[R] creating list with 200 identical objects

[Wacek Kusnierczyk]

Rainer M Krug wrote:
> Thanks a lot Wacek for this clear description of the problem - I was
> not aware, that it is that complex.
> I definitely did not consider the initialize() function in writing my code.
>
> But as I only want to allocate the space for the objects, it does not
> matter here. But when I write a simulation and want to initialize it
> with two hundred different individuals, this definitely becomes
> crucial - thanks again for this clarification and the very useful
> references (especially [2] for the moment).
>
> Am I right that all assignments of classes via <- or -> are by value
> and NOT by reference? So when
>
> setClass('foo',
>        representation=representation(content='character'),
>        prototype=list(content='foo'))
>
> A <- new("foo")
> B <- A
>   

i guess [sic] that what happens at this point is the usual thing:  B
refers to the *same* object, which now has an increased accession counter.

> B is a different object then A, i.e. changes in B are not reflected in
> A and vice versa, but they have the same content.
>   

well.  i guess [sic] that when you assign to a slot of b, it is that
object that is reproduced, which also triggers a reproduction of the
instance object itself (note:  without 'new' and any initializer being
called).  however, this does not happen if the slot is an environment,
because r won't reproduce an environment on assignment.  in this case,
you still have A and B being the same object, with modified content:

    setClass('foo',
        representation=representation(foo='environment'),
        prototype=list(foo=new.env()))

    a = new('foo')
    b = a
    a at foo$bar = 'bar'
    b at foo$bar
    # "bar"


to be sure, you may want to check with the sources -- i haven't done it
yet, hence the guesswork.


> It seems that if foo has a slot O containing an object of class
> "fooInFoo", this object O also copied by value. I.e. when, following
> above,
>
> setClass('foo2',
>        representation=representation(content='character'),
>        prototype=list(content='fooInFoo'))
> setClass('foo',
>        representation=representation(content='character', O='foo2'),
>        prototype=list(content='foo', O=new('foo2')))
> A <- new("foo")
> B <- A
>
> A at O@content <- "AAAAAAAAAAAAAA"
> A at O@content
>   [1] "AAAAAAAAAAAAAA"
> B at O@content
>   [1] "fooInFoo"
>
> Is this always the case? and is there a way of copying by reference
> (or passing an object to a function by reference)?
>   

using environments provides a straightforward way to pass by reference,
because that's how they are passed.  (the claim that r passes by value,
made here and there, is incorrect and misleading, for this and other
reasons.)

otherwise, r's object systems -- s3 and s4 -- are so good that people
have been inventing their own ways to deal with the trouble.  For
example, R.oo, available from cran, is a package with

"Methods and classes for object-oriented programming in R with or
without references. [...] This is a cross-platform package implemented
in pure R that defines standard S3 classes without any tricks."

vQ