[R] Replacement in an expression - can't use parse()

[Marc Schwartz]

On Tue, 2007-03-27 at 15:56 +0200, Daniel Berg wrote:
> Dear all,
> 
> Suppose I have a very long expression e. Lets assume, for simplicity, that it is
> 
> e = expression(u1+u2+u3)
> 
> Now I wish to replace u2 with x and u3 with 1. I.e. the 'new'
> expression, after replacement, should be:
> 
> > e
> expression(u1+x+1)
> 
> My question is how to do the replacement?
> 
> I have tried using:
> 
> > e = parse(text=gsub("u2","x",e))
> > e = parse(text=gsub("u3",1,e))
> 
> Even though this works fine in this simple example, the use of parse
> when e is very long will fail since parse has a maximum line length
> and will cut my expressions. I need to keep mode(e)=expression since I
> will use e further in symbolic derivation and division.
> 
> Any suggestions are most welcome.
> 
> Thank you.
> 
> Regards,
> Daniel Berg

Here is one possibility, depending upon how complicated the
substitutions end up being:

e <- expression(u1+u2+u3)

TMP <- unlist(strsplit(as.character(e), " "))

> TMP
[1] "u1" "+"  "u2" "+"  "u3"

TMP <- gsub("u2", "x", TMP)

TMP <- gsub("u3", "i", TMP)

> TMP
[1] "u1" "+"  "x"  "+"  "i" 

> as.expression(paste(TMP, collapse = ""))
expression("u1+x+i")

HTH,

Marc Schwartz