R help - May 2001 - Symbolic substitution of expressions into expressions

In the following

titletxt <- substitute(paste(tx, tilde(y) == sqrt(y)),
  list(tx="Replace y by "))

I'd like to be able to replace e.g. sqrt(y) by a symbol or
symbolic expression that has the effect of inserting sqrt(y)
into the place that it occupies.  This would allow me to put an
arbitrary expression into that position.  [One can use text() or
mtext() etc. to place titletxt on a graph.]

Any clues would be most welcome.
John Maindonald.
John Maindonald               email : john.maindonald at anu.edu.au        
Statistical Consulting Unit,  phone : (6125)3998        
c/o CMA, SMS,                 fax   : (6125)5549  
John Dedman Mathematical Sciences Building
Australian National University
Canberra ACT 0200
Australia
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John Maindonald <john.maindonald at anu.edu.au> writes:
> In the following
> 
> titletxt <- substitute(paste(tx, tilde(y) == sqrt(y)),
>   list(tx="Replace y by "))
> 
> I'd like to be able to replace e.g. sqrt(y) by a symbol or
> symbolic expression that has the effect of inserting sqrt(y)
> into the place that it occupies.  This would allow me to put an
> arbitrary expression into that position.  [One can use text() or
> mtext() etc. to place titletxt on a graph.]
> 
> Any clues would be most welcome.
titletxt <- substitute(paste(tx, tilde(y) == my.y),
   list(tx="Replace y by ",my.y=quote(sqrt(y))))


-- 
   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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I inquired
> In the following
> 
> titletxt <- substitute(paste(tx, tilde(y) == sqrt(y)),
>   list(tx="Replace y by "))
> 
> I'd like to be able to replace e.g. sqrt(y) by a symbol or
> symbolic expression that has the effect of inserting sqrt(y)
> into the place that it occupies.  This would allow me to put an
> arbitrary expression into that position.  [One can use text() or
> mtext() etc. to place titletxt on a graph.]
> 
> Any clues would be most welcome.
to which Peter Dalgaard helpfully replied
<<
titletxt <- substitute(paste(tx, tilde(y) == my.y),
   list(tx="Replace y by
",my.y=quote(sqrt(y))))
>>
This does not quite go as far as I want.
> tx2 <- expression(x^2)
> qx2 <- quote(x^2)
> tx2
expression(x^2)
> qx2
x^2

Setting my.y = qx2 places the latexed version of x^2
into the text string and hence on the graph, while 
my.y=quote(tx2) places "expression(x^2)" into the
text string.  Is there any way to get from 
expression(x^2) or the text string "x^2" to quote(x^2)?

John Maindonald.
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