On May 21, 2012, at 05:25 , Duncan Murdoch wrote:
> On 12-05-20 10:28 PM, Bert Gunter wrote:
>> Well, that's not very comforting, Duncan. It's like saying that you
>> have to read the engineering specs to drive the car successfully.
>
> I think Robert's message that I responded to was asking for a deeper
> understanding than simply driving the car. He appeared to want to know why
> the car worked the way it did, and describing that entirely in terms of
> things you can see without opening the hood is hard.
There are levels, though. For basic car driving, it might be sufficient to know
that turning the steering wheel left makes the car change direction towards
left. Rather soon, you will realize that it is imortant that it does so by
turning the front wheels; this explains why you need to reverse into a
parallel-parking space. At some point, it may become useful to know that the
wheels are tangential to the curve that the car follows and that it therefore
turns around a point on the line trough the rear wheels (not that that ever
helped me to parallel park...).
In R, it is important to have some reasonably accurate mental image of its
internal structures. For quote() and friends, the thing that you really need is
the notion of a _parse tree_, i.e. the fact that expressions are not evaluated
as-is, but first converted (parsed) to an internal structure that is equivalent
to a list of lists:
> e <- quote(2/(3+a))
> e[[1]]
`/`
> e[[2]]
[1] 2
> e[[3]]
(3 + a)
> e[[3]][[1]]
`(`
> e[[3]][[2]]
3 + a
> e[[3]][[2]][[1]]
`+`
> e[[3]][[2]][[2]]
[1] 3
> e[[3]][[2]][[3]]
a
or, graphically (mailer permitting)
`/` +--2
|
+--`(`--`+` +-- 3
|
+-- a
Once you have this concept in mind, it should become fairly clear that the
string constant "a" is fundamentally different from the variable name a.
--
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd....@cbs.dk Priv: pda...@gmail.com
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