[R] overloading the generic primitive functions "+" and "["

[Christofer Bogaso]

Dear Barry, this is really interesting. However I could not understand
this line:

Ops.ss=function(e1,e2){paste(e1,e2)}

Where you have told R to behave "+" function differently when it faces
"ss" class?
What should be the ideal approach if I what to use "*" function?

Thanks,

On Thu, Oct 28, 2010 at 2:47 PM, Barry Rowlingson
<b.rowlingson at lancaster.ac.uk> wrote:
> On Thu, Oct 28, 2010 at 9:31 AM, Mark Heckmann <mark.heckmann at gmx.de> wrote:
>>
>> How can I add a new method to the generic function "[" or "+"
>>
>> I want to create a S3 and S4 class that will use the "[" and "+" method in a different way.
>> How can I overload the generic primitive "[" or "+" so the method dispatch will work correctly for my class?
>>
>> I saw an example of this in the ggplot2 package where different objects are joined using the "+" primitive.
>> Still, I could not figure out how to do that.
>>
>> Can someone give a small code example to show how that works?
>
>  With S3 method stuff, to override "+" its as easy as writing Ops.{class}:
>
>  > a="hello"
>  > b="world"
>  > a+b
>  Error in a + b : non-numeric argument to binary operator
>  > class(a)="ss"
>  > class(b)="ss"
>  > Ops.ss=function(e1,e2){paste(e1,e2)}
>  > a+b
>  [1] "hello world"
>
>  better make sure I haven't broken anything:
>
>  > c=1
>  > d=2
>  > c+d
>  [1] 3   *phew*
>
> The Ops function should really have a handler for all possible
> operators (*, /, - etc) or fail with an error - see Ops.Date for
> example.
>
> With S4 classes... umm.. signatures... setMethod... setGeneric...
> errr. something... representations... setClass this that... yeah.
> Something like that.
>
> Barry
>
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