Ivan Krylov [krylov.r...@gmail.com] said:
> Instead you can either close over X:
>
> X <- c(...)
> holly <- function(p) (p$a * X^2) / (p:b^2 + X^2)
> # function holly now "contains" the vector X
That would not be an accurate statement as written.
The function only contains an unevaluated call referencing X; not the vector X.
If X is not defined inside the function or its arguments, scoping rules take
over and R goes looking in the function's environment, using the first thing
called "X" that it finds.
So
X<-1:5
h <- function(p=2) p*X
h()
# works, but
X <- pi
h()
# Not the same answer. If the function 'contained' the vector, the result would
be 2*(1:5) as above.
# This is why it's not wise to rely on scoping rules in functions, unless you
_completely_ control the environment.
# and
rm(X)
h()
# returns an error because X is no longer defined, in the function or out of
it, even though X was defined at the time h() was defined.
BUT
X <- pi/2
fh <- function(p=2) {
X <- 7
h(p)
}
fh()
# returns pi and not 14 because h() was bound to the global environment on
creation and still is when R finds it on evaluating h() in the fh() function
body.
Moral: if you want to be safe, pass it as an argument.
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