[R] String replacement in an expression

[William Dunlap]

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com  

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Wacek Kusnierczyk
> Sent: Thursday, May 28, 2009 12:31 PM
> To: Caroline Bazzoli
> Cc: r-help at r-project.org
> Subject: Re: [R] String replacement in an expression
> 
> Caroline Bazzoli wrote:
> > Dear R-experts,
> >
> > I need to replace in an expression the character "Cl" by "Cl+beta"
> >
> > But in the following case:
> >
> > form<-expression((Cl-(V *ka)  ) +(V   *Vm   *exp(-(Clm/Vm)   *t)))
> >
> > gsub("Cl","(Cl+beta)",as.character(form))
> >
> > We obtain:
> >
> > [1] "((Cl+beta) - (V * ka)) + (V * Vm * exp(-((Cl+beta)m/Vm) * t))"
> >
> >
> > the character "Clm" has been also replaced.
> >
> >
> > How could I avoid this unwanted replacement ?
> 
> try '\\bCl\\b' as the pattern, which says 'match Cl as a 
> separate word'.

That works in this case, but \\b idea of what a word is not
same as R's idea of what a name is.  E..g, \\b thinks that
a period is not in a word but R thinks periods in names are
fine.
   > gsub("\\bC1\\b", "(C1+beta)", "C1 * exp(C1.5 / C2.5)")
   [1] "(C1+beta) * exp((C1+beta).5 / C2.5)"
This is one more reason to use substitute(), which directly
edits an expression to produce a new one.  It avoids
the deparse-edit-parse cycle that can corrupt things
(even if you don't do any editing).
   > substitute(C1 * exp(C1.5 / C2.5), list(C1=Quote(C1+beta)))
   (C1 + beta) * exp(C1.5/C2.5)

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com 

> 
> 
> vQ
> 
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