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

[Gabor Grothendieck]

Try substitute:

e <- expression(u1 + u2 + u3)
L <- list(u2 = as.name("x"), u3 = 1)
as.expression(do.call(substitute, list(as.call(e), L))[[1]])


On 3/27/07, Daniel Berg <daniel at danielberg.no> 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
>
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