[R] A technical question on methods for "+"
bgunter@4567 @end|ng |rom gm@||@com
Thu Dec 2 22:16:56 CET 2021
Those are both helpful comments. Would it not be useful to say
something along these lines in ?class. ?
The key point I missed is that there often(but not always) must be an
*explicit* class **attribute** for method dispatch; and class() does
not indicate whether its value is explicit or not (just recapitulating
what you both said). Ops methods are such a case (for anything but
classes inheriting from numeric?), and perhaps a mention there might
be useful, too.
Again, thanks for the help.
On Thu, Dec 2, 2021 at 12:40 PM Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
> The reason for this behaviour is that finding methods is a lot slower
> than just evaluating the built-in function. So R takes the time to
> determine if there's an attribute named "class" attached, but doesn't go
> searching further if there isn't one.
> Duncan Murdoch
> On 02/12/2021 3:10 p.m., Andrew Simmons wrote:
> > This is because + dispatches on the class attribute, which a string like
> > "test" has set to NULL, so it doesn't dispatch. You can add the class
> > yourself like structure("test", class = "character") and that should work.
> > I'm not sure where it's explained, but most primitive functions dispatch on
> > the class attribute, which is different from UseMethod which calls class()
> > if the class attribute is NULL.
> > I think if you want to define something like what you have written, you
> > could write a function `%+%` use that instead
> > On Thu, Dec 2, 2021, 14:32 Bert Gunter <bgunter.4567 using gmail.com> wrote:
> >> ... and probably a dumb one and almost certainly not of interest to
> >> most R users. But anyway...
> >> ?"+" says:
> >> "The unary and binary arithmetic operators are generic functions:
> >> methods can be written for them individually or via the Ops group
> >> generic function. "
> >> So:
> >> "+.character" <- function(e1, e2) paste0(e1, e2)
> >> ## but this doesn't 'work':
> >>> "a" + "b"
> >> Error in "a" + "b" : non-numeric argument to binary operator
> >> ## but explicitly invoking the method does 'work' :
> >>> "+.character"('a','b')
> >>  "ab"
> >> ##Note also:
> >>> methods("+")
> >>  +.character +.Date +.IDate* +.POSIXt +.trellis*
> >> So what am I failing to understand?
> >> Thanks.
> >> Bert Gunter
> >> ______________________________________________
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