[R] Names of Greek letters stored as character strings; plotmath.

[Bert Gunter]

Yet again:Thank you Peter and Duncan. I appreciate your comments and insights.

I agree wholeheartedly with Peter's comments below about understanding
what a parsed expression is in R. In R -- and in functional
programming in general, I believe -- computing on the language is
extremely handy, even for relatively basic programming.  I found V&R's
discussion in "S Programming" on "Computing on the Language" very
helpful in this regard, especially their table 3.2,  which dissects
the different but cognate situation of formulas,  functions, and
function calls. Perhaps something similar could be done for
expression(), parse(), and quote(). I think there are subtleties here
that deserve explicit mention and elaboration.

As a feeble attempt in this direction, which I hope illuminates more
than it obfuscates -- and I would greatly appreciate prompt
corrections of any errors -- perhaps the following might be useful:
########################
a <- 3
x <- quote(a+2)
y <- parse(text = "a+2")
z <- expression(a+2)
##
eval(x) ## 5
eval(y) ## the same
eval(z) ## the same
##
## Now note:
as.list(x) ## quote(a+2) the parse tree for the expression a+2
##
as.list(y) ## looks like a list whose component is the parse tree for
the expression
## as demonstrated by:
##
identical(x,y[[1]]) ##TRUE
##
as.list(z) ## appears to be the same as z. And, indeed,
##
identical(y[[1]],z[[1]])  ##TRUE. The parse tree for the expression again.
##
### However
##
identical(y,z) ## FALSE !!
## To see what's going on, use str()
str(x) ## language a + 2
str(y[[1]]) ## the same
str(z[[1]]) ## the same
##
## But
str(y) ## complex structure with attributes
str(z) ## simple expression
##########################################

To me, this reinforces Bill Dunlap's and Thomas Lumley's counsels:
avoid explicit parsing and evaluation via eval(parse(...)) in favor of
working with the parsed expression via substitute() and bquote().

HTH,

Bert


On Mon, May 21, 2012 at 12:20 AM, peter dalgaard <pdalgd at gmail.com> wrote:
>
> 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.mes at cbs.dk  Priv: PDalgd at gmail.com
>
>
>
>
>
>
>
>



-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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