Petr, thanks for pointing that out. Jim, you are exactly right! Thank you
for catching that. I did not realize in the other replies that they were
using log and not ln.

David, thank you for the lessons. I will improve my question asking skills.

Thanks to all,

Xu

On Tue, Nov 29, 2011 at 9:31 AM, David Winsemius <dwinsemius@comcast.net>wrote:

>
> On Nov 29, 2011, at 2:30 AM, Xu Wang wrote:
>
>  David,
>>
>> Did my reply get orphaned
>>
>
> All replies are "orphaned".  You are asked to include context if your
> question relies on code that has previously been posted.
>
>
>  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.?).
>>
>
> It appears you need to review the help page for the `log` function.
>
>
>  But I expected it to produce
>> the same as the following line:
>> eval(parse(text="ln(a+b)"))
>>
>
> It did.
>
> > eval(parse(text="ln(a+b)"))
>
> Error in eval(expr, envir, enclos) : could not find function "ln"
>
>
>> 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))
>>>>
>>>>
> They both "worked" as expected. An error was appropriately reported.
>
> > ln(a+b)
> Error: could not find function "ln"
>
> > log(a+b)
> [1] 1.098612
>
>
>
>
>
>>> Can you think of anything else we might need to know in order to
>>> answer that question?
>>>
>>
>
> Some Nabble users seem to expect that the rest of Rhelp sees what they
> see. They are delusional when they do so.
>
> --
> David.
>
>  --
>> View this message in context: http://r.789695.n4.nabble.com/**
>> Transforming-a-string-into-a-**command-tp4112183p4118294.html<http://r.789695.n4.nabble.com/Transforming-a-string-into-a-command-tp4112183p4118294.html>
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________**________________
>>
>
> David Winsemius, MD
> West Hartford, CT
>
>

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