I'm still going over old emails and trying to get my head around
evaluation so I'm persistent if nothing else.
A while back , an expert sent me below as an exercise in understanding
and I only got around to it tonight. I understand some of the output but
not all of it and I put "Why not Zero ?" next to the ones that I don't
understand based on my reading of the various functions in the help
pages. It's either my reading comprehension or the evaluation subtleties
in R but I just can't understand some of them. If any of the expeRts has
time to explain the ones that I marked with "WHY NOT ZERO ?", it would
be much appreciated. Obviously, I don't expect a long explanation but I
think my problem is that I keep thinking that eval.parent and
eval(whatever, parent.frame) go back to the function that called
with.options so f() and do the evaluation in there but that doesn't
always seem to be the case. I'm also not so clear on the difference
between print(x) and L[[len]]. Thanks a lot in advance to anyone who can
be bothered with below.
with.options <- function(...) {
L <- as.list(match.call())[-1]
len <- length(L)
print(L)
eval.parent(L[[len]]) # =0 MAKES SENSE
eval(L[[len]]) # =1 MAKES SENSE
eval(L[[len]],parent.frame()) # =0 MAKES SENSE
eval.parent(print(x)) # =1 WHY NOT ZERO ? Somehow this is
different from eval.parent(L[[len]])
eval(print(x)) # =1 MAKES SENSE
eval(print(x),parent.frame()) # =1 # WHY NOT ZERO ? Somehow this is
different from eval(L[[len]],parent.frame)
evalq(print(x)) # =1 MAKES SENSE
evalq(print(x),parent.frame()) # =1 MAKES SENSE
print("====================")
x <- 2
eval.parent(L[[len]]) # =0 MAKES SENSE
eval(L[[len]]) # =2 MAKES SENSE
eval(L[[len]],parent.frame()) # =0 MAKES SENSE
eval.parent(print(x)) # =2 WHY NOT ZERO ? Somehow this is different
from eval.parent(L[[len]])
eval(print(x)) # 2 MAKES SENSE
eval(print(x),parent.frame()) # 2 WHY NOT ZERO ? Somehow this is
different from eval(L[[len]], parent.frame)
evalq(print(x)) # 2 MAKES SENSE
evalq(print(x),parent.frame()) # 1 WHY NOT ZERO ?
print("====================")
}
x <- 1
f <- function() {
x <- 0
with.options(width = 40, print(x))
}
f()
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