Duncan Murdoch wrote:
Liaw, Andy wrote:
A colleague and I were trying to understand all the possible things one
can do with for loops in R, and found some surprises. I think we've
done sufficient detective work to have a good guess as to what's going
on "underneath", but it would be nice to get some confirmation, and
better yet, perhaps documentation in the R-lang manual. Basically, the
question is, how/what does R do with the loop index variable? Below are
some examples:
I think it is documented in the ?Control topic that a copy of the seq
argument (the 1:2 in your first example) is made at the beginning, and
that
altering var (your i) doesn't affect the loop.
almost true, in that "copy of the seq" does not necessarily mean a deep
copy, though, and you *can* affect subsequent iterations by altering
var, as in this trivialized example:
e = new.env()
e$a = 'a'
seq = list(e, e)
for (var in seq)
var[[var$a]] = 'b'
as.list(e)
# list(a = 'b', b = 'b')
vQ
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