[R] How to dynamically replace the text behind dollar sign: '$'

[David Winsemius]

On Feb 5, 2013, at 10:48 PM, 谢一鸣 wrote:

> Thanks David, your solution perfectly solve my problem. But Do you  
> know what exactly the mechanism of "Computing on the language" in  
> pdf file: "R language definition", Can that "THING" do the same job?
>

I wouldn't consider that to be "computing on the language. There is an  
entire Section 6 regarding "computing on the Language". I don't see  
evidence that you have read it yet. "Computing on the language"  
generally refers to manipulation objects of 'language' class before  
they get evaluated.

-- 
David.

> Thank you very much!
> Xie YM
>
> 2013-02-06 14:22:57，"David Winsemius" <dwinsemius at comcast.net> 写 
> 道：
>> The "$" construction is only a syntactic shortcut for  
>> "[["argument"]].
>> What you are looking for is described in the ?"[[" page. You should  
>> be
>> constructing a formal that is assigned a syntactically legal column
>> name ...  NOT "ANYTHING I WANT". Perhaps:
>>
>> function(dfrm , colname = "V1") {
>>        return( dfrm[[ colname ]] )
>> # the argument will get evaluated as "V1" unless an alternate colname
>> argument is supplied
>>        }
>>
>> ( You can't get "$" to evaluate a character column name.)
>>
>> --
>> David Winsemius, MD
>> Alameda, CA, USA
>>

David Winsemius, MD
Alameda, CA, USA