[R] mathematical expressions in main

[Peter Dalgaard BSA]

Peter Baker <p.baker at qut.edu.au> writes:

> Hi
> 
> I've tried and tried and I presume it is very simple but .....
> 
> I want some varying titles for plots using greek symbols
> eg using an expression like
> 
> for (true in c(0:5))
>    plot(...,main = expression(paste(phi," = ",true,sigma)),...)
> 
> where phi and sigma are greek symbols.  Instead of \phi=2\sigma I get
> \phi=true\sigma as true is taken as a text string - not a variable.
> 
> Any help much appreciated

Problem is not in plot but in the fact that expression takes
everything inside literally: 

> expression(paste(phi," = ",true,sigma))
expression(paste(phi, " = ", true, sigma))

Try creative uses of substitute:

> substitute(expression(paste(phi, " = ", true, sigma)),list(true=5))
expression(paste(phi, " = ", 5, sigma))

and then

> plot(0,0,main=substitute(expression(paste(phi, " = ", true, 
+ sigma)),list(true=5)))

...which doesn't quite work, because the result is an object of mode
"call" (namely to the primitive function "expression"), not the result
of such a call (which is of mode "expression", although it prints
identically -- yes, this is confusing!). 

What does work is either to evaluate the call, yielding an object of
mode "expression":

> plot(0,0,main=eval(substitute(expression(paste(phi, " = ", true,
+ sigma)),list(true=5))))

...or to use the fact that an object of mode "call" is also allowed for
math graphics and dump the expression bit:

> plot(0,0,main=substitute(paste(phi, " = ", true, sigma),list(true=5)))


-- 
   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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