[Rd] Set operation generics

[Hervé Pagès]

Hi Hadley,

On 10/22/2013 07:54 AM, Hadley Wickham wrote:
>>> Would anyone be interested in reviewing a patch to make the set
>>> operations (union, intersect, setdiff, setequal, is.element) generic?
>>
>> S3 generics, S4 generics, or primitives?
>
> I would expect S3. Can you even have an S4 generic in the base
> package? (i.e. before the methods package is loaded)

Probably not. But the patch could be trying to put them in stats4.

>
>> Note that all those things heavily rely on match() behind the scene.
>> If match() itself was an S4 generic (or a primitive like c() and [)
>> then union(), intersect(), setdiff(), is.element() could be defined
>> with something like:
>>
>>
>>    union <- function(x, y)
>>    {
>>      xy <- c(x, y)
>>      sm <- match(xy, xy)
>>      xy[sm == seq_along(sm)]
>>    }
>>
>>    intersect <- function(x, y)
>>    {
>>      sm <- match(x, x)
>>      x <- x[sm == seq_along(sm)]
>>      m <- match(x, y)
>>      x[!is.na(m)]
>>    }
>>
>>    setequal <- function(x, y)
>>    {
>>      !(anyNA(match(x, y)) || anyNA(match(x, y)))
>>    }
>
> Although I suspect R-core would prefer a minimal change where it's
> easier to see that existing behaviour is preserved.
>
>> For the record, match(), union(), intersect(), and setdiff() are S4
>> generics in the BiocGenerics package. But there is no doubt it would
>> be a better/cleaner situation if base::match() itself was an S4 generic
>> or primitive.
>
> By primitive, you mean internal generic?

Yes.

Thanks,
H.

>
> Hadley
>

-- 
Hervé Pagès

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