On 13-06-27 8:18 AM, Terry Therneau wrote:
I second Ellison sentiments of "almost never". One main reason is readability
on later
viewing.
Yes, as Duncan says global variables can sometimes be handy and make functions
quick to
write, but using a formal argument in the call is always clearer.
I didn't say that. I said that in most cases global variables other
than functions should not be used.
I would generalize that a little: global constants are okay, changing
globals is usually a bad idea.
Duncan Murdoch
Terry Therneau
On 06/27/2013 05:00 AM, r-help-requ...@r-project.org wrote:
On 13-06-26 6:57 AM, S Ellison wrote:
-----Original Message-----
It may be helpful not to worry about the technical details,
just to look at the source code defining the function: if it
is defined in a place where a variable can be seen, it can
see that variable.
I too find R's lexical scoping rules straightforward.
However, I'd say that if your code relies on lexical scoping to find
something, you should probably rewrite your code.
The number of times I've seen new R users get unexpected results because they
haven't noticed that their function is referencing a parent environment instead
of a locally defined variable or argument is past counting.
Of course there are times when it's useful and sensible to have globally
defined variables that can be accessed within a function. But they are very
rare; as a default, I'd recommend avoiding it if at all possible. If your
function needs something from outside, pass it as an argument.
I would say the meaning of "probably" in your 2nd sentence depends quite
a bit on the user. For beginners, it's "almost certainly". For people
who are comfortable with the concept, it's just "maybe".
I would agree that in most cases the only global objects that functions
should reference are other functions, but small nested functions are
quite safe, and it's sometimes useful to create functions in a local
environment so they have persistent memory.
Duncan Murdoch
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