R help - Mar 2003 - lists of lists

I have a function that returns multiple elements in a list (for the
sake of the example here, this will simply be a list of two elements,
a and b, but it is actually five named elements).

This functions computes these values in an iterative manner, and I
would like to start with an empty list and append the list of return
values to this list (so that they would remain lists). The Scheme code
for the resulting list would look something like

(list (list 1 2) (list 3 4) (list 4 5))

I tried to do the same thing in R:
> l <- list()
> l <- list(unlist(l, recursive=FALSE), list(a=1, b=2)) # OK
> l <- list(unlist(l, recursive=FALSE), list(a=3, b=4)) # OK
> l <- list(unlist(l, recursive=FALSE), list(a=5, b=6)) # fails here
I would like to access the elements with eg l[[2]]$a.

Strangely enough (at least to me), I find that the following works:

l <- NULL
l <- c(l, list(list(a=1, b=2))) # for any values of 1 and 2, of course ;)

Could somebody please explain why the first way doesn't work, and why
the second does? There seems to be an implicit assumption I am making
about lists in R which is clearly wrong. Comparisons with Scheme would
be welcome (does c() "unquote" the lists it is given?)

Is there a better way to solve this problem (using another data
structure)?

Regards,

Tamas Papp

-- 
Tam?s K. Papp
E-mail: tpapp at axelero.hu (preferred, especially for large messages)
        tpapp at westel900.net
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On Sun, 9 Mar 2003, Tamas Papp wrote:
> I have a function that returns multiple elements in a list (for the
> sake of the example here, this will simply be a list of two elements,
> a and b, but it is actually five named elements).
>
> This functions computes these values in an iterative manner, and I
> would like to start with an empty list and append the list of return
> values to this list (so that they would remain lists). The Scheme code
> for the resulting list would look something like
>
> (list (list 1 2) (list 3 4) (list 4 5))
>
> I tried to do the same thing in R:
>
> > l <- list()
> > l <- list(unlist(l, recursive=FALSE), list(a=1, b=2)) # OK
> > l <- list(unlist(l, recursive=FALSE), list(a=3, b=4)) # OK
> > l <- list(unlist(l, recursive=FALSE), list(a=5, b=6)) # fails here
>
> I would like to access the elements with eg l[[2]]$a.
>
> Strangely enough (at least to me), I find that the following works:
>
> l <- NULL
> l <- c(l, list(list(a=1, b=2))) # for any values of 1 and 2, of course
;)
>
> Could somebody please explain why the first way doesn't work, and why
> the second does? There seems to be an implicit assumption I am making
> about lists in R which is clearly wrong. Comparisons with Scheme would
> be welcome (does c() "unquote" the lists it is given?)
I think you can usefully regard c() as the vector analogue of (cons).
That is, if you were adding to the head of a list you would expect
  (cons (list 3 4) l)
to do what you want, and
  c(list(3,4), l)
is the S equivalent.  Because S lists are generic vectors rather than
pairlists the natural pasting operating is more symmetric, and
  c(l, list(3,4))
adds one element, a list, to the end of l (where (cons) would do something
different).

The unlist() solution doesn't work because recursive=FALSE does one more
level of unlisting than you expect.

> Is there a better way to solve this problem (using another data
> structure)?
Not unless you know in advance how many lists you have.

	-thomas