On Mon, 28 Apr 2008, Kenn Konstabel wrote:
On Sun, Apr 27, 2008 at 4:43 AM, Greg Snow <[EMAIL PROTECTED]> wrote:
What if mylist <- list( 1:10, 101:110 , some.other.things) so the first 2
elements are vectors of length 10. then mylist[1:2] makes sense as still
being a list with the 2 vectors. What should mylist[[1:2]] return in this
case? One vector of length 20? or should it return a matrix with 2 columns
and 10 rows. Both of those make sense, how should the computer decide
between them (it may be obvious to you knowing the
Note that the R language definition says,
"for a list, the index can be a vector and each element of the vector is
applied in turn to the list, the selected component, the selected component
of that component, and so on. The result is still a single element. "
So mylist[[1:2]] will return something in this case! -- the result is the
second element of the first element of the list, or the same as
mylist[[1]][[2]] or mylist[[1]][2].
I don't find this particularly useful, and for better understanding, I would
almost always prefer mylist[[1]][2] to mylist[[c(1,2)]] but there is a
theoretical possibility that someone might find a use for this behavior. For
these people, making list[[vector]] return something different may be
disturbing.
Not theoretical -- it was an user wishlist request for compatibility with
S. And if the nesting is of variable depth, it is very useful.
This is also on the help page for [[, so there is little excuse for not
being aware of it.
Kenn
context, but how can the computer decide). You can do either of these in
R by giving the computer a bit more information (as.matrix or unlist). What
if one of the vectors is character and one is numeric, what should the
return object be? What if the first element of mylist is the return object
from "lm" and the second element is a function, what should mylist[[1:2]]
return then?
If you can come up with a set of rules that will cover every possible
case, then someone may be willing to implement those rules. But while it is
not obvious what to return without giving extra information, it is better to
require the extra information through other functions.
________________________________
From: [EMAIL PROTECTED] on behalf of Olivier Lefevre
Sent: Sat 4/26/2008 9:43 AM
To: [EMAIL PROTECTED]
Subject: Re: [R] matrix from list
Olivier Lefevre wrote:
Anyway you are right that it would still return the kind of object, only
subsetted, which is not I want.
I mean [] would do that; I know [[]] doesn't. Yet I still don't see why
one
accepts vector arguments but not the other: they are both indexing
operators. It is such inconsistencies that make languages hard to learn.
-- O.L.
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--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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