[R] Difference between a[[i]] and a[i]
Rolf Turner
r.turner at auckland.ac.nz
Tue Feb 3 03:44:16 CET 2009
On 3/02/2009, at 12:45 PM, David Epstein wrote:
>
> I'm sure I've read about the difference between a[[i]] and a[i] in
> R, but I
> cannot recall what I read. Even more disturbing is the fact that I
> don't
> know how to search the newsgroup for this. All the different
> combinations I
> tried were declared not to be valid search syntax.
Essentially: a is a list; a[i] is a list of length 1 whose sole entry is
the i-th entry of a; a[[i]]] is the i-th entry of a. If you are of a
mathematical bent it may be illuminating to think of this as being
analogous
to, say, {3} being a subset of the set {1,2,3,4,5} and 3 being an
element
of this set.
> 1. What sort of object can the operators [] and [[]] be applied to?
> How do
> they differ? I mean objects in standard R, not in packages that
> provide
> conceivable overloading of these operators (if that's possible).
Essentially lists. Note that any vector can be considered to be a list.
Note that data frames are lists.
> 2. Meta-question: how could one search for an answer to this
> question in the
> newsgroup?
RSiteSearch("[") or RSiteSearch("[[")
> 3. Meta-question: how could one search for an answer to this in the
> FAQ?
Doesn't seem to be there, AFAICS.
> 4. Meta-question: how could one search for an answer to this in any
> of the
> links one is shown after typing help.start()?
Click on ``Search Engine and Keywords''; then search on ``[[''. Then
click on ``Extract''. This last is less than totally perspicuous. ;-)
Instead of trying to follow these links, just do
?"[" or ?"[["
HTH.
cheers,
Rolf Turner
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