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

[Duncan Murdoch]

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