R help - Jul 2007 - The "$" operator and vectors

Hi all,

I've run into a slightly illogical (to me) behaviour with the "$"
subsetting function.

consider:
> Test
  A B
1 1 Q
2 2 R
> Test$A
[1] 1 2
> vector<-"A"
> Test$vector
NULL
> Test$"A"
[1] 1 2
> Test[,vector]
[1] 1 2


Is there a reason for the $ operator not evaluating the vector before executing?

best,

Gustaf


-- 
Gustaf Rydevik, M.Sci.
tel: +46(0)703 051 451 /+46(0)8 760 52 93
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skype:gustaf_rydevik

Gustaf Rydevik <gustaf.rydevik <at> gmail.com> writes:
 
> Is there a reason for the $ operator not evaluating
> the vector before executing?
Yes. Consider:
> x <- list(a = 1, b = 'n')
> a <- 'b'
> x[[a]] == 'n'
[1] TRUE
> x[['a']] == 1
[1] TRUE
> x$a == ???
If the result is 'n', then all bare $ references would require
quotes to avoid name collisions.

THK
> 
> best,
> 
> Gustaf
>

Gustaf Rydevik wrote:
> Hi all,
>
> I've run into a slightly illogical (to me) behaviour with the
"$"
> subsetting function.
>
> consider:
>   
>> Test
>>     
>   A B
> 1 1 Q
> 2 2 R
>
>   
>> Test$A
>>     
> [1] 1 2
>
>   
>> vector<-"A"
>> Test$vector
>>     
> NULL
>
>   
>> Test$"A"
>>     
> [1] 1 2
>
>   
>> Test[,vector]
>>     
> [1] 1 2
>
>
> Is there a reason for the $ operator not evaluating the vector before
executing?
>   
Yes, the evaluation rule for "$" is like that....

Notice that it also didn't go looking for an object called A when you
said test$A.


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