[R] "a"+"b"

[Prof Brian Ripley]

On Tue, 2 May 2006, Matthew Dowle wrote:

> Thanks for the information.  By 'not easily' it sounds like it is possible,
> but how?

Change the C code.  R is Open Source, so anything is possible.

> I'm happy to allow 'errors' like "2"+"3"="23", is that what an
> erroneous use could be?

2 + x, for example, where x is not numeric.

> I tried the following, given the info in your reply, which works.  But is
> there a way to allow "a"+"b"="ab"?
>> x = "a"
>> `+.character` <- function(e1, e2) paste(e1,e2, sep="")
>> attr(x,"class")="character"
>> x+x
> [1] "aa"
>>
>
>> -----Original Message-----
>> From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk]
>> Sent: 02 May 2006 12:03
>> To: Matthew Dowle
>> Cc: 'r-help at stat.math.ethz.ch'
>> Subject: Re: [R] "a"+"b"
>>
>>
>> Not easily.  There's a comment in ?Ops that for efficiency the group
>> generics only dispatch on objects with a "class" attribute:
>> otherwise you
>> could use an S3 method like
>>
>> `+.character` <- function(e1, e2) paste(e1,e2, sep="")
>>
>> S4 methods are built on top of S3 methods as far as internal
>> dispatch is
>> concerned, so the same comment will apply there AFAICS.
>>
>> I would think that the intention was also to positively
>> discourage messing
>> with the basics of R, as if you were able to do this
>> erroneous uses would
>> likely not get caught.
>>
>> On Tue, 2 May 2006, Matthew Dowle wrote:
>>
>>>
>>> Hi,
>>>
>>> Is there a way to define "+" so that "a"+"b" returns "ab" ?
>>>
>>>> setMethod("+",c("character","character"),paste)
>>> Error in setMethod("+", c("character", "character"), paste) :
>>>        the method for function '+' and signature e1="character",
>>> e2="character" is sealed and cannot be re-defined
>>>> "a"+"b"
>>> Error in "a" + "b" : non-numeric argument to binary operator
>>>
>>> Thanks!
>>>
>>>
>>>
>>> 	[[alternative HTML version deleted]]
>>>
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>>
>> --
>> Brian D. Ripley,                  ripley at stats.ox.ac.uk
>> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
>> University of Oxford,             Tel:  +44 1865 272861 (self)
>> 1 South Parks Road,                     +44 1865 272866 (PA)
>> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>>
>
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595