R help - Jun 1999 - text-plotting math & regular mixed ?

Maybe embarassing, but I can't quickly find out how to do this:

Mix expression() text and regular text __in a variable__ for plotting:

I can easily do

plot(1,type="n",main=expression(paste("Plot of ", x ^ alpha,
"; ", alpha==2.3)

However, if alpha is really just in a variable, I don't know what to do

  alpha <- 2.3
  ch.a <- format(alpha)

and now do the above but only use  `ch.a' ?

i.e., can you ``fix'' the following function ?

p.ex <- function(alpha = .05) {

     x <- seq(0,1, len=101)
     ch.a <- formatC(alpha,wid=1)
     plot(x, x^alpha, main= 
	  expression(paste("Power plot of ", x ^ alpha, " for ",
alpha == {})))
     ## Where can I pass  ch.a ??			  
     invisible()			  
}

Martin Maechler <maechler at stat.math.ethz.ch>
http://stat.ethz.ch/~maechler/
Seminar fuer Statistik, ETH-Zentrum SOL G1;	Sonneggstr.33
ETH (Federal Inst. Technology)	8092 Zurich	SWITZERLAND
phone: x-41-1-632-3408		fax: ...-1086			<><
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Martin Maechler <maechler at stat.math.ethz.ch> writes:
> i.e., can you ``fix'' the following function ?
> 
> p.ex <- function(alpha = .05) {
> 
>      x <- seq(0,1, len=101)
>      ch.a <- formatC(alpha,wid=1)
>      plot(x, x^alpha, main= 
> 	  expression(paste("Power plot of ", x ^ alpha, " for
", alpha == {})))
>      ## Where can I pass  ch.a ??			  
>      invisible()			  
> }
Use substitute(). Something like 

e<-substitute(expression(paste("Power plot of ", x ^ alpha, "
for ",
alpha == ch.a)), list(ch.a=formatC(alpha,wid=1)))

plot(x, x^alpha, main=e)

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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