- function() {
show2.a <- function() a
environment(show2.a) <- parent.frame()
show2.a()
}
evalq(show2.a(), my.env) # 200
or you can create a function which evaluates its body in the parent.frame:
show3.a <- function() eval.parent(substitute(a))
evalq(show3.a(), my.env) # 200
Also, depending on what you actually want to do, the proto package
may be applicable.
On 5/13/05, Whit Armstrong <whit at twinfieldscapital.com>
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]]
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>
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]] > > ______________________________________________ > R-help at stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html >-- 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