[R] Defining binary indexing operators

[Tony Plate]

Excuse me!  I misunderstood the question, and indeed, it is necessary be 
that complicated when you try to make x$y behave the same as foo(x,y), 
rather than foo(x,"y") (doing the former would be inadvisible, as I 
think someelse pointed out too.)

Tony Plate wrote:
> It's not necessary to be that complicated, is it?  AFAIK, the '$' 
> operator is treated specially by the parser so that its RHS is treated 
> as a string, not a variable name.  Hence, a method for "$" can just take 
> the indexing argument directly as given -- no need for any fancy 
> language tricks (eval(), etc.)
> 
>  > x <- structure(3, class = "myclass")
>  > y <- 5
>  > foo <- function(x,y) paste(x, " indexed by '", y, "'", sep="")
>  > foo(x, y)
> [1] "3 indexed by '5'"
>  > "$.myclass" <- foo
>  > x$y
> [1] "3 indexed by 'y'"
>  >
> 
> The point of the above example is that foo(x,y) behaves differently from 
> x$y even when both call the same function: foo(x,y) uses the value of 
> the variable 'y', whereas x$y uses the string "y".  This is as desired 
> for an indexing operator "$".
> 
> -- Tony Plate
> 
> 
> 
> Gabor Grothendieck wrote:
> 
>> On 4/27/05, Ali - <saveez at hotmail.com> wrote:
>>
>>> Assume we have a function like:
>>>
>>> foo <- function(x, y)
>>>
>>> how is it possible to define a binary indexing operator, denoted by 
>>> $, so
>>> that
>>>
>>> x$y
>>>
>>> functions the same as
>>>
>>> foo(x, y)
>>
>>
>>
>>   Here is an example. Note that $ does not evaluate y so you have
>> to do it yourself:
>>
>> x <- structure(3, class = "myclass")
>> y <- 5
>> foo <- function(x,y) x+y
>> "$.myclass" <- function(x, i) { i <- eval.parent(parse(text=i)); 
>> foo(x, i) }
>> x$y # structure(8, class = "myclass")
>>
>>     [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help at stat.math.ethz.ch mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide! 
>> http://www.R-project.org/posting-guide.html
>>
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
>