[R] overloading + operator for chars

[Martin Morgan]

On 11/02/2011 06:52 AM, Albert-Jan Roskam wrote:
> Hello,
>
> I would like to overload the "+" operator so that it can be used to concatenate two strings, e.g "John" + "Doe" = "JohnDoe".
> How can I 'unseal' the "+" method?
>> setMethod("+", signature(e1="character", e2="character"), function(e1, e2) paste(e1, e2, sep="") )
> Error in setMethod("+", signature(e1 = "character", e2 = "character"),  :
>    the method for function "+" and signature e1="character", e2="character" is sealed and cannot be re-defined
>>
>

Hi -- I think the two issues are that "+" is part of the "Arith" group 
generic (?Methods, ?Arith) and that `+` (actually, members of the Ops 
group) for primitive types dispatches directly without doing method 
look-up. Personally I might

setClass("Character", contains="character")

Character <- function(...) new("Character", ...)

setMethod("Arith", c("Character", "Character"), function(e1, e2) {
     switch(.Generic,
            "+"=Character(paste(e1, e2, sep="")),
            stop("unhandled 'Arith' operator '", .Generic, "'"))
})

and then

 > Character(c("foo", "bar")) + Character("baz")
[1] "foobaz" "barbaz"

Some might point to

 > `%+%` <- function(e1, e2) paste(e1, e2, sep="")
 > "foo" %+% "bar"
[1] "foobar"

Martin

> Cheers!!
> Albert-Jan
>
>
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