If `inherits' is `FALSE', only the first frame of the specified
environment is inspected. If `inherits' is `TRUE', the search is
continued up through the parent frames until a bound value of the
right mode is found.Should this read "parent environments" instead of "parent frames"? I'm taking this from R 1.7.1.
-roger
Prof Brian Ripley wrote:
On Wed, 8 Oct 2003, Roger D. Peng wrote:
It seems like you want in fnB
get(AA$first, envir = parent.frame(1))
but I'm entirely clear on why your original function doesn't work. My understanding was that get() should search through the parent frames.
Where did you get that idea from? Argument `inherits' to get() defaults to TRUE and is defined as
inherits: should the enclosing frames of the environment be inspected?
Note `enclosing', not `parent'. So the normal R scope rules apply when looking for an object from the frame of fnB, which as its environment (aka enclosing frame) is the workspace (.GlobalEnv) is to look in the local frame and then along the search path.
[This does seem a fairly common misconception, so if you do have any idea in which document it arises it would be good to know.]
Peter Alspach wrote:
Dear List members:
I'm using R1.7.1 (Windows 2000) and having difficulty with scoping. I've studied the FAQ and on-line manuals and think I have identified
the
source of my difficulty, but cannot work out the solution.
For the purposes of illustration. I have three functions as defined below:
fnA <- function(my.x) { list(first=as.character(substitute(my.x)), second=sqrt(my.x)) }
fnB <- function(AA) { tmp.x <- get(AA$first) tmp.x^2 }
fnC <- function() { x <- 1:2 y <- fnA(x) z <- fnB(y) c(x,y,z) }
fnA() has a vector as an argument and returns the name of the vector and the square root of its elements in a list. fn(B) takes the result of fn(A) as its argument, gets the appropriate vector and computes the square of its elements. These work fine when called at the command line.
fnC() defines a local vector x and calls fnA() which operates on this vector. Then fnB() is called, but it operates on a global vector x in GlobalEnv (or returns an error is x doesn't exist there) - but I want it to operate on the local vector.
I am not sure what you really want to do here, but R works best if you pass functions an object to work on, and not the name of an object.
Normally this sort of thing is best avoided, but if it is really needed it is normally simpler to find the object in the calling function (your fnC).
I think this is related to the enclosing environment of all three functions being GlobalEnv (since they were created at the command line), but the parent environment of fnB() being different when invoked from within fnC().
My questions:
1 Have I correctly understood the issue ? 2 How do I make fnB() operate on the local vector rather than the global one ? 3 And, as an aside, I have used as.character(substitute(my.x)) to pass the name - but deparse(substitute(my.x)) also works. Is there any reason to prefer one over the other?
deparse() is preferred. One subtle reason is what happens with very long expressions (and note deparse may give more than one line of output), but the main reason is what happens with expressions such as calls:
fn1 <- function(x) as.character(substitute(x)) fn2 <- function(x) deparse(substitute(x)) fn1(log(x))
[1] "log" "x"
fn2(log(x))
[1] "log(x)"
It is normally dangerous to use get() on the result of deparse(substitute()), as the latter may be the character representation of an expression and not a simple name.
______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help
