On 03/01/2013 06:13 AM, Simon Zehnder wrote:
Dear R-users,
I am working on a project that uses S4-classes. At some point I encountered the
problem - well known to R - that I have to pass 3 different objects to a
function, that should modify several slots of them and of course there is no
passing by reference in R.
Hi Simon --
for this problem it is helpful to recognize that the default 'initialize' method
is actually a copy constructor, so
.A = setClass("A", representation(x="numeric", y="numeric", z="numeric"))
a = .A(x=1:5)
initialize(a, y=1:2, z=2:1) # update y, z
> initialize(a, y=1:2, z=2:1)
An object of class "A"
Slot "x":
[1] 1 2 3 4 5
Slot "y":
[1] 1 2
Slot "z":
[1] 2 1
Write your own initialize methods to obey this contract by (a) not writing an
initialize method (there is none for "A", above)! or (b) using named arguments
with default values taken from .Object and specified after the ..., e.g.,
setMethod(initialize, "A", function(.Object, ..., y=.Object@y, z=.Object@z) {
## some additional initialization, e.g.,
y <- abs(y)
callNextMethod(.Object, ..., y=y, z=z)
})
Then I read this thread by Steve Lianoglou:
https://stat.ethz.ch/pipermail/r-help/2010-August/250468.html, which offers
from viewpoint of OOP an elegant solution. But I do not use an Object Method,
but a regular function, pass in several objects and modify them.
The thread was written before 'reference classes' were introduced; see
?ReferenceClasses for this style of programming.
Hope that helps,
Martin
Here is an example with 1 Object using Steve's approach:
setClass("example",
representation(
par = "matrix",
.cache = "environment")
)
setMethod("initialize", "example", function(.Object, ..., .cache = new.env()) {
callNextMethod(.Object, .cache = .cache, ...)
}
)
ex <- new("example", par = matrix())
fmodify <- function(O1) {
pm <- mean(rnorm(100))
pm <- matrix(pm)
O1@.cache$par <- pm
}
fmodify(ex)
So far so good. Everything worked nicely. But of course the value computed in
the function 'fmodify' is now in the slot ex@.cache$par:
ex@.cache$par
and not in ex@par. Is there a possibility to get it into ex@par?
Best
Simon
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