# [R] questions about setMethod("Arith", ...)

Matthias Kohl Matthias.Kohl at uni-bayreuth.de
Tue Jul 6 13:17:28 CEST 2004

```Hi,

we have some questions concerning the definition of new arithmetic methods.

In our package "distr" (on CRAN) we define some new arithmetic methods
for "+", "-", "*", "/".
we define a new
class and also a new method for one of the arithmetic methods "+", "-",
"*", "/", still everything works fine. But, if we then define new
methods for the whole group "Arith", the arithmetic methods of "distr"
no longer work. (for more details see example code below)
What are we missing?

Moreover, there is a certain precendence; i.e., if one defines a single
arithmetic method (e.g., "/") and alterwards defines a method for the
whole group "Arith", the "old" method "/" remains valid.
However, if we first define a method for the whole group "Arith" and
afterwards define a new single arithmetic method (e.g., "+") the new one
is valid. (for more details see example code below).
Is this intended?

Matthias, Thomas

###########################################################
## Example code
###########################################################
require(distr)
getMethods("/")  # shows the corresponding methods of "distr"

## now define a new class "track" (see Chambers (1998))
## and define "/"

setClass("track", representation(x = "numeric", y = "numeric"))
setMethod("/", signature("track", "numeric"),
function(e1, e2){ e1 at y = e1 at y/e2; e1 })

getMethods("/")  # shows the corresponding methods
# of "distr" and class "track"

(N <- Norm()) # creates an object of standard normal distribution
(N1 <- N/3) # works
(tr <- new("track", x = 1:3, y = 4:6))
(tr1 <- tr/3) # works

## now define new methods for "Arith"
setMethod("Arith", signature("track", "numeric"),
function(e1, e2){
e1 at x = callGeneric(e1 at x, e2)
e1
})

getMethods("/") # "/" for "distr" is lost
N2 <- N/3 # fails
(tr2 <- tr/3) # works, "but" still the "old" method
tr + 2 # works

## now a new method "+"
setMethod("+", signature("track", "numeric"),
function(e1, e2){ e1 at y = e1 at y+e2; e1 })

tr + 2 # works, "but" with the "new" method

```