[R] Defining binary indexing operators

Tony Plate tplate at blackmesacapital.com
Thu Apr 28 01:26:35 CEST 2005

```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]]
>>
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>
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