[R] expression, strsplit, ...

[baptiste Auguié]

On 25 Jun 2008, at 19:45, Gabor Grothendieck wrote:

> Try this:
>
> plot(1, xlab = ~ alpha / V * m^-3 * kg ^-2 * l^4)
>


Thanks, I would never have expected this code to work, this is a  
mystery to me! Actually, I thought xlab wanted an expression, but it  
seems to be happy with a formula. Also, the handling of exponents is  
cleverer than I naively assumed. I think an example like the one  
above would make a nice addition to the plotmath documentation.

One small thing bothers me: can one use a similar syntax and use  
something like bquote to substitute a variable?

say,

a <- " some text "

> plot(1, xlab = ~ alpha *  `a` / V * m^-3 * kg ^-2 * l^4)

to produce,


> plot(1, xlab = ~ alpha * " some text " / V * m^-3 * kg ^-2 * l^4)

but I can't get either bquote, deparse, substitute, "``" to produce  
the desired substitution in this formula.

Many thanks,

baptiste




>
> On Wed, Jun 25, 2008 at 1:06 PM, baptiste Auguié  
> <ba208 at exeter.ac.uk> wrote:
>> DeaR list,
>>
>> I'm a bit lost in the behavior of substitute and co.
>> I often use fairly long axis labels in my graphs (long to write,  
>> that is).
>> Typically, they would contain some greek letters and units with  
>> exponents,
>> as in:
>>
>>>        xlab=expression(paste("text ", alpha, " / ", V,".", m^ 
>>> {-3}, ".",
>>> kg^{-2}, ".", l^{4}))
>>
>>
>> To make this a bit prettier, I've attempted to mimic the behavior  
>> of the
>> SIstyle latex package which defines a macro that cleverly parses an
>> expression for units, exponents, etc. My code fails in putting  
>> together the
>> unit, as seen in the example below:
>>
>>>
>>> makeUnits <- function(expr){ # formats the units
>>>
>>> units.list <- strsplit(expr, "[[:blank:]]", perl=F)
>>> expr.list <- strsplit(unlist(units.list), "\\^", perl=T)
>>>
>>> units <- unlist(lapply(expr.list, function(unit) {
>>>                if (length(unit) == 2)
>>> paste(unit[1],"^{",unit[2],"}",sep="")
>>>                else paste(unit,sep="")
>>>                }))
>>>        cat(units, sep=".")
>>> }
>>>
>>> expr <- "V m^-3 kg^-2 l^4"
>>> makeUnits(expr) # this works, and produces:
>>
>> # V.m^{-3}.kg^{-2}.l^{4}
>>>
>>> silab <- function(..., units=NULL){ # creates a valid expression  
>>> for xlab
>>> and co.
>>>        dotCalls <- substitute(list(...))
>>>        nArgs <- length(dotCalls)
>>> if (!is.null(units))    myUnits <- makeUnits(units)
>>> if (!is.null(units)) return(substitute(paste(..., " / ",  myUnits)))
>>> if (is.null(units)) return(substitute(paste(...)))
>>> }
>>>
>>> silab("text", alpha,  units = "V m^-3 kg^-2 l^4") # the result is
>>> obviously not what I want
>>>
>>> par(mfrow=c(2, 1)) # comparison of the desired output and the  
>>> current one
>>> plot(1:10, 1:10,
>>>        xlab=silab("text ", alpha, units = "V m^-3 kg^-2 l^4"),
>>>        ylab=silab("simple text"))
>>>
>>> plot(1:10, 1:10,
>>>        xlab=expression(paste("text ", alpha, " / ", V,".", m^ 
>>> {-3}, ".",
>>> kg^{-2}, ".", l^{4})),
>>>        ylab="simple text")
>>
>>
>>
>>
>> Any thoughts welcome!
>>
>> Sincerely,
>>
>> baptiste
>>
>> _____________________________
>>
>> Baptiste Auguié
>>
>> Physics Department
>> University of Exeter
>> Stocker Road,
>> Exeter, Devon,
>> EX4 4QL, UK
>>
>> Phone: +44 1392 264187
>>
>> http://newton.ex.ac.uk/research/emag
>> http://projects.ex.ac.uk/atto
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting- 
>> guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>

_____________________________

Baptiste Auguié

Physics Department
University of Exeter
Stocker Road,
Exeter, Devon,
EX4 4QL, UK

Phone: +44 1392 264187

http://newton.ex.ac.uk/research/emag
http://projects.ex.ac.uk/atto