[R] expression, strsplit, ...

[baptiste Auguié]

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