[Rd] "for" loop wiht S4 Objects

[Douglas Bates]

On Fri, Jan 23, 2009 at 1:25 AM, Biczok Rudolf
<r.biczok at dkfz-heidelberg.de> wrote:
> Hi all,

> I'm working with the S4-Class system and I have a little problem with

> Implementing iteration functionality in my S4 class but it don't work:

>> setClass("foo",representation(bar="list"))
>> x <- new("foo",bar=list(1,2,3))
>>for(e in x) cat(e)
> invalid type/length (S4/1) in vector allocation

I'm not sure why you would expect a for loop to work there.  Wouldn't
a function in the "apply" family be more natural?  At least in my code
it is much more common to use lapply to loop over the elements of a
list than it is to use 'for'.

I once suggested to John Chambers that it would be a good idea to have
a way of applying a function to the slots of an S4 object and he
persuaded me otherwise.  However, you can generate something like that
with

lapply(slotNames(x), function(nm) cat(slot(x, nm)))

or create a list out of the S4 object

S4toL <- function(x) {nms <- slotNames(x); names(nms) <- nms;
lapply(nms, slot, object = x)}

then use lapply on the result of S4toL

>
>
>
>
>
> But when I extend from a primitive vector it works:
>
>
>
>> setClass("foo",contains="list")
>
>
>
>> x <- new("foo",.Data=list(1,2,3))
>
>
>
>>for(e in x) cat(e)
>
> 123
>
>
>
> This is ok, but is there any other way to do this (for e.g. with a
> generic function)?
>
>
>
> Thanks,
>
> Rudolf
>
>
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