[R] Defining binary indexing operators

[Tony Plate]

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")
> 
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