[R] Transforming a string into a command

[Petr PIKAL]

Hi

> 
> Did my reply get orphaned or are you trying to help me realize that 
asking
> why something does not work is not a straightforward question? I'll try 
to
> cover both bases.
> 
> I'll focus just on the first case that I don't understand. Suppose we 
have
> s<- "ln(a+b)" 
> a<-1 
> b<-2
> eval(parse(text=s))
> Error in eval(expr, envir, enclos) : could not find function "ln"

What is function ln supposed to do and to what package it belongs. I get

> ?ln
No documentation for ‘ln’ in specified packages and libraries:
you could try ‘??ln’

but it does not mean that ln is not used elsewhere.

Regards
Petr


> 
> Perhaps it's because I don't understand eval well (any good references 
for
> reading up on eval, parse, substitute, etc.?). But I expected it to 
produce
> the same as the following line:
> eval(parse(text="ln(a+b)"))
> 
> Xu
> 
> 
> David Winsemius wrote
> > 
> > On Nov 29, 2011, at 2:00 AM, Xu Wang wrote:
> > 
> >> Why don't the following two commands work?
> >>
> >> eval(parse(text=s))
> >> eval(as.expression(s))
> >>
> > 
> > Can you think of anything else we might need to know in order to 
> > answer that question?
> > 
> > -- 
> > 
> > David Winsemius, MD
> > West Hartford, CT
> > 
> > ______________________________________________
> > R-help@ mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> > 
> 
> 
> --
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> a-string-into-a-command-tp4112183p4118294.html
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> 
> ______________________________________________
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> PLEASE do read the posting guide 
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> and provide commented, minimal, self-contained, reproducible code.