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

[Christos Hatzis]

A way to do this is through substitute:

e1 <- substitute(expression(u1 + u2 + u3), list(u2=quote(x), u3=1))

But I am not sure whether you will run into similar limitations regarding
the length of the expression to be substituted. 

-Christos

Christos Hatzis, Ph.D.
Nuvera Biosciences, Inc.
400 West Cummings Park
Suite 5350
Woburn, MA 01801
Tel: 781-938-3830
www.nuverabio.com
 


> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Daniel Berg
> Sent: Tuesday, March 27, 2007 9:57 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Replacement in an expression - can't use parse()
> 
> 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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