On 25/06/2013 9:32 AM, Dan Murphy wrote:
I am having difficulty understanding the envir argument of do.call.
The help page says
envir an environment within which to evaluate the call.
so I thought that in the following toy example x would be found in the
environment e and f would return 4 via do.call:
> e <- new.env()
> e$x <- 2
> f <- function() x^2
> do.call(f, list(), envir = e)
Error in (function () : object 'x' not found
Thanks in advance for clarifying my misunderstanding.
do.call will construct the expression f(), then evaluate it in e. It
will try to look up f there, and not finding it, will go to the parent
environment and find it.
When evaluating the function, the environment in which it was evaluated
is used for looking up arguments, but f() has none, so e is not used at
all. R will use the environment attached to f, which is the global
environment, since you created f by evaluating its definition there.
To get what you want, you could use the sequence
e <- new.env()
e$x <- 2
f <- function() x^2
environment(f) <- e
f()
An alternative way to do the 3rd and 4th lines is
f <- with(e, function() x^2)
because that would evaluate the creation of f within e.
A third approach (which might be the nicest one, depending on what else
you are doing) is never to name e:
f <- local({
x <- 2
function() x^2
})
Duncan Murdoch
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