R help - May 1999 - mathematical expressions in main

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

Thanks
Peter Baker

========================================================================|| Peter
Baker  email: p.baker at fsc.qut.edu.au    ph: 07 3864 1293  (W)  ||
||              Mail:  School of Mathematics, Queensland University     ||
||                     of Technology,  GPO Box 2434, Brisbane  QLD 4001 ||
========================================================================

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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   
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