[R] Transforming a string into a command

[Xu Wang]

David,

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"

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
> 
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