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

[Sundar Dorai-Raj]

Daniel Berg said the following on 3/27/2007 6:56 AM:
> 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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Take a look at ?substitute (or ?bquote)

 > as.expression(substitute(u1+u2+u3, list(u1 = quote(x))))
expression(x + u2 + u3)
 > as.expression(bquote(u1+u2+.(u3), list(u3 = 1)))
expression(u1 + u2 + 1)

HTH,

--sundar