[R] plotting an expression

[Peter Dalgaard BSA]

apjaworski at mmm.com writes:

> This is confusing, since
> 
>      substitiue(expression(cos(x)))
> 
> returns just
> 
>      expression(cos(x)).
> 

> Must be some side effects I do not understand.  What is even more confusing
> is that the above code (with one line commented out) does not work when
> called as
> 
>      curve(parse(text="cos(x)"))
> 
> although
> 
>      parse(text="cos(x)")
> 
> returns
> 
>      expression(cos(x)).

You need to be very careful with expression() 

For instance

> q <- quote(expression(cos(pi)))
> e <- eval(q)
> v <- eval(e)
> q
expression(cos(pi))
> e
expression(cos(pi))
> v
[1] -1
> mode(q)
[1] "call"
> mode(e)
[1] "expression"

Notice that q and e look similar, but behave differently and have
different modes. q is a call to the expression() function, e is an
object of mode expression. The object is being represented by the call
that generates it, (numeric(0)/quote(numeric(0)) is a similar
example). You can get to see what is inside as follows

> e[[1]]
cos(pi)
> q[[1]]
expression
> q[[2]]  
cos(pi)


-- 
   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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