Names of Greek letters stored as character strings; plotmath.

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 am certainly ignorant of the internals, but I honestly believe
well-written documentation on these points would help. I learned a lot
when I read V&R's "S Programming" some years ago, so I think that's an
existence theorem.

Unfortunately, writing such documentation is hard, maybe harder than
writing the code even (my own experience documenting my own code). And
your logical response -- "Bert, R is open source and would welcome
your efforts" -- is not going to get much traction. Like you, I don't
have the time (nor, in my case, the smarts).

Nevertheless, I agree with Robert's sentiment that it could be done.
In particular, plotmath docs really should expand their explanation of
appropriate syntax, imho.

As always, I end by acknowledging the extraordinary efforts of R's
developers. Your success is what has led to such carping. A response
of "Aw shaddup" would therefore not be out of line.

Best,
Bert

On Sun, May 20, 2012 at 5:00 PM, Duncan Murdoch
<murdoch.duncan at gmail.com>  wrote:

On 12-05-20 6:53 PM, Robert Baer wrote:

-----Original Message-----
From: William Dunlap
Sent: Saturday, May 19, 2012 11:07 AM
To: Rolf Turner
Cc: r-help
Subject: Re: [R] Names of Greek letters stored as character
strings;plotmath.

parse(text=paste(...)) works in simple cases but not in others.  The
fortune about it is there because it is tempting to use but if you bury it
in a general purpose function it will cause problems when people
start using nonstandard names for variables.  bquote(), substitute(),
call(), and relatives work in all cases.  E.g.,

   >    par(mfrow=c(2,1))
   >    power<- "gamma" ; x<- "Waist" ; y<- "Weight" # valid R variable names
   >    plot(0, main=bquote(.(as.name(x))^.(as.name(power))/.(as.name(y))))
   >    plot(0, main=parse(text=paste0(x, "^", power, "/", y))) # same as

previous

   >
   >    power<- "gamma" ; x<- "Waist Size (cm)" ; y<- "Weight (kg)" # invalid

R names

   >    plot(0, main=bquote(.(as.name(x))^.(as.name(power))/.(as.name(y))))
   >    plot(0, main=parse(text=paste0(x, "^", power, "/", y))) # whoops

   Error in parse(text = paste0(x, "^", power, "/", y)) :
     <text>:1:7: unexpected symbol
   1: Waist Size
            ^

Now you might say that serves me right for using weird variable names,
but some of us use R as a back end to a GUI system (one not designed
around R) and don't want to inflict on users R's rules for names when
we do not have to.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf
Of Bert Gunter
Sent: Saturday, May 19, 2012 7:24 AM
To: Gabor Grothendieck
Cc: r-help
Subject: Re: [R] Names of Greek letters stored as character strings;
plotmath.

... and here is another incantation that may be  informative.

xnm<- as.name("gamma')  ## This does the parsing
plot(0, xlab =bquote(.(xnm))

The initial puzzle is that if you just set
xnm<- "gamma"

bquote will insert the string "gamma" rather than the symbol. After
all, that's what plotmath sees for xnm. So the key is telling plotmath
that it's a symbol, not a string. This can either be done before, as
above, or inline, as you and Gabor showed. Unsurprisingly. this also
does it, since as.name() is doing the parsing:

xnm<- "gamma"
  plot(0,xlab=bquote(.(as.name(xnm))))

AND we are adhering to Thomas's dictum: bquote is a wrapper for
substitute(), which is what he recommends as the preferable
alternative to eval(parse(...)) . But, heck -- all such software
principles are just guidelines. Whatever works (robustly).

HTH.

Cheers,
Bert

On Sat, May 19, 2012 at 3:17 AM, Gabor Grothendieck
<ggrothendieck at gmail.com>    wrote:

On Sat, May 19, 2012 at 1:18 AM, Rolf Turner<rolf.turner at xtra.co.nz>
wrote:

I had such good luck with my previous question to r-help, (a few
minutes
ago) that I thought I would try again with the following query:

Suppose I have

    xNm<- "gamma"

I would like to be able to do

    plot(1:10,xlab =<something involving xNm">)

and get the x axis label to be the Greek letter gamma
(rather than the literal text string "gamma").

Is this possible?  I've messed around with substitute()
and bquote() and got nowhere.

Then, just before clicking on "Send", I had one more thimk, and blow
me down, I got something that worked:

plot(1:10,xlab=eval(expression(parse(text=xNm))))

That can be shortened to:

plot(0, xlab = parse(text = xNm))

-- snip --


This discussion has been exceedingly helpful, sort of.

Every time I try to do a task involving this I read the documentation for
bquote(), expression(), plotmath(), etc.,  over and over, and I still fail
to get the big picture of how R parses things under the hood.  Typically,
I
only succeed each time by frustrating trial and error.   Can I ask how you
guys got a handle on the bigger (besides your usual brilliance<G>)?

Is there more comprehensive documentation in the developer literature or
is
there a user wiki that you would recommend for those who never quite get
the
big picture?  If not, this would be a worthy topic for an R Journal
article
if someone has knowledge and the time to do it.  Wish I were knowledgeable
enough to do it myself.

I think you have to try writing C code to work with R objects to really
understand what's going on.  Reading those sections of the Writing R
Extensions manual will probably help, but actually writing C code to do it
may be necessary.

It's not really a very complicated system, it's just that things that are
obviously different in C (e.g. the R symbol name "gamma" versus the string
"gamma") look confusingly similar in R.

You might be able to figure this out by studying the result of str(x) (or
the low level .Internal(inspect(x))) for lots of different x objects, but I
don't think it's going to make sense unless you know what's going on behind
the curtain.

Duncan Murdoch