# [R] expression, strsplit, ...

Prof Brian Ripley ripley at stats.ox.ac.uk
Thu Jun 26 13:07:29 CEST 2008

```On Thu, 26 Jun 2008, baptiste Auguié wrote:

>
> 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.

>From ?plotmath

In most cases other language objects (names and calls) are coerced
to expressions and so can also be used.

A formula is a call:

> mode(~ alpha / V * m^-3 * kg ^-2 * l^4)
[1] "call"

That was just GG saving himself a few key strokes at the expense of
clarity:

plot(1, xlab = expression(alpha / V * m^-3 * kg ^-2 * l^4))

is what this is coerced to.

> Also, the handling of exponents is cleverer than I naively assumed.

It is exactly as documented.

> 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?

You use substitute() to do that:

> substitute(alpha *  aaa / V * m^-3 * kg ^-2 * l^4, list(aaa=a))
alpha * " some text "/V * m^-3 * kg^-2 * l^4

> 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.

You can't 'substitute' in a formula, but you can in the RHS of a 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
>>> 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
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>
> _____________________________
>
> Baptiste Auguié
>
> Physics Department
> University of Exeter
> 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